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commit 7398cee4c3e6aea1ba07a6449e5533ecd0b92cdd upstream. Some targets introduce delays when handshaking the response to certain commands. For example, a disk may send a 96-byte response to an INQUIRY command (or a 24-byte response to a MODE SENSE command) too slowly. Apparently the first 12 or 14 bytes are handshaked okay but then the system bus error timeout is reached while transferring the next word. Since the scsi bus phase hasn't changed, the driver then sets the target borken flag to prevent further PDMA transfers. The driver also logs the warning, "switching to slow handshake". Raise the PDMA threshold to 512 bytes so that PIO transfers will be used for these commands. This default is sufficiently low that PDMA will still be used for READ and WRITE commands. The existing threshold (16 bytes) was chosen more or less at random. However, best performance requires the threshold to be as low as possible. Those systems that don't need the PIO workaround at all may benefit from mac_scsi.setup_use_pdma=1 Cc: Michael Schmitz <schmitzmic@gmail.com> Cc: stable@vger.kernel.org # v4.14+ Fixes: 3a0f64bfa907 ("mac_scsi: Fix pseudo DMA implementation") Signed-off-by: Finn Thain <fthain@telegraphics.com.au> Tested-by: Stan Johnson <userm57@yahoo.com> Tested-by: Michael Schmitz <schmitzmic@gmail.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.