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V4L/DVB (8893): pvrusb2: Add comment elaborating on direct use of swab32()

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Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Mike Isely 2008-08-30 15:09:31 -03:00 committed by Mauro Carvalho Chehab
parent 83512e2074
commit 5f33df1443
1 changed files with 11 additions and 0 deletions
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11
drivers/media/video/pvrusb2/pvrusb2-hdw.c
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@ -1313,6 +1313,17 @@ int pvr2_upload_firmware2(struct pvr2_hdw *hdw)
if (bcnt > FIRMWARE_CHUNK_SIZE) bcnt = FIRMWARE_CHUNK_SIZE;
memcpy(fw_ptr, fw_entry->data + fw_done, bcnt);
/* Usbsnoop log shows that we must swap bytes... */
/* Some background info: The data being swapped here is a
firmware image destined for the mpeg encoder chip that
lives at the other end of a USB endpoint. The encoder
chip always talks in 32 bit chunks and its storage is
organized into 32 bit words. However from the file
system to the encoder chip everything is purely a byte
stream. The firmware file's contents are always 32 bit
swapped from what the encoder expects. Thus the need
always exists to swap the bytes regardless of the endian
type of the host processor and therefore swab32() makes
the most sense. */
for (icnt = 0; icnt < bcnt/4 ; icnt++)
((u32 *)fw_ptr)[icnt] = swab32(((u32 *)fw_ptr)[icnt]);