[PATCH RFT v2 5/8] media: hantro: jpeg: Add COM segment to JPEG header to align image scan

[Chen-Yu Tsai <wenst@chromium.org>]

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 | 23 +++++++++++++++++++++-
 drivers/staging/media/hantro/hantro_jpeg.h |  2 +-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/media/hantro/hantro_jpeg.c b/drivers/staging/media/hantro/hantro_jpeg.c
index 7d4018bd6876..e63eeef3952d 100644
--- a/drivers/staging/media/hantro/hantro_jpeg.c
+++ b/drivers/staging/media/hantro/hantro_jpeg.c
@@ -6,6 +6,9 @@
  * Copyright (C) Jean-Francois Moine (http://moinejf.free.fr)
  * Copyright (C) 2014 Philipp Zabel, Pengutronix
  */
+
+#include <linux/align.h>
+#include <linux/build_bug.h>
 #include <linux/dma-mapping.h>
 #include <linux/kernel.h>
 #include <linux/string.h>
@@ -140,7 +143,7 @@ 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,
 
@@ -247,11 +250,29 @@ 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,
 };
 
+/*
+ * JPEG_HEADER_SIZE is used in other parts of the driver in lieu of
+ * "sizeof(hantro_jpeg_header)". The two must be equal.
+ */
+static_assert(sizeof(hantro_jpeg_header) == JPEG_HEADER_SIZE);
+
+/*
+ * hantro_jpeg_header is padded with a COM segment, so that the payload
+ * of the SOS segment (the entropy-encoded image scan), which should
+ * trail the whole header, is 8-byte aligned for the hardware to write
+ * to directly.
+ */
+static_assert(IS_ALIGNED(sizeof(hantro_jpeg_header), 8),
+	      "Hantro JPEG header size needs to be 8-byte aligned.");
+
 static unsigned char jpeg_scale_qp(const unsigned char qp, int scale)
 {
 	unsigned int temp;
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
 #define JPEG_QUANT_SIZE		64
 
 struct hantro_jpeg_ctx {
-- 
2.34.1.575.g55b058a8bb-goog