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From: Rafal Jaworowski <raj@FreeBSD.org>
Date: Tue, 12 Feb 2008 11:03:29 +0000 (UTC)
To: src-committers@FreeBSD.org, cvs-src@FreeBSD.org, cvs-all@FreeBSD.org
X-FreeBSD-CVS-Branch: HEAD
Cc: 
Subject: cvs commit: src/sys/dev/usb usbdi.c
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raj         2008-02-12 11:03:29 UTC

  FreeBSD src repository

  Modified files:
    sys/dev/usb          usbdi.c 
  Log:
  Eliminate BUS_DMA <-> cache incoherencies in USB transfers.
  
  With write-allocate cache we get into the following scenario:
  
  1. data has been updated in the memory by the USB HC, but
  2. D-cache holds an un-flushed value of it
  3. when affected cache line is being replaced, the old (un-flushed) value is
  flushed and overwrites the newly arrived
  
  This is possible due to how write-allocate works with virtual caches (ARM for
  example).
  
  In case of USB transfers it leads to fatal tags discrepancies in umass(4)
  operation, which look like the following:
  
  umass0: Invalid CSW: tag 1 should be 2
  (probe0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
  (probe0:umass-sim0:0:0:0): Retrying Command
  umass0: Invalid CSW: tag 1 should be 3
  (probe0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
  (probe0:umass-sim0:0:0:0): Retrying Command
  umass0: Invalid CSW: tag 1 should be 4
  (probe0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
  (probe0:umass-sim0:0:0:0): Retrying Command
  umass0: Invalid CSW: tag 1 should be 5
  (probe0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
  (probe0:umass-sim0:0:0:0): Retrying Command
  umass0: Invalid CSW: tag 1 should be 6
  (probe0:umass-sim0:0:0:0): Request completed with CAM_REQ_CMP_ERR
  (probe0:umass-sim0:0:0:0): error 5
  (probe0:umass-sim0:0:0:0): Retries Exausted
  
  To eliminate this, a BUS_DMASYNC_PREREAD sync operation is required in
  usbd_start_transfer().
  
  Credits for nailing this down go to Grzegorz Bernacki gjb AT semihalf DOT com.
  
  Reviewed by:    imp
  Approved by:    cognet (mentor)
  
  Revision  Changes    Path
  1.104     +2 -1      src/sys/dev/usb/usbdi.c