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<!-- Copyright (C) 2002 Red Hat, Inc. -->
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<!-- permission is obtained from the copyright holder. -->
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<HTML
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><HEAD
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><TITLE
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>Devtab Entries</TITLE
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><meta name="MSSmartTagsPreventParsing" content="TRUE">
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CONTENT="Modular DocBook HTML Stylesheet Version 1.64
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"><LINK
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REL="HOME"
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TITLE="eCos USB Slave Support"
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HREF="io-usb-slave.html"><LINK
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REL="PREVIOUS"
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TITLE="Starting up a USB Device"
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HREF="usbs-start.html"><LINK
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REL="NEXT"
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TITLE="Receiving Data from the Host"
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HREF="usbs-start-rx.html"></HEAD
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><BODY
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CLASS="REFENTRY"
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BGCOLOR="#FFFFFF"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>eCos USB Slave Support</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="usbs-start.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="usbs-start-rx.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="USBS-DEVTAB"
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>Devtab Entries</A
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></H1
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><DIV
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CLASS="REFNAMEDIV"
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NAME="AEN200"
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></A
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><H2
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>Name</H2
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>Devtab Entries -- Data endpoint data structure</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN203"
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></A
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><H2
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>Synopsis</H2
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><TABLE
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BORDER="0"
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><TD
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><PRE
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CLASS="SYNOPSIS"
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>/dev/usb0c
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/dev/usb1r
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/dev/usb2w</PRE
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN205"
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></A
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><H2
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>Devtab Entries</H2
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><P
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>USB device drivers provide two ways of transferring data between host
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and peripheral. The first involves USB-specific functionality such as
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<A
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HREF="usbs-start-rx.html"
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><TT
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CLASS="FUNCTION"
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>usbs_start_rx_buffer</TT
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></A
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>.
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This provides non-blocking I/O: a transfer is started, and some time
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later the device driver will call a supplied completion function. The
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second uses the conventional I/O model: there are entries in the
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device table corresponding to the various endpoints. Standard calls
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such as <TT
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CLASS="FUNCTION"
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>open</TT
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> can then be used to get a suitable
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handle. Actual I/O happens via blocking <TT
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CLASS="FUNCTION"
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>read</TT
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> and
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<TT
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CLASS="FUNCTION"
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>write</TT
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> calls. In practice the blocking operations
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are simply implemented using the underlying non-blocking
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functionality.</P
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><P
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>Each endpoint will have its own devtab entry. The exact names are
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controlled by the device driver package, but typically the root will
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be <TT
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CLASS="LITERAL"
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>/dev/usb</TT
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>. This is followed by one or more
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decimal digits giving the endpoint number, followed by
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<TT
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CLASS="LITERAL"
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>c</TT
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> for a control endpoint, <TT
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CLASS="LITERAL"
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>r</TT
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> for
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a receive endpoint (host to peripheral), and <TT
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CLASS="LITERAL"
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>w</TT
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> for
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a transmit endpoint (peripheral to host). If the target hardware
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involves more than one USB device then different roots should be used,
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for example <TT
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CLASS="LITERAL"
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>/dev/usb0c</TT
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> and
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<TT
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CLASS="LITERAL"
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>/dev/usb1_0c</TT
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>. This may require explicit
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manipulation of device driver configuration options by the application
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developer.</P
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><P
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>At present the devtab entry for a control endpoint does not support
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any I/O operations. </P
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><DIV
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CLASS="REFSECT2"
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><A
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NAME="AEN221"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>write</TT
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> operations</H3
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><P
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><TT
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CLASS="FUNCTION"
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>cyg_io_write</TT
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> and similar functions in
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higher-level packages can be used to perform a transfer from
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peripheral to host. Successive write operations will not be coalesced.
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For example, when doing a 1000 byte write to an endpoint that uses the
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bulk transfer protocol this will involve 15 full-size 64-byte packets
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and a terminating 40-byte packet. USB device drivers are not expected
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to do any locking, and if higher-level code performs multiple
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concurrent write operations on a single endpoint then the resulting
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behaviour is undefined.</P
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><P
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>A USB <TT
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CLASS="FUNCTION"
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>write</TT
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> operation will never transfer less
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data than specified. It is the responsibility of higher-level code to
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ensure that the amount of data being transferred is acceptable to the
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host-side code. Usually this will be defined by a higher-level
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protocol. If an attempt is made to transfer more data than the host
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expects then the resulting behaviour is undefined.</P
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><P
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>There are two likely error conditions. <TT
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CLASS="LITERAL"
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>EPIPE</TT
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>
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indicates that the connection between host and target has been broken.
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<TT
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CLASS="LITERAL"
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>EAGAIN</TT
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> indicates that the endpoint has been
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stalled, either at the request of the host or by other activity
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| 225 |
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inside the peripheral.</P
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| 226 |
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></DIV
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><DIV
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| 228 |
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CLASS="REFSECT2"
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><A
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NAME="AEN231"
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></A
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><H3
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><TT
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CLASS="FUNCTION"
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>read</TT
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> operations</H3
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><P
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| 238 |
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><TT
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| 239 |
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CLASS="FUNCTION"
|
| 240 |
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>cyg_io_read</TT
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| 241 |
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|
> and similar functions in higher-level
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packages can be used to perform a transfer from host to peripheral.
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This should be a complete transfer: higher-level protocols should
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define an upper bound on the amount of data being transferred, and the
|
| 245 |
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<TT
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| 246 |
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CLASS="FUNCTION"
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>read</TT
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| 248 |
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> operation should involve at least this
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| 249 |
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amount of data. The return value will indicate the actual transfer
|
| 250 |
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size, which may be less than requested.</P
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| 251 |
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><P
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| 252 |
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>Some device drivers may support partial reads, but USB device drivers
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| 253 |
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are not expected to perform any buffering because that involves both
|
| 254 |
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memory and code overheads. One technique that may work for bulk
|
| 255 |
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transfers is to exploit the fact that such transfers happen in 64-byte
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packets. It is possible to <TT
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| 257 |
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CLASS="FUNCTION"
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| 258 |
|
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>read</TT
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| 259 |
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> an initial 64
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| 260 |
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bytes, corresponding to the first packet in the transfer. These 64
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bytes can then be examined to determine the total transfer size, and
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| 262 |
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the remaining data can be transferred in another
|
| 263 |
|
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<TT
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| 264 |
|
|
CLASS="FUNCTION"
|
| 265 |
|
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>read</TT
|
| 266 |
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> operation. This technique is not guaranteed
|
| 267 |
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to work with all USB hardware. Also, if the delay between accepting
|
| 268 |
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the first packet and the remainder of the transfer is excessive then
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| 269 |
|
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this could cause timeout problems for the host-side software. For
|
| 270 |
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these reasons the use of partial reads should be avoided.</P
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| 271 |
|
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><P
|
| 272 |
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>There are two likely error conditions. <TT
|
| 273 |
|
|
CLASS="LITERAL"
|
| 274 |
|
|
>EPIPE</TT
|
| 275 |
|
|
>
|
| 276 |
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|
indicates that the connection between host and target has been broken.
|
| 277 |
|
|
<TT
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| 278 |
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|
CLASS="LITERAL"
|
| 279 |
|
|
>EAGAIN</TT
|
| 280 |
|
|
> indicates that the endpoint has been
|
| 281 |
|
|
stalled, either at the request of the host or by other activity
|
| 282 |
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|
inside the peripheral.</P
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| 283 |
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|
><P
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| 284 |
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>USB device drivers are not expected to do any locking. If higher-level
|
| 285 |
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code performs multiple concurrent read operations on a single endpoint
|
| 286 |
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then the resulting behaviour is undefined.</P
|
| 287 |
|
|
></DIV
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| 288 |
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><DIV
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| 289 |
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|
CLASS="REFSECT2"
|
| 290 |
|
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><A
|
| 291 |
|
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NAME="AEN244"
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></A
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><H3
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| 294 |
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><TT
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| 295 |
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CLASS="FUNCTION"
|
| 296 |
|
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>select</TT
|
| 297 |
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|
> operations</H3
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| 298 |
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><P
|
| 299 |
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>Typical USB device drivers will not provide any support for
|
| 300 |
|
|
<TT
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| 301 |
|
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CLASS="FUNCTION"
|
| 302 |
|
|
>select</TT
|
| 303 |
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|
>. Consider bulk transfers from the host to
|
| 304 |
|
|
the peripheral. At the USB device driver level there is no way of
|
| 305 |
|
|
knowing in advance how large a transfer will be, so it is not feasible
|
| 306 |
|
|
for the device driver to buffer the entire transfer. It may be
|
| 307 |
|
|
possible to buffer part of the transfer, for example the first 64-byte
|
| 308 |
|
|
packet, and copy this into application space at the start of a
|
| 309 |
|
|
<TT
|
| 310 |
|
|
CLASS="FUNCTION"
|
| 311 |
|
|
>read</TT
|
| 312 |
|
|
>, but this adds code and memory overheads.
|
| 313 |
|
|
Worse, it means that there is an unknown but potentially long delay
|
| 314 |
|
|
between a peripheral accepting the first packet of a transfer and the
|
| 315 |
|
|
remaining packets, which could confuse or upset the host-side
|
| 316 |
|
|
software.</P
|
| 317 |
|
|
><P
|
| 318 |
|
|
>With some USB hardware it may be possible for the device driver to
|
| 319 |
|
|
detect OUT tokens from the host without actually accepting the data,
|
| 320 |
|
|
and this would indicate that a <TT
|
| 321 |
|
|
CLASS="FUNCTION"
|
| 322 |
|
|
>read</TT
|
| 323 |
|
|
> is likely to
|
| 324 |
|
|
succeed. However, it would not be reliable since the host-side I/O
|
| 325 |
|
|
operation could time out. A similar mechanism could be used to
|
| 326 |
|
|
implement <TT
|
| 327 |
|
|
CLASS="FUNCTION"
|
| 328 |
|
|
>select</TT
|
| 329 |
|
|
> for outgoing data, but again
|
| 330 |
|
|
this would not be reliable.</P
|
| 331 |
|
|
><P
|
| 332 |
|
|
>Some device drivers may provide partial support for
|
| 333 |
|
|
<TT
|
| 334 |
|
|
CLASS="FUNCTION"
|
| 335 |
|
|
>select</TT
|
| 336 |
|
|
> anyway, possibly under the control of a
|
| 337 |
|
|
configuration option. The device driver's documentation should be
|
| 338 |
|
|
consulted for further information. It is also worth noting that the
|
| 339 |
|
|
USB-specific non-blocking API can often be used as an alternative to
|
| 340 |
|
|
<TT
|
| 341 |
|
|
CLASS="FUNCTION"
|
| 342 |
|
|
>select</TT
|
| 343 |
|
|
>.</P
|
| 344 |
|
|
></DIV
|
| 345 |
|
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><DIV
|
| 346 |
|
|
CLASS="REFSECT2"
|
| 347 |
|
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><A
|
| 348 |
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NAME="AEN256"
|
| 349 |
|
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></A
|
| 350 |
|
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><H3
|
| 351 |
|
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><TT
|
| 352 |
|
|
CLASS="FUNCTION"
|
| 353 |
|
|
>get_config</TT
|
| 354 |
|
|
> and
|
| 355 |
|
|
<TT
|
| 356 |
|
|
CLASS="FUNCTION"
|
| 357 |
|
|
>set_config</TT
|
| 358 |
|
|
> operations</H3
|
| 359 |
|
|
><P
|
| 360 |
|
|
>There are no <TT
|
| 361 |
|
|
CLASS="FUNCTION"
|
| 362 |
|
|
>set_config</TT
|
| 363 |
|
|
> or
|
| 364 |
|
|
<TT
|
| 365 |
|
|
CLASS="FUNCTION"
|
| 366 |
|
|
>get_config</TT
|
| 367 |
|
|
> (also known as
|
| 368 |
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|
<TT
|
| 369 |
|
|
CLASS="FUNCTION"
|
| 370 |
|
|
>ioctl</TT
|
| 371 |
|
|
>) operations defined for USB devices.
|
| 372 |
|
|
Some device drivers may provide hardware-specific facilities this way. </P
|
| 373 |
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><DIV
|
| 374 |
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CLASS="NOTE"
|
| 375 |
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><BLOCKQUOTE
|
| 376 |
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CLASS="NOTE"
|
| 377 |
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><P
|
| 378 |
|
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><B
|
| 379 |
|
|
>Note: </B
|
| 380 |
|
|
>Currently the USB-specific functions related to <A
|
| 381 |
|
|
HREF="usbs-halt.html"
|
| 382 |
|
|
>halted endpoints</A
|
| 383 |
|
|
> cannot be accessed readily
|
| 384 |
|
|
via devtab entries. This functionality should probably be made
|
| 385 |
|
|
available via <TT
|
| 386 |
|
|
CLASS="FUNCTION"
|
| 387 |
|
|
>set_config</TT
|
| 388 |
|
|
> and
|
| 389 |
|
|
<TT
|
| 390 |
|
|
CLASS="FUNCTION"
|
| 391 |
|
|
>get_config</TT
|
| 392 |
|
|
>. It may also prove useful to provide
|
| 393 |
|
|
a <TT
|
| 394 |
|
|
CLASS="FUNCTION"
|
| 395 |
|
|
>get_config</TT
|
| 396 |
|
|
> operation that maps from the
|
| 397 |
|
|
devtab entries to the underlying endpoint data structures.</P
|
| 398 |
|
|
></BLOCKQUOTE
|
| 399 |
|
|
></DIV
|
| 400 |
|
|
></DIV
|
| 401 |
|
|
><DIV
|
| 402 |
|
|
CLASS="REFSECT2"
|
| 403 |
|
|
><A
|
| 404 |
|
|
NAME="AEN270"
|
| 405 |
|
|
></A
|
| 406 |
|
|
><H3
|
| 407 |
|
|
>Presence</H3
|
| 408 |
|
|
><P
|
| 409 |
|
|
>The devtab entries are optional. If the USB device is accessed
|
| 410 |
|
|
primarily by class-specific code such as the USB-ethernet package and
|
| 411 |
|
|
that package uses the USB-specific API directly, the devtab entries
|
| 412 |
|
|
are redundant. Even if application code does need to access the USB
|
| 413 |
|
|
device, the non-blocking API may be more convenient than the blocking
|
| 414 |
|
|
I/O provided via the devtab entries. In these cases the devtab entries
|
| 415 |
|
|
serve no useful purpose, but they still impose a memory overhead. It
|
| 416 |
|
|
is possible to suppress the presence of these entries by disabling the
|
| 417 |
|
|
configuration option
|
| 418 |
|
|
<TT
|
| 419 |
|
|
CLASS="LITERAL"
|
| 420 |
|
|
>CYGGLO_IO_USB_SLAVE_PROVIDE_DEVTAB_ENTRIES</TT
|
| 421 |
|
|
>.</P
|
| 422 |
|
|
></DIV
|
| 423 |
|
|
></DIV
|
| 424 |
|
|
><DIV
|
| 425 |
|
|
CLASS="NAVFOOTER"
|
| 426 |
|
|
><HR
|
| 427 |
|
|
ALIGN="LEFT"
|
| 428 |
|
|
WIDTH="100%"><TABLE
|
| 429 |
|
|
WIDTH="100%"
|
| 430 |
|
|
BORDER="0"
|
| 431 |
|
|
CELLPADDING="0"
|
| 432 |
|
|
CELLSPACING="0"
|
| 433 |
|
|
><TR
|
| 434 |
|
|
><TD
|
| 435 |
|
|
WIDTH="33%"
|
| 436 |
|
|
ALIGN="left"
|
| 437 |
|
|
VALIGN="top"
|
| 438 |
|
|
><A
|
| 439 |
|
|
HREF="usbs-start.html"
|
| 440 |
|
|
>Prev</A
|
| 441 |
|
|
></TD
|
| 442 |
|
|
><TD
|
| 443 |
|
|
WIDTH="34%"
|
| 444 |
|
|
ALIGN="center"
|
| 445 |
|
|
VALIGN="top"
|
| 446 |
|
|
><A
|
| 447 |
|
|
HREF="io-usb-slave.html"
|
| 448 |
|
|
>Home</A
|
| 449 |
|
|
></TD
|
| 450 |
|
|
><TD
|
| 451 |
|
|
WIDTH="33%"
|
| 452 |
|
|
ALIGN="right"
|
| 453 |
|
|
VALIGN="top"
|
| 454 |
|
|
><A
|
| 455 |
|
|
HREF="usbs-start-rx.html"
|
| 456 |
|
|
>Next</A
|
| 457 |
|
|
></TD
|
| 458 |
|
|
></TR
|
| 459 |
|
|
><TR
|
| 460 |
|
|
><TD
|
| 461 |
|
|
WIDTH="33%"
|
| 462 |
|
|
ALIGN="left"
|
| 463 |
|
|
VALIGN="top"
|
| 464 |
|
|
>Starting up a USB Device</TD
|
| 465 |
|
|
><TD
|
| 466 |
|
|
WIDTH="34%"
|
| 467 |
|
|
ALIGN="center"
|
| 468 |
|
|
VALIGN="top"
|
| 469 |
|
|
> </TD
|
| 470 |
|
|
><TD
|
| 471 |
|
|
WIDTH="33%"
|
| 472 |
|
|
ALIGN="right"
|
| 473 |
|
|
VALIGN="top"
|
| 474 |
|
|
>Receiving Data from the Host</TD
|
| 475 |
|
|
></TR
|
| 476 |
|
|
></TABLE
|
| 477 |
|
|
></DIV
|
| 478 |
|
|
></BODY
|
| 479 |
|
|
></HTML
|
| 480 |
|
|
>
|
