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 7fa903ad26bb86d47e8b3b0ed393250ea6a0a8e3..c8dedbab6e39c8ea0f092395fd4168d98a02c434 100644 (file)
--- 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]);