From: Finn Thain fthain@telegraphics.com.au
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
--- drivers/scsi/mac_scsi.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/drivers/scsi/mac_scsi.c +++ b/drivers/scsi/mac_scsi.c @@ -52,7 +52,7 @@ static int setup_cmd_per_lun = -1; module_param(setup_cmd_per_lun, int, 0); static int setup_sg_tablesize = -1; module_param(setup_sg_tablesize, int, 0); -static int setup_use_pdma = -1; +static int setup_use_pdma = 512; module_param(setup_use_pdma, int, 0); static int setup_hostid = -1; module_param(setup_hostid, int, 0); @@ -305,7 +305,7 @@ static int macscsi_dma_xfer_len(struct N struct scsi_cmnd *cmd) { if (hostdata->flags & FLAG_NO_PSEUDO_DMA || - cmd->SCp.this_residual < 16) + cmd->SCp.this_residual < setup_use_pdma) return 0;
return cmd->SCp.this_residual;