Re: [PATCH] scsi: libsas: Reset num_scatter if libata mark qc as NODATA

From: John Garry
Date: Thu Mar 18 2021 - 12:20:19 EST

On 18/03/2021 00:24, Jolly Shah wrote:
Hi John,

Thanks for the review.

On Wed, Mar 17, 2021 at 4:44 AM John Garry <john.garry@xxxxxxxxxx> wrote:

On 16/03/2021 19:39, Jolly Shah wrote:
When the cache_type for the scsi device is changed, the scsi layer
issues a MODE_SELECT command. The caching mode details are communicated
via a request buffer associated with the scsi command with data
direction set as DMA_TO_DEVICE (scsi_mode_select). When this command
reaches the libata layer, as a part of generic initial setup, libata
layer sets up the scatterlist for the command using the scsi command
(ata_scsi_qc_new). This command is then translated by the libata layer
into ATA_CMD_SET_FEATURES (ata_scsi_mode_select_xlat). The libata layer
treats this as a non data command (ata_mselect_caching), since it only
needs an ata taskfile to pass the caching on/off information to the
device. It does not need the scatterlist that has been setup, so it does
not perform dma_map_sg on the scatterlist (ata_qc_issue).

So if we don't perform the dma_map_sg() on the sgl at this point, then
it seems to me that we should not perform for_each_sg() on it either,
right? That is still what happens in sas_ata_qc_issue() in this case.

Hi Jolly Shah,

Yes that's right. To avoid that, I can add elseif block for
ATA_PROT_NODATA before for_each_sg() is performed. Currently there's
existing code block for ATA_PROT_NODATA after for_each_sg() is
reused that to reset num_scatter. Please suggest.

How about just combine the 2x if-else statements into 1x if-elif-else statement, like:

if (ata_is_atapi(qc->tf.protocol)) {
memcpy(task->ata_task.atapi_packet, qc->cdb, qc->dev->cdb_len);
task->total_xfer_len = qc->nbytes;
task->num_scatter = qc->n_elem;
task->data_dir = qc->dma_dir;
} else if (qc->tf.protocol == ATA_PROT_NODATA) {
task->data_dir = DMA_NONE;
} else {
for_each_sg(qc->sg, sg, qc->n_elem, si)
xfer += sg_dma_len(sg);

task->total_xfer_len = xfer;
task->num_scatter = si;
task->data_dir = qc->dma_dir;

when this command reaches the libsas layer(sas_ata_qc_issue), libsas
layer sees it as a non data command with a scatterlist. It cannot
extract the correct dma length, since the scatterlist has not been
mapped with dma_map_sg for a DMA operation. When this partially
constructed SAS task reaches pm80xx LLDD, it results in below warning.

"pm80xx_chip_sata_req 6058: The sg list address
start_addr=0x0000000000000000 data_len=0x0end_addr_high=0xffffffff
end_addr_low=0xffffffff has crossed 4G boundary"

This patch assigns appropriate value to num_sectors for ata non data

Signed-off-by: Jolly Shah <jollys@xxxxxxxxxx>
drivers/scsi/libsas/sas_ata.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/libsas/sas_ata.c b/drivers/scsi/libsas/sas_ata.c
index 024e5a550759..94ec08cebbaa 100644
--- a/drivers/scsi/libsas/sas_ata.c
+++ b/drivers/scsi/libsas/sas_ata.c
@@ -209,10 +209,12 @@ static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
task->num_scatter = si;

- if (qc->tf.protocol == ATA_PROT_NODATA)
+ if (qc->tf.protocol == ATA_PROT_NODATA) {
task->data_dir = DMA_NONE;
- else
+ task->num_scatter = 0;

task->num_scatter has already been set in this function. Best not set it

Sure. Please suggest if I should update patch to above suggested
approach. That will avoid setting num_scatter twice.


BTW, could we add a fixes tag?