Subversion Repositories openrisc

/*{{{  Banner                                                   */

/*=================================================================

//

//        common.c

//

//        USB testing - code common to host and target

//

//==========================================================================

//####ECOSGPLCOPYRIGHTBEGIN####

// -------------------------------------------

// This file is part of eCos, the Embedded Configurable Operating System.

// Copyright (C) 1998, 1999, 2000, 2001, 2002 Red Hat, Inc.

//

// eCos is free software; you can redistribute it and/or modify it under

// the terms of the GNU General Public License as published by the Free

// Software Foundation; either version 2 or (at your option) any later version.

//

// eCos is distributed in the hope that it will be useful, but WITHOUT ANY

// WARRANTY; without even the implied warranty of MERCHANTABILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// You should have received a copy of the GNU General Public License along

// with eCos; if not, write to the Free Software Foundation, Inc.,

// 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.

//

// As a special exception, if other files instantiate templates or use macros

// or inline functions from this file, or you compile this file and link it

// with other works to produce a work based on this file, this file does not

// by itself cause the resulting work to be covered by the GNU General Public

// License. However the source code for this file must still be made available

// in accordance with section (3) of the GNU General Public License.

//

// This exception does not invalidate any other reasons why a work based on

// this file might be covered by the GNU General Public License.

//

// Alternative licenses for eCos may be arranged by contacting Red Hat, Inc.

// at http://sources.redhat.com/ecos/ecos-license/

// -------------------------------------------

//####ECOSGPLCOPYRIGHTEND####

//==========================================================================

//#####DESCRIPTIONBEGIN####

//

// This module contains some definitions and functions that are common to

// both the host and target side of USB testing, for example filling in

// a buffer with well-known data and validating the contents at the other end.

// The module is #include'd by other code rather than compiled separately,

// which simplifies the build process.

//

// Author(s):     bartv

// Date:          2001-08-14

//####DESCRIPTIONEND####

//==========================================================================

*/

/*}}}*/

/*{{{  Simple data pack and unpack operations                   */

// ----------------------------------------------------------------------------

// Utilities to pack and unpack data into buffers. 

//

// Integers are transferred with 32 bits of precision, irrespective

// of the capabilities of either target and host.

static inline void

pack_int(int datum, unsigned char* buffer, int* index_ptr)

{

    int index = *index_ptr;

    buffer[index++] = (datum >>  0) & 0x0FF;

    buffer[index++] = (datum >>  8) & 0x0FF;

    buffer[index++] = (datum >> 16) & 0x0FF;

    buffer[index++] = (datum >> 24) & 0x0FF;

    *index_ptr = index;

}

static inline int

unpack_int(unsigned char* buffer, int* index_ptr)

{

    int index   = *index_ptr;

    int result;

    result  = (buffer[index++] <<  0);

    result |= (buffer[index++] <<  8);

    result |= (buffer[index++] << 16);

    result |= (buffer[index++] << 24);

    *index_ptr = index;

    return result;

}

/*}}}*/

/*{{{  Buffer data and validation                               */

// ----------------------------------------------------------------------------

// The data required for a given test. For some test cases, for

// example when trying to achieve maximum throughput, it does not

// matter what data is transferred. For other tests it is important to

// validate that the data sent and received match up, and there should

// be some control over the actual data: some tests might want to send

// a long sequence of byte 0, while others want to send more random data

// for which a simple random number generator is useful.

//

// Exactly the same routines are used on both host and target to fill in

// and check buffers, and they are sufficiently simple that the routines

// should get compiled in compatible ways.

//

// There is no support at present for sending specific data, e.g. a

// specific ethernet packet that appears to be causing problems. Knowledge

// of specific data cannot be compiled into the test code, so the only

// way to implement something like this would be to transfer the

// problematical data over the USB bus in order to determine whether or

// not the bus is capable of reliably transferring this data. That is

// not entirely impossible (checksums, use of alternative endpoints),

// but it is not implemented.

//

// An alternative approach would be to support a bounce operation

// involving both an IN and an OUT endpoint, doing validation only on

// the host. Again that is not yet implemented.

//

// The byte_fill and int_fill options are actually redundant because the

// same effect can be achieved using a multiplier of 1 and an increment

// of 0, but they can be implemented much more efficiently so may be

// useful for benchmarks.

typedef enum usbtestdata {

    usbtestdata_none        = 0,       // There is nothing useful in the data

    usbtestdata_bytefill    = 1,       // The data consists of a single byte, repeated

    usbtestdata_wordfill    = 2,       // Or a single integer

    usbtestdata_byteseq     = 3,       // Or a pseudo-random sequence (a * seed) + b

    usbtestdata_wordseq     = 4        // as either bytes or integers

} usbtestdata;

typedef struct UsbTestData {

    usbtestdata     format;

    int             seed;

    int             multiplier; // 1103515245

    int             increment;  // 12345

    int             transfer_seed_multiplier;

    int             transfer_seed_increment;

    int             transfer_multiplier_multiplier;

    int             transfer_multiplier_increment;

    int             transfer_increment_multiplier;

    int             transfer_increment_increment;

} UsbTestData;

static void

usbtest_fill_buffer(UsbTestData* how, unsigned char* buffer, int length)

{

    switch(how->format)

    {

      case usbtestdata_none:

        return;

      case usbtestdata_bytefill:

        // Leave it to the system to optimise memset().

        memset(buffer, (how->seed & 0x0FF), length);

        break;

      case usbtestdata_wordfill:

      {

          // The buffer may not be a multiple of four bytes, so the last entry is always

          // zero'd.

          int i;

          int index = 0;

          for (i = 0; i < (length / 4); i++) {

              pack_int(how->seed, buffer, &index);

          }

          pack_int(0, buffer, &index);

          break;

      }

      case usbtestdata_byteseq:

      {

          int i;

          for (i = 0; i < length; i++) {

              buffer[i] = (how->seed & 0x00FF);

              how->seed *= how->multiplier;

              how->seed += how->increment;

          }

          break;

      }

      case usbtestdata_wordseq:

      {

          int i;

          int index = 0;

          for (i = 0; i < (length / 4); i++) {

              pack_int(how->seed, buffer, &index);

              how->seed *= how->multiplier;

              how->seed += how->increment;

          }

          pack_int(0, buffer, &index);

          break;

      }

    }

    // After each transfer update the seed, multiplier and increment

    // ready for the next one.

    how->seed       *= how->transfer_seed_multiplier;

    how->seed       += how->transfer_seed_increment;

    how->multiplier *= how->transfer_multiplier_multiplier;

    how->multiplier += how->transfer_multiplier_increment;

    how->increment  *= how->transfer_increment_multiplier;

    how->increment  += how->transfer_increment_increment;

}

static int

usbtest_check_buffer(UsbTestData* how, unsigned char* buffer, int length)

{

    int result  = 1;

    switch(how->format) {

      case usbtestdata_none:

        break;

      case usbtestdata_bytefill:

      {

          int i;

          result = 1;

          for (i = 0; i < length; i++) {

              if (buffer[i] != (how->seed & 0x00FF)) {

                  result = 0;

                  break;

              }

          }

          break;

      }

      case usbtestdata_wordfill:

      {

          int i;

          int index = 0;

          for (i = 0; i < (length / 4); i++) {

              int datum = unpack_int(buffer, &index);

              if (datum != (how->seed & 0x0FFFFFFFF)) {

                  result = 0;

                  break;

              }

          }

          for (i = 4 * i; result && (i < length); i++) {

              if (0 != buffer[i]) {

                  result = 0;

                  break;

              }

          }

          break;

      }

      case usbtestdata_byteseq:

      {

          int i;

          for (i = 0; i < length; i++) {

              if (buffer[i] != (how->seed & 0x00FF)) {

                  result = 0;

                  break;

              }

              how->seed *= how->multiplier;

              how->seed += how->increment;

          }

          break;

      }

      case usbtestdata_wordseq:

      {

          int   i;

          int   index = 0;

          for (i = 0; i < (length / 4); i++) {

              int datum = unpack_int(buffer, &index);

              if (datum != (how->seed & 0x0FFFFFFFF)) {

                  result = 0;

                  break;

              }

              how->seed *= how->multiplier;

              how->seed += how->increment;

          }

          for (i = 4 * i; result && (i < length); i++) {

              if (0 != buffer[i]) {

                  result = 0;

                  break;

              }

          }

          break;

      }

    }

    // After each transfer update the seed, multiplier and increment

    // ready for the next transfer.

    how->seed       *= how->transfer_seed_multiplier;

    how->seed       += how->transfer_seed_increment;

    how->multiplier *= how->transfer_multiplier_multiplier;

    how->multiplier += how->transfer_multiplier_increment;

    how->increment  *= how->transfer_increment_multiplier;

    how->increment  += how->transfer_increment_increment;

    return result;

}

#ifdef HOST

static void

pack_usbtestdata(UsbTestData* data, unsigned char* buf, int* index)

{

    pack_int((int)data->format,                         buf, index);

    pack_int((int)data->seed,                           buf, index);

    pack_int((int)data->multiplier,                     buf, index);

    pack_int((int)data->increment,                      buf, index);

    pack_int((int)data->transfer_seed_multiplier,       buf, index);

    pack_int((int)data->transfer_seed_increment,        buf, index);

    pack_int((int)data->transfer_multiplier_multiplier, buf, index);

    pack_int((int)data->transfer_multiplier_increment,  buf, index);

    pack_int((int)data->transfer_increment_multiplier,  buf, index);

    pack_int((int)data->transfer_increment_increment,   buf, index);

}

#endif

#ifdef TARGET

static void

unpack_usbtestdata(UsbTestData* data, unsigned char* buf, int* index)

{

    data->format                        = (usbtestdata) unpack_int(buf, index);

    data->seed                          = unpack_int(buf, index);

    data->multiplier                    = unpack_int(buf, index);

    data->increment                     = unpack_int(buf, index);

    data->transfer_seed_multiplier      = unpack_int(buf, index);

    data->transfer_seed_increment       = unpack_int(buf, index);

    data->transfer_multiplier_multiplier= unpack_int(buf, index);

    data->transfer_multiplier_increment = unpack_int(buf, index);

    data->transfer_increment_multiplier = unpack_int(buf, index);

    data->transfer_increment_increment  = unpack_int(buf, index);

}

#endif

/*}}}*/

/*{{{  Testcase definitions                                     */

// ----------------------------------------------------------------------------

// Definitions of the supported test cases. The actual implementations need

// to vary between host and target.

typedef enum usbtest {

    usbtest_invalid     = 0,

    usbtest_bulk_out    = 1,

    usbtest_bulk_in     = 2,

    usbtest_control_in  = 3

} usbtest;

// What I/O mechanism should be used on the target to process data?

typedef enum usb_io_mechanism {

    usb_io_mechanism_usb    = 1,        // The low-level USB-specific API

    usb_io_mechanism_dev    = 2         // cyg_devio_cread() et al

} usb_io_mechanism;

// Bulk transfers. The same structure can be used for IN and OUT transfers.

// The endpoint number will be or'd with either USB_DIR_IN or USB_DIR_OUT,

// or the equivalent under eCos.

typedef struct UsbTest_Bulk {

    int                 number_packets;

    int                 endpoint;

    int                 tx_size;

    int                 tx_size_min;

    int                 tx_size_max;

    int                 tx_size_multiplier;

    int                 tx_size_divisor;

    int                 tx_size_increment;

    int                 rx_size;

    int                 rx_size_min;

    int                 rx_size_max;

    int                 rx_size_multiplier;

    int                 rx_size_divisor;

    int                 rx_size_increment;

    int                 rx_padding;

    int                 tx_delay;

    int                 tx_delay_min;

    int                 tx_delay_max;

    int                 tx_delay_multiplier;

    int                 tx_delay_divisor;

    int                 tx_delay_increment;

    int                 rx_delay;

    int                 rx_delay_min;

    int                 rx_delay_max;

    int                 rx_delay_multiplier;

    int                 rx_delay_divisor;

    int                 rx_delay_increment;

    usb_io_mechanism    io_mechanism;

    UsbTestData         data;

} UsbTest_Bulk;

#ifdef HOST

static void

pack_usbtest_bulk(UsbTest_Bulk* test, unsigned char* buffer, int* index)

{

    pack_int(test->number_packets,          buffer, index);

    pack_int(test->endpoint,                buffer, index);

    pack_int(test->tx_size,                 buffer, index);

    pack_int(test->tx_size_min,             buffer, index);

    pack_int(test->tx_size_max,             buffer, index);

    pack_int(test->tx_size_multiplier,      buffer, index);

    pack_int(test->tx_size_divisor,         buffer, index);

    pack_int(test->tx_size_increment,       buffer, index);

    pack_int(test->rx_size,                 buffer, index);

    pack_int(test->rx_size_min,             buffer, index);

    pack_int(test->rx_size_max,             buffer, index);

    pack_int(test->rx_size_multiplier,      buffer, index);

    pack_int(test->rx_size_divisor,         buffer, index);

    pack_int(test->rx_size_increment,       buffer, index);

    // There is no need to transfer the padding field. It is only of

    // interest on the host, and this message is being packed

    // for the target side.

    pack_int(test->tx_delay,                buffer, index);

    pack_int(test->tx_delay_min,            buffer, index);

    pack_int(test->tx_delay_max,            buffer, index);

    pack_int(test->tx_delay_multiplier,     buffer, index);

    pack_int(test->tx_delay_divisor,        buffer, index);

    pack_int(test->tx_delay_increment,      buffer, index);

    pack_int(test->rx_delay,                buffer, index);

    pack_int(test->rx_delay_min,            buffer, index);

    pack_int(test->rx_delay_max,            buffer, index);

    pack_int(test->rx_delay_multiplier,     buffer, index);

    pack_int(test->rx_delay_divisor,        buffer, index);

    pack_int(test->rx_delay_increment,      buffer, index);

    pack_int((int)test->io_mechanism,       buffer, index);

    pack_usbtestdata(&(test->data),         buffer, index);

}

#endif

#ifdef TARGET

static void

unpack_usbtest_bulk(UsbTest_Bulk* test, unsigned char* buffer, int* index)

{

    test->number_packets            = unpack_int(buffer, index);

    test->endpoint                  = unpack_int(buffer, index);

    test->tx_size                   = unpack_int(buffer, index);

    test->tx_size_min               = unpack_int(buffer, index);

    test->tx_size_max               = unpack_int(buffer, index);

    test->tx_size_multiplier        = unpack_int(buffer, index);

    test->tx_size_divisor           = unpack_int(buffer, index);

    test->tx_size_increment         = unpack_int(buffer, index);

    test->rx_size                   = unpack_int(buffer, index);

    test->rx_size_min               = unpack_int(buffer, index);

    test->rx_size_max               = unpack_int(buffer, index);

    test->rx_size_multiplier        = unpack_int(buffer, index);

    test->rx_size_divisor           = unpack_int(buffer, index);

    test->rx_size_increment         = unpack_int(buffer, index);

    test->tx_delay                  = unpack_int(buffer, index);

    test->tx_delay_min              = unpack_int(buffer, index);

    test->tx_delay_max              = unpack_int(buffer, index);

    test->tx_delay_multiplier       = unpack_int(buffer, index);

    test->tx_delay_divisor          = unpack_int(buffer, index);

    test->tx_delay_increment        = unpack_int(buffer, index);

    test->rx_delay                  = unpack_int(buffer, index);

    test->rx_delay_min              = unpack_int(buffer, index);

    test->rx_delay_max              = unpack_int(buffer, index);

    test->rx_delay_multiplier       = unpack_int(buffer, index);

    test->rx_delay_divisor          = unpack_int(buffer, index);

    test->rx_delay_increment        = unpack_int(buffer, index);

    test->io_mechanism              = (usb_io_mechanism) unpack_int(buffer, index);

    unpack_usbtestdata(&(test->data), buffer, index);

}

#endif

// A macro for moving on the next packet size. This also has to be shared between host

// and target, if the two got out of synch then testing would go horribly wrong.

//

// The new packet size is determined using a multiplier and increment,

// so to e.g. increase packet sizes by 4 bytes each time the

// multiplier would be 1 and the increment would be 4, or to double

// packet sizes the multiplier would be 2 and the increment would be

// 0. On underflow or overflow the code tries to adjust the packet size

// back to within the accepted range.

#define USBTEST_NEXT_TX_SIZE(_x_)                               \

    do {                                                        \

        _x_.tx_size *= _x_.tx_size_multiplier;                  \

        _x_.tx_size /= _x_.tx_size_divisor;                     \

        _x_.tx_size += _x_.tx_size_increment;                   \

        if (_x_.tx_size < _x_.tx_size_min) {                    \

            if (_x_.tx_size_min == _x_.tx_size_max) {           \

                _x_.tx_size = _x_.tx_size_min;                  \

            } else {                                            \

                int tmp  = _x_.tx_size_min - _x_.tx_size;       \

                tmp     %= _x_.tx_size_max - _x_.tx_size_min;   \

                _x_.tx_size = tmp + _x_.tx_size_min;            \

            }                                                   \

        } else if (_x_.tx_size > _x_.tx_size_max) {             \

            if (_x_.tx_size_min == _x_.tx_size_max) {           \

                _x_.tx_size = _x_.tx_size_max;                  \

            } else {                                            \

                int tmp  = _x_.tx_size - _x_.tx_size_max;       \

                tmp     %= _x_.tx_size_max - _x_.tx_size_min;   \

                _x_.tx_size = tmp + _x_.tx_size_min;            \

            }                                                   \

        }                                                       \

    } while ( 0 )

// A similar macro for moving on to the next receive size. This is less

// critical since care is taken to always receive at least the current

// tx size plus padding.

// Note that padding needs to be added by the calling code, not here,

// since padding is only applicable on the host-side and this macro

// is used on both host and target.

#define USBTEST_NEXT_RX_SIZE(_x_)                               \

    do {                                                        \

        _x_.rx_size *= _x_.rx_size_multiplier;                  \

        _x_.rx_size /= _x_.rx_size_divisor;                     \

        _x_.rx_size += _x_.rx_size_increment;                   \

        if (_x_.rx_size < _x_.rx_size_min) {                    \

            if (_x_.rx_size_min == _x_.rx_size_max) {           \

                _x_.rx_size = _x_.rx_size_min;                  \

            } else {                                            \

                int tmp  = _x_.rx_size_min - _x_.rx_size;       \

                tmp     %= _x_.rx_size_max - _x_.rx_size_min;   \

                _x_.rx_size = tmp + _x_.rx_size_min;            \

            }                                                   \

        } else if (_x_.rx_size > _x_.rx_size_max) {             \

            if (_x_.rx_size_min == _x_.rx_size_max) {           \

                _x_.rx_size = _x_.rx_size_max;                  \

            } else {                                            \

                int tmp  = _x_.rx_size - _x_.rx_size_max;       \

                tmp     %= _x_.rx_size_max - _x_.rx_size_min;   \

                _x_.rx_size = tmp + _x_.rx_size_min;            \

            }                                                   \

        }                                                       \

    } while ( 0 )

// And a macro for adjusting the transmit delay.

#define USBTEST_NEXT_TX_DELAY(_x_)                              \

    do {                                                        \

        _x_.tx_delay *= _x_.tx_delay_multiplier;                \

        _x_.tx_delay /= _x_.tx_delay_divisor;                   \

        _x_.tx_delay += _x_.tx_delay_increment;                 \

        if (_x_.tx_delay < _x_.tx_delay_min) {                  \

            if (_x_.tx_delay_min == _x_.tx_delay_max) {         \

                _x_.tx_delay = _x_.tx_delay_min;                \

            } else {                                            \

                int tmp  = _x_.tx_delay_min - _x_.tx_delay;     \

                tmp     %= _x_.tx_delay_max - _x_.tx_delay_min; \

                _x_.tx_delay = tmp + _x_.tx_delay_min;          \

            }                                                   \

        } else if (_x_.tx_delay > _x_.tx_delay_max) {           \

            if (_x_.tx_delay_min == _x_.tx_delay_max) {         \

                _x_.tx_delay = _x_.tx_delay_max;                \

            } else {                                            \

                int tmp  = _x_.tx_delay - _x_.tx_delay_max;     \

                tmp     %= _x_.tx_delay_max - _x_.tx_delay_min; \

                _x_.tx_delay = tmp + _x_.tx_delay_min;          \

            }                                                   \

        }                                                       \

    } while ( 0 )

#define USBTEST_NEXT_RX_DELAY(_x_)                              \

    do {                                                        \

        _x_.rx_delay *= _x_.rx_delay_multiplier;                \

        _x_.rx_delay /= _x_.rx_delay_divisor;                   \

        _x_.rx_delay += _x_.rx_delay_increment;                 \

        if (_x_.rx_delay < _x_.rx_delay_min) {                  \

            if (_x_.rx_delay_min == _x_.rx_delay_max) {         \

                _x_.rx_delay = _x_.rx_delay_min;                \

            } else {                                            \

                int tmp  = _x_.rx_delay_min - _x_.rx_delay;     \

                tmp     %= _x_.rx_delay_max - _x_.rx_delay_min; \

                _x_.rx_delay = tmp + _x_.rx_delay_min;          \

            }                                                   \

        } else if (_x_.rx_delay > _x_.rx_delay_max) {           \

            if (_x_.rx_delay_min == _x_.rx_delay_max) {         \

                _x_.rx_delay = _x_.rx_delay_max;                \

            } else {                                            \

                int tmp  = _x_.rx_delay - _x_.rx_delay_max;     \

                tmp     %= _x_.rx_delay_max - _x_.rx_delay_min; \

                _x_.rx_delay = tmp + _x_.rx_delay_min;          \

            }                                                   \

        }                                                       \

    } while ( 0 )

#define USBTEST_BULK_NEXT(_bulk_)                               \

    USBTEST_NEXT_TX_SIZE(_bulk_);                               \

    USBTEST_NEXT_RX_SIZE(_bulk_);                               \

    USBTEST_NEXT_TX_DELAY(_bulk_);                              \

    USBTEST_NEXT_RX_DELAY(_bulk_);

// Control transfers, receives

typedef struct UsbTest_ControlIn {

    int         number_packets;

    int         packet_size_initial;

    int         packet_size_min;

    int         packet_size_max;

    int         packet_size_multiplier;

    int         packet_size_increment;

    UsbTestData data;

} UsbTest_ControlIn;

#ifdef HOST

static void

pack_usbtest_control_in(UsbTest_ControlIn* test, unsigned char* buffer, int* index)

{

    pack_int(test->number_packets,          buffer, index);

    pack_int(test->packet_size_initial,     buffer, index);

    pack_int(test->packet_size_min,         buffer, index);

    pack_int(test->packet_size_max,         buffer, index);

    pack_int(test->packet_size_multiplier,  buffer, index);

    pack_int(test->packet_size_increment,   buffer, index);

    pack_usbtestdata(&(test->data),         buffer, index);

}

#endif

#ifdef TARGET

static void

unpack_usbtest_control_in(UsbTest_ControlIn* test, unsigned char* buffer, int* index)

{

    test->number_packets            = unpack_int(buffer, index);

    test->packet_size_initial       = unpack_int(buffer, index);

    test->packet_size_min           = unpack_int(buffer, index);

    test->packet_size_max           = unpack_int(buffer, index);

    test->packet_size_multiplier    = unpack_int(buffer, index);

    test->packet_size_increment     = unpack_int(buffer, index);

    unpack_usbtestdata(&(test->data), buffer, index);

}

#endif

// For now control packet sizes are adjusted in exactly the same way as bulk transfers.

#define USBTEST_CONTROL_NEXT_PACKET_SIZE(_packet_size_, _control_)                                          \

    _packet_size_ = (_packet_size_ * _control_.packet_size_multiplier) + _control_.packet_size_increment;   \

    if (_packet_size_ < _control_.packet_size_min) {                                                        \

        _packet_size_ += _control_.packet_size_max - _control_.packet_size_min;                             \

        if (_packet_size_ < _control_.packet_size_min) {                                                    \

            _packet_size_ = _control_.packet_size_initial;                                                  \

        }                                                                                                   \

    } else if (_packet_size_ > _control_.packet_size_max) {                                                 \

        _packet_size_ -= _control_.packet_size_max - _control_.packet_size_min;                             \

        if (_packet_size_ > _control_.packet_size_max) {                                                    \

            _packet_size_ = _control_.packet_size_initial;                                                  \

        }                                                                                                   \

    }

/*}}}*/

/*{{{  Recovery support                                         */

// ----------------------------------------------------------------------------

// When things go wrong threads on either the host or the target may get

// locked up waiting for further communication that never happens, because

// the other side has already raised an error. Recovery is possible by

// performing an extra I/O operation. For example, if a thread on the

// target is blocked waiting on an OUT endpoint then recovery is possible

// by the host sending some data to that endpoint. Similarly if a thread

// on the host is blocked then recovery involves the target either sending

// or receiving some additional data. There are alternative approaches such

// as stalling endpoints, but making sure that the requested communication

// actually happens involves fewer dependencies on exactly how those

// operations behave.