[Qemu-devel] [PATCH 4/6] ahci: unify sglist preparation

[John Snow <jsnow@redhat.com>]

The intent of this patch is to further unify the creation and
deletion of the sglist used for all AHCI transfers, including
emulated PIO, ATAPI R/W, and native DMA R/W.

By replacing ahci_start_transfer's call to ahci_populate_sglist
with ahci_dma_prepare_buf, we reduce the number of direct calls
where we manipulate the scatter-gather list in the AHCI code.

To make this switch, the constant "0" passed as an offset
in ahci_dma_prepare_buf is adjusted to use io_buffer_offset.

For DMA pathways, this has no effect: io_buffer_offset is always
updated to 0 at the beginning of a DMA transfer loop regardless.
DMA pathways through ide_dma_cb() update the io_buffer_offset
accordingly, and for circumstances where we might make several
trips through this loop, this may actually correct a design flaw.

For PIO pathways, the newly updated ahci_dma_prepare_buf will
now prepare the sglist at the correct offset. It will also set
io_buffer_size, but this is not used in the cmd_read_pio or
cmd_write_pio pathways.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 hw/ide/ahci.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
index db1d226..16cd248 100644
--- a/hw/ide/ahci.c
+++ b/hw/ide/ahci.c
@@ -1091,7 +1091,7 @@ static void ahci_start_transfer(IDEDMA *dma)
         goto out;
     }
 
-    if (!ahci_populate_sglist(ad, &s->sg, s->io_buffer_offset)) {
+    if (ahci_dma_prepare_buf(dma, is_write)) {
         has_sglist = 1;
     }
 
@@ -1143,7 +1143,7 @@ static int ahci_dma_prepare_buf(IDEDMA *dma, int is_write)
     AHCIDevice *ad = DO_UPCAST(AHCIDevice, dma, dma);
     IDEState *s = &ad->port.ifs[0];
 
-    ahci_populate_sglist(ad, &s->sg, 0);
+    ahci_populate_sglist(ad, &s->sg, s->io_buffer_offset);
     s->io_buffer_size = s->sg.size;
 
     DPRINTF(ad->port_no, "len=%#x\n", s->io_buffer_size);
-- 
1.9.3