scsi: aha152x: use sg helper to iterate over scatterlist [Linux 5.3]

This Linux kernel change "scsi: aha152x: 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 09:37:52 2019 +0800. The commit for this change in Linux stable tree is a7a253b (patch).

scsi: aha152x: 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.

Finn added the change to replace SCp.buffers_residual with
sg_is_last() for fixing updating it, and the similar change has been
applied on NCR5380.c

[mkp: clarified commit message]

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

There are 46 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/scsi/aha152x.c | 46 +++++++++++++++++++++++-----------------------
 1 file changed, 23 insertions(+), 23 deletions(-)

diff --git a/drivers/scsi/aha152x.c b/drivers/scsi/aha152x.c
index 9787283..f07f3fa 100644
--- a/drivers/scsi/aha152x.c
+++ b/drivers/scsi/aha152x.c
@@ -948,7 +948,6 @@ static int aha152x_internal_queue(struct scsi_cmnd *SCpnt,
       SCp.ptr              : buffer pointer
       SCp.this_residual    : buffer length
       SCp.buffer           : next buffer
-      SCp.buffers_residual : left buffers in list
       SCp.phase            : current state of the command */

    if ((phase & resetting) || !scsi_sglist(SCpnt)) {
@@ -956,13 +955,11 @@ static int aha152x_internal_queue(struct scsi_cmnd *SCpnt,
        SCpnt->SCp.this_residual = 0;
        scsi_set_resid(SCpnt, 0);
        SCpnt->SCp.buffer           = NULL;
-       SCpnt->SCp.buffers_residual = 0;
    } else {
        scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
        SCpnt->SCp.buffer           = scsi_sglist(SCpnt);
        SCpnt->SCp.ptr              = SG_ADDRESS(SCpnt->SCp.buffer);
        SCpnt->SCp.this_residual    = SCpnt->SCp.buffer->length;
-       SCpnt->SCp.buffers_residual = scsi_sg_count(SCpnt) - 1;
    }

    DO_LOCK(flags);
@@ -2030,10 +2027,9 @@ static void datai_run(struct Scsi_Host *shpnt)
                }

                if (CURRENT_SC->SCp.this_residual == 0 &&
-                   CURRENT_SC->SCp.buffers_residual > 0) {
+                   !sg_is_last(CURRENT_SC->SCp.buffer)) {
                    /* advance to next buffer */
-                   CURRENT_SC->SCp.buffers_residual--;
-                   CURRENT_SC->SCp.buffer++;
+                   CURRENT_SC->SCp.buffer = sg_next(CURRENT_SC->SCp.buffer);
                    CURRENT_SC->SCp.ptr           = SG_ADDRESS(CURRENT_SC->SCp.buffer);
                    CURRENT_SC->SCp.this_residual = CURRENT_SC->SCp.buffer->length;
                }
@@ -2136,10 +2132,10 @@ static void datao_run(struct Scsi_Host *shpnt)
            CMD_INC_RESID(CURRENT_SC, -2 * data_count);
        }

-       if(CURRENT_SC->SCp.this_residual==0 && CURRENT_SC->SCp.buffers_residual>0) {
+       if (CURRENT_SC->SCp.this_residual == 0 &&
+           !sg_is_last(CURRENT_SC->SCp.buffer)) {
            /* advance to next buffer */
-           CURRENT_SC->SCp.buffers_residual--;
-           CURRENT_SC->SCp.buffer++;
+           CURRENT_SC->SCp.buffer = sg_next(CURRENT_SC->SCp.buffer);
            CURRENT_SC->SCp.ptr           = SG_ADDRESS(CURRENT_SC->SCp.buffer);
            CURRENT_SC->SCp.this_residual = CURRENT_SC->SCp.buffer->length;
        }
@@ -2158,22 +2154,26 @@ static void datao_run(struct Scsi_Host *shpnt)
 static void datao_end(struct Scsi_Host *shpnt)
 {
    if(TESTLO(DMASTAT, DFIFOEMP)) {
-       int data_count = (DATA_LEN - scsi_get_resid(CURRENT_SC)) -
-           GETSTCNT();
+       u32 datao_cnt = GETSTCNT();
+       int datao_out = DATA_LEN - scsi_get_resid(CURRENT_SC);
+       int done;
+       struct scatterlist *sg = scsi_sglist(CURRENT_SC);

-       CMD_INC_RESID(CURRENT_SC, data_count);
+       CMD_INC_RESID(CURRENT_SC, datao_out - datao_cnt);

-       data_count -= CURRENT_SC->SCp.ptr -
-           SG_ADDRESS(CURRENT_SC->SCp.buffer);
-       while(data_count>0) {
-           CURRENT_SC->SCp.buffer--;
-           CURRENT_SC->SCp.buffers_residual++;
-           data_count -= CURRENT_SC->SCp.buffer->length;
+       done = scsi_bufflen(CURRENT_SC) - scsi_get_resid(CURRENT_SC);
+       /* Locate the first SG entry not yet sent */
+       while (done > 0 && !sg_is_last(sg)) {
+           if (done < sg->length)
+               break;
+           done -= sg->length;
+           sg = sg_next(sg);
        }
-       CURRENT_SC->SCp.ptr = SG_ADDRESS(CURRENT_SC->SCp.buffer) -
-           data_count;
-       CURRENT_SC->SCp.this_residual = CURRENT_SC->SCp.buffer->length +
-           data_count;
+
+       CURRENT_SC->SCp.buffer = sg;
+       CURRENT_SC->SCp.ptr = SG_ADDRESS(CURRENT_SC->SCp.buffer) + done;
+       CURRENT_SC->SCp.this_residual = CURRENT_SC->SCp.buffer->length -
+           done;
    }

    SETPORT(SXFRCTL0, CH1|CLRCH1|CLRSTCNT);
@@ -2501,7 +2501,7 @@ static void get_command(struct seq_file *m, struct scsi_cmnd * ptr)

    seq_printf(m, "); resid=%d; residual=%d; buffers=%d; phase |",
        scsi_get_resid(ptr), ptr->SCp.this_residual,
-       ptr->SCp.buffers_residual);
+       sg_nents(ptr->SCp.buffer) - 1);

    if (ptr->SCp.phase & not_issued)
        seq_puts(m, "not issued|");

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