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<!--     disk_mmc.sgml                                               -->
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<!--     Documentation for the MMC disk device driver                -->
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<!-- Author(s):   bartv                                              -->
<!-- Contact(s):  bartv                                              -->
<!-- Date:        2004/04/25                                         -->
<!-- Version:     0.01                                               -->
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<part id="devs-disk-mmc-part"><title>MMC MultiMedia Card Disk Driver</title>

<refentry id="devs-disk-mmc">
  <refmeta>
    <refentrytitle>Device Driver for MMC MultiMedia Cards</refentrytitle>
  </refmeta>
  <refnamediv>
    <refname><varname>CYGPKG_DEVS_DISK_MMC</varname></refname>
    <refpurpose>eCos Support for MMC MultiMedia Cards</refpurpose>
  </refnamediv>

  <refsect1 id="devs-disk-mmc-description"><title>Description</title>
    <para>
This package provides a disk device driver for MultiMediaCards (MMC).
A MultiMediaCard provides non-volatile storage in a small footprint
(24mm &ast; 32mm &ast; 1.4mm), and weighing less than 2 grams. Typical
card sizes are 16MB to 128MB, with an upper limit of 4GB. It
should be noted that these sizes are measured in millions of bytes,
not 2^20. The <ulink url="http://www.mmca.org">MultiMediaCard
Association</ulink> defines the standard for these cards.
    </para>
    <para>
At the hardware level there are two ways of accessing an MMC card,
either using a custom interface or via an SPI bus. A card will detect
the interface in use at run-time. The custom interface allows for
better performance but requires additional hardware. Currently only
SPI mode is supported by this package.
    </para>
    <para>
Theoretically an MMC card can be used with any file system. In
practice all cards are formatted for PC compatibility, with a
partition table in the first block and a single FAT file system on the
rest of the card. Currently this package checks the format of the MMC
card and will only allow access to a card if it is formatted this way.
    </para>
    <para>
An MMC socket allows cards to be removed and inserted at any time. The
device driver will detect removal events when the next I/O operation
happens and will report them to higher-level code via an error code
<literal>ENODEV</literal>. However there are no guarantees that the
higher-level code will be able to recover from this error. The
expected usage is that application code will explicitly
<function>mount</function> the card before attempting any file I/O,
and will <function>umount</function> the card before it is removed.
Between these operations the system is likely to keep some disk blocks
cached, for performance reasons. If the card is removed before the
<function>umount</function> then it may end up with a corrupted file
system, and the disk subsystem may be left in an inconsistent state.
Regular uses of <function>sync</function> will reduce the risk
of file system corruption.
    </para>
  </refsect1>

  <refsect1 id="devs-disk-mmc-config"><title>Configuration Options</title>
    <para>
<varname>CYGPKG_DEVS_DISK_MMC</varname> is a hardware package which
should get loaded automatically when you configure for a suitable eCos
target platform. In this case suitable means that the hardware has an
MMC socket connected to an SPI bus, that an SPI bus driver package
exists and is also automatically loaded, and that the platform HAL
provides <link linkend="devs-disk-mmc-porting">information</link>
on how the card is connected to the SPI bus.
    </para>
    <para>
The package depends on support from the generic disk package
<varname>CYGPKG_IO_DISK</varname>. That will not be loaded
automatically: the presence of an MMC socket on the board does not
mean that the application has any need for a file system. Hence by
default <varname>CYGPKG_DEVS_DISK_MMC</varname> will be inactive and
will not contribute any code or data to the application's memory
footprint. To activate the driver it will be necessary to add one or
more packages to the configuration using
<command>ecosconfig&nbsp;add</command> or the graphical configuration
tool: the generic disk support <varname>CYGPKG_IO_DISK</varname>;
usually a file system, <varname>CYGPKG_FS_FAT</varname>; support for
the file I/O API <varname>CYGPKG_IO_FILEIO</varname>; and possibly
additional support packages that may be needed by the file system, for
example <varname>CYGPKG_LINUX_COMPAT</varname>. Depending on the
template used to create the initial configuration some of these may be
loaded already.
    </para>
    <para>
The package provides two main configuration options.
<varname>CYGDAT_DEVS_DISK_MMC_SPI_DISK0_NAME</varname> specifies the
name of the raw disk device, for example <literal>/dev/hd0</literal>.
Allowing for partition tables that makes <literal>/dev/hd0/1</literal>
the first argument that shoul be passed to a
<function>mount</function> call. If the hardware has multiple disk
devices then each one will need a unique name.
<varname>CYGIMP_DEVS_DISK_MMC_SPI_POLLED</varname> controls whether
the SPI bus will be accessed in interrupt-driven or polled mode. It
will default to interrupt-driven if the application is multi-threaded,
which is assumed to be the case if the kernel is present. If the
kernel is absent, for example in a RedBoot configuration, then the
driver will default to polled mode. With some hardware polled mode may
significantly increase disk throughput even in a multi-threaded
application, but will consume cpu cycles that could be used by other
threads.
    </para>
  </refsect1>
  
  <refsect1 id="devs-disk-mmc-extra"><title>Additional Functionality</title>
    <para>
The disk driver package exports a variable
<varname>cyg_mmc_spi_polled</varname>. This defaults to true or false
depending on the configuration option
<varname>CYGIMP_DEVS_DISK_MMC_SPI_POLLED</varname>. If the default
mode is interrupt-driven then file I/O, including mount operations,
are only allowed when the scheduler has started and interrupts have
been enabled. Any attempts at file I/O earlier during system
initialization, for example inside a C++ static constructor, will lock
up. If it is necessary to perform file I/O at this time then the
driver can be temporarily switched to polling mode before the I/O
operation by setting <varname>cyg_mmc_spi_polled</varname>, and
clearing it again after the I/O. Alternatively the default mode can be
changed to polling by editing the configuration, and then the
<function>main()</function> thread can change the mode to
interrupt-driven once the scheduler has started.
    </para>
  </refsect1>

  <refsect1 id="devs-disk-mmc-porting"><title>Porting to New Hardware</title>
    <para>
Assuming that the MMC connector is hooked up to a standard SPI bus and
that there is already an eCos SPI bus driver, porting the MMC disk
driver package should be straightforward. Some other package, usually
the platform HAL, should provide a <type>cyg_spi_device</type>
structure <varname>cyg_spi_mmc_dev0</varname>. That structure contains
the information needed by this package to interact with the MMC card
via the usual SPI interface, for example how to activate the
appropriate chip select. The platform HAL should also implement the
CDL interface <varname>CYGINT_DEVS_DISK_MMC_SPI_CONNECTORS</varname>.
    </para>
    <para>
When defining <varname>cyg_spi_mmc_dev0</varname> special care must be
taken with the chip select. The MMC protocol is transaction-oriented.
For example a read operation involves an initial command sent to the
card, then a reply, then the actual data, and finally a checksum. The
card's chip select must be kept asserted for the entire operation, and
there can be no interactions with other devices on the same SPI bus
during this time.
    </para>
    <para>
Optionally the platform HAL may define a macro
<function>HAL_MMC_SPI_INIT</function> which will be invoked during a
mount operation. This can take any hardware-specific actions that may
be necessary, for example manipulating GPIO pins. Usually no such
macro is needed because the hardware is set up during platform
initialization.
    </para>
    <para>
Currently the package does not provide any support for accessing MMC
cards using an interface other than SPI. On some targets there may be
additional hardware to detect events such as card insertion or
removal, but there is no support for exploiting such hardware at
present.
    </para>
    <para>
Only a single MMC socket is supported. Extending the package to
support multiple sockets would be straightforward but it seems
unlikely that any hardware would come with multiple MMC sockets. Given
the nature of SPI buses there is a problem if the MMC socket is hooked
up via an expansion connector rather than being attached to the main
board. The platform HAL would not know about the socket so would not
implement the CDL interface
<varname>CYGINT_DEVS_DISK_MMC_SPI_CONNECTORS</varname>, and the
<database>ecos.db</database> target entry would not include
<varname>CYGPKG_DEVS_DISK_MMC</varname>. Because this is a hardware
package it cannot easily be added by hand. Instead this scenario would
require some editing of the existing platform HAL and target entry.
    </para>
  </refsect1>

</refentry>  
</part>