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>

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 7fa903ad26bb..c8dedbab6e39 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]);