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>
diff --git a/drivers/media/video/pvrusb2/pvrusb2-hdw.c b/drivers/media/video/pvrusb2/pvrusb2-hdw.c
index 7fa903a..c8dedba 100644
--- a/drivers/media/video/pvrusb2/pvrusb2-hdw.c
+++ b/drivers/media/video/pvrusb2/pvrusb2-hdw.c
@@ -1313,6 +1313,17 @@
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]);