scsi: usb: image: microtek: use sg helper to iterate over scatterlist [Linux 5.3]

This Linux kernel change "scsi: usb: image: microtek: use sg helper to iterate over scatterlist" is included in the Linux 5.3 release. This change is authored by Ming Lei <ming.lei [at] redhat.com> on Tue Jun 18 09:37:48 2019 +0800. The commit for this change in Linux stable tree is 1194b5c (patch).

scsi: usb: image: microtek: 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: Oliver Neukum <oliver@neukum.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

There are 22 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/usb/image/microtek.c | 20 ++++++++------------
 drivers/usb/image/microtek.h |  2 +-
 2 files changed, 9 insertions(+), 13 deletions(-)

diff --git a/drivers/usb/image/microtek.c b/drivers/usb/image/microtek.c
index 607be1f..0a57c2c 100644
--- a/drivers/usb/image/microtek.c
+++ b/drivers/usb/image/microtek.c
@@ -488,7 +488,6 @@ static void mts_command_done( struct urb *transfer )

 static void mts_do_sg (struct urb* transfer)
 {
-   struct scatterlist * sg;
    int status = transfer->status;
    MTS_INT_INIT();

@@ -500,13 +499,12 @@ static void mts_do_sg (struct urb* transfer)
        mts_transfer_cleanup(transfer);
         }

-   sg = scsi_sglist(context->srb);
-   context->fragment++;
+   context->curr_sg = sg_next(context->curr_sg);
    mts_int_submit_urb(transfer,
               context->data_pipe,
-              sg_virt(&sg[context->fragment]),
-              sg[context->fragment].length,
-              context->fragment + 1 == scsi_sg_count(context->srb) ?
+              sg_virt(context->curr_sg),
+              context->curr_sg->length,
+              sg_is_last(context->curr_sg) ?
               mts_data_done : mts_do_sg);
 }

@@ -526,22 +524,20 @@ static void mts_do_sg (struct urb* transfer)
 mts_build_transfer_context(struct scsi_cmnd *srb, struct mts_desc* desc)
 {
    int pipe;
-   struct scatterlist * sg;
-   
+
    MTS_DEBUG_GOT_HERE();

    desc->context.instance = desc;
    desc->context.srb = srb;
-   desc->context.fragment = 0;

    if (!scsi_bufflen(srb)) {
        desc->context.data = NULL;
        desc->context.data_length = 0;
        return;
    } else {
-       sg = scsi_sglist(srb);
-       desc->context.data = sg_virt(&sg[0]);
-       desc->context.data_length = sg[0].length;
+       desc->context.curr_sg = scsi_sglist(srb);
+       desc->context.data = sg_virt(desc->context.curr_sg);
+       desc->context.data_length = desc->context.curr_sg->length;
    }

diff --git a/drivers/usb/image/microtek.h b/drivers/usb/image/microtek.h
index 66685e5..7bd5f46 100644
--- a/drivers/usb/image/microtek.h
+++ b/drivers/usb/image/microtek.h
@@ -21,7 +21,7 @@ struct mts_transfer_context
    void *data;
    unsigned data_length;
    int data_pipe;
-   int fragment;
+   struct scatterlist *curr_sg;

    u8 *scsi_status; /* status returned from ep_response after command completion */
 };

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