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authorMike Isely <isely@pobox.com>2008-08-30 15:09:31 -0300
committerMauro Carvalho Chehab <mchehab@redhat.com>2008-10-12 09:36:57 -0200
commit5f33df1443cd53283279d017b45ed1b1bc66f622 (patch)
tree49a4fc26ff01fd0a9e5834a07f043c52fbad153c /drivers
parent83512e207479d0bdb83ee1e000a306af9e6e870e (diff)
V4L/DVB (8893): pvrusb2: Add comment elaborating on direct use of swab32()
Signed-off-by: Mike Isely <isely@pobox.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/media/video/pvrusb2/pvrusb2-hdw.c11
1 files changed, 11 insertions, 0 deletions
diff --git a/drivers/media/video/pvrusb2/pvrusb2-hdw.c b/drivers/media/video/pvrusb2/pvrusb2-hdw.c
index 7fa903ad26b..c8dedbab6e3 100644
--- a/drivers/media/video/pvrusb2/pvrusb2-hdw.c
+++ b/drivers/media/video/pvrusb2/pvrusb2-hdw.c
@@ -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]);