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[media] usbtv: improve a comment 

Patrick Keshishian improved the explanation of the protocol when porting
the driver to OpenBSD. Given it's a reverse engineering one and there's
no documetnation it might be helpful to whoever hacks on the driver.

Signed-off-by: Patrick Keshishian <sidster@boxsoft.com>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Lubomir Rintel 2016-06-01 10:04:07 -03:00 committed by Mauro Carvalho Chehab
parent 5ae1e2b24f
commit 8ed50eb420
1 changed files with 17 additions and 2 deletions
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19
drivers/media/usb/usbtv/usbtv-video.c
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@ -265,8 +265,23 @@ static int usbtv_setup_capture(struct usbtv *usbtv)
/* Copy data from chunk into a frame buffer, deinterlacing the data
* into every second line. Unfortunately, they don't align nicely into
* 720 pixel lines, as the chunk is 240 words long, which is 480 pixels.
* Therefore, we break down the chunk into two halves before copyting,
* so that we can interleave a line if needed. */
* Therefore, we break down the chunk into two halves before copying,
* so that we can interleave a line if needed.
*
* Each "chunk" is 240 words; a word in this context equals 4 bytes.
* Image format is YUYV/YUV 4:2:2, consisting of Y Cr Y Cb, defining two
* pixels, the Cr and Cb shared between the two pixels, but each having
* separate Y values. Thus, the 240 words equal 480 pixels. It therefore,
* takes 1.5 chunks to make a 720 pixel-wide line for the frame.
* The image is interlaced, so there is a "scan" of odd lines, followed
* by "scan" of even numbered lines.
*
* Following code is writing the chunks in correct sequence, skipping
* the rows based on "odd" value.
* line 1: chunk[0][ 0..479] chunk[0][480..959] chunk[1][ 0..479]
* line 3: chunk[1][480..959] chunk[2][ 0..479] chunk[2][480..959]
* ...etc.
*/
static void usbtv_chunk_to_vbuf(u32 *frame, __be32 *src, int chunk_no, int odd)
{
int half;