scsi: NCR5380: use sg helper to iterate over scatterlist [Linux 5.3]

This Linux kernel change "scsi: NCR5380: use sg helper to iterate over scatterlist" is included in the Linux 5.3 release. This change is authored by Finn Thain <fthain [at] telegraphics.com.au> on Tue Jun 18 09:37:57 2019 +0800. The commit for this change in Linux stable tree is 0e9fdd2 (patch).

scsi: NCR5380: use sg helper to iterate over scatterlist

Unlike the legacy I/O path, scsi-mq preallocates a large array to hold
the scatterlist for each request. This static allocation can consume
substantial amounts of memory on modern controllers which support a
large number of concurrently outstanding requests.

To facilitate a switch to a smaller static allocation combined with a
dynamic allocation for requests that need it, we need to make sure all
SCSI drivers handle chained scatterlists correctly.

Convert remaining drivers that directly dereference the scatterlist
array to using the iterator functions.

[mkp: clarified commit message]

Cc: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

There are 41 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/scsi/NCR5380.c | 41 ++++++++++++++++++-----------------------
 1 file changed, 18 insertions(+), 23 deletions(-)

diff --git a/drivers/scsi/NCR5380.c b/drivers/scsi/NCR5380.c
index fe0535af..4ef44fa 100644
--- a/drivers/scsi/NCR5380.c
+++ b/drivers/scsi/NCR5380.c
@@ -149,12 +149,10 @@ static inline void initialize_SCp(struct scsi_cmnd *cmd)

    if (scsi_bufflen(cmd)) {
        cmd->SCp.buffer = scsi_sglist(cmd);
-       cmd->SCp.buffers_residual = scsi_sg_count(cmd) - 1;
        cmd->SCp.ptr = sg_virt(cmd->SCp.buffer);
        cmd->SCp.this_residual = cmd->SCp.buffer->length;
    } else {
        cmd->SCp.buffer = NULL;
-       cmd->SCp.buffers_residual = 0;
        cmd->SCp.ptr = NULL;
        cmd->SCp.this_residual = 0;
    }
@@ -163,6 +161,17 @@ static inline void initialize_SCp(struct scsi_cmnd *cmd)
    cmd->SCp.Message = 0;
 }

+static inline void advance_sg_buffer(struct scsi_cmnd *cmd)
+{
+   struct scatterlist *s = cmd->SCp.buffer;
+
+   if (!cmd->SCp.this_residual && s && !sg_is_last(s)) {
+       cmd->SCp.buffer = sg_next(s);
+       cmd->SCp.ptr = sg_virt(cmd->SCp.buffer);
+       cmd->SCp.this_residual = cmd->SCp.buffer->length;
+   }
+}
+
 /**
  * NCR5380_poll_politely2 - wait for two chip register values
  * @hostdata: host private data
@@ -1672,12 +1681,7 @@ static void NCR5380_information_transfer(struct Scsi_Host *instance)
                sun3_dma_setup_done != cmd) {
                int count;

-               if (!cmd->SCp.this_residual && cmd->SCp.buffers_residual) {
-                   ++cmd->SCp.buffer;
-                   --cmd->SCp.buffers_residual;
-                   cmd->SCp.this_residual = cmd->SCp.buffer->length;
-                   cmd->SCp.ptr = sg_virt(cmd->SCp.buffer);
-               }
+               advance_sg_buffer(cmd);

                count = sun3scsi_dma_xfer_len(hostdata, cmd);

@@ -1727,15 +1731,11 @@ static void NCR5380_information_transfer(struct Scsi_Host *instance)
                 * scatter-gather list, move onto the next one.
                 */

-               if (!cmd->SCp.this_residual && cmd->SCp.buffers_residual) {
-                   ++cmd->SCp.buffer;
-                   --cmd->SCp.buffers_residual;
-                   cmd->SCp.this_residual = cmd->SCp.buffer->length;
-                   cmd->SCp.ptr = sg_virt(cmd->SCp.buffer);
-                   dsprintk(NDEBUG_INFORMATION, instance, "%d bytes and %d buffers left\n",
-                            cmd->SCp.this_residual,
-                            cmd->SCp.buffers_residual);
-               }
+               advance_sg_buffer(cmd);
+               dsprintk(NDEBUG_INFORMATION, instance,
+                   "this residual %d, sg ents %d\n",
+                   cmd->SCp.this_residual,
+                   sg_nents(cmd->SCp.buffer));

                /*
                 * The preferred transfer method is going to be
@@ -2136,12 +2136,7 @@ static void NCR5380_reselect(struct Scsi_Host *instance)
    if (sun3_dma_setup_done != tmp) {
        int count;

-       if (!tmp->SCp.this_residual && tmp->SCp.buffers_residual) {
-           ++tmp->SCp.buffer;
-           --tmp->SCp.buffers_residual;
-           tmp->SCp.this_residual = tmp->SCp.buffer->length;
-           tmp->SCp.ptr = sg_virt(tmp->SCp.buffer);
-       }
+       advance_sg_buffer(tmp);

        count = sun3scsi_dma_xfer_len(hostdata, tmp);

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