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SubjectRe: [PATCH RFT 5/7] media: hantro: jpeg: Add COM segment to JPEG header to align image scan
Hi,

On Fri, Dec 24, 2021 at 04:42:46PM +0800, Chen-Yu Tsai wrote:
> The JPEG header size is not 64-bit aligned. This makes the driver
> require a bounce buffer for the encoded JPEG image scan output.
>
> Add a COM (comment) segment to the JPEG header so that the header size
> is a multiple of 64 bits. This will then allow dropping the use of the
> bounce buffer, and instead have the hardware write out to the capture
> buffer directly.
>
> Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
> ---
> drivers/staging/media/hantro/hantro_jpeg.c | 3 +++
> drivers/staging/media/hantro/hantro_jpeg.h | 2 +-
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/staging/media/hantro/hantro_jpeg.c b/drivers/staging/media/hantro/hantro_jpeg.c
> index 7d4018bd6876..51e67e5cf86f 100644
> --- a/drivers/staging/media/hantro/hantro_jpeg.c
> +++ b/drivers/staging/media/hantro/hantro_jpeg.c
> @@ -247,6 +247,9 @@ static const unsigned char hantro_jpeg_header[JPEG_HEADER_SIZE] = {
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
>
> + /* COM */
> + 0xff, 0xfe, 0x00, 0x03, 0x00,
> +
> /* SOS */
> 0xff, 0xda, 0x00, 0x0c, 0x03, 0x01, 0x00, 0x02,
> 0x11, 0x03, 0x11, 0x00, 0x3f, 0x00,
> diff --git a/drivers/staging/media/hantro/hantro_jpeg.h b/drivers/staging/media/hantro/hantro_jpeg.h
> index f33c492134e4..0b49d0b82caa 100644
> --- a/drivers/staging/media/hantro/hantro_jpeg.h
> +++ b/drivers/staging/media/hantro/hantro_jpeg.h
> @@ -1,6 +1,6 @@
> /* SPDX-License-Identifier: GPL-2.0+ */
>
> -#define JPEG_HEADER_SIZE 619
> +#define JPEG_HEADER_SIZE 624

Can we add some compile-time check for the 8-byte alignment,
so this is always enforced?

Perhaps getting rid of the JPEG_HEADER_SIZE macro,
something like this....


@@ -140,7 +140,7 @@ static const unsigned char chroma_ac_table[] = {
* and we'll use fixed offsets to change the width, height
* quantization tables, etc.
*/
-static const unsigned char hantro_jpeg_header[JPEG_HEADER_SIZE] = {
+static const unsigned char hantro_jpeg_header[] = {
/* SOI */
0xff, 0xd8,

@@ -304,8 +304,13 @@ void hantro_jpeg_header_assemble(struct hantro_jpeg_ctx *ctx)
{
char *buf = ctx->buffer;

- memcpy(buf, hantro_jpeg_header,
- sizeof(hantro_jpeg_header));
+ /*
+ * THE JPEG buffer is prepended with the JPEG header,
+ * so 64-bit alignment is needed for DMA.
+ */
+ BUILD_BUG_ON(!IS_ALIGNED(sizeof(hantro_jpeg_header), 8));
+
+ memcpy(buf, hantro_jpeg_header, sizeof(hantro_jpeg_header));

Thanks,
Ezequiel

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