[PATCH v3] media: atomisp: csi2: Fix DPCM decompression for source pad format

[Debjeet Banerjee <debjeetbanerjee48@gmail.com>]

The CSI2 source pad format was set by blindly copying the sink pad
format, regardless of whether the sink pad is receiving a DPCM-compressed
mbus code. This was marked with a FIXME comment.

When a sensor transmits DPCM-compressed data over MIPI CSI-2 (e.g.
MEDIA_BUS_FMT_SGRBG10_DPCM8_1X8), the Atom ISP hardware decompresses it
before passing it to the ISP pipeline. Therefore the source pad must
expose the uncompressed format (e.g. MEDIA_BUS_FMT_SGRBG10_1X10), not
the compressed one.

atomisp_subdev_uncompressed_code() already implements this mapping via
the compressed_codes[] table in atomisp_subdev.c, which pairs each
DPCM mbus code with its uncompressed counterpart. For codes that are not
DPCM-compressed, the function returns the code unchanged, so this change
is a no-op for all non-DPCM formats.

Signed-off-by: Debjeet Banerjee <debjeetbanerjee48@gmail.com>
---
v3:
- Drop "staging:" prefix in subject (per Andy Shevchenko)

v2:
- Rebased onto latest media tree (fix CI apply failure)
- Fix missing linux-staging recipient (typo in v1)
- Fix indentation
---
 drivers/staging/media/atomisp/pci/atomisp_csi2.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/media/atomisp/pci/atomisp_csi2.c b/drivers/staging/media/atomisp/pci/atomisp_csi2.c
index 95b9113d75e9..e723304bec80 100644
--- a/drivers/staging/media/atomisp/pci/atomisp_csi2.c
+++ b/drivers/staging/media/atomisp/pci/atomisp_csi2.c
@@ -107,10 +107,17 @@ int atomisp_csi2_set_ffmt(struct v4l2_subdev *sd,
 					     &tmp_ffmt);
 	}
 
-	/* FIXME: DPCM decompression */
-	*actual_ffmt = *ffmt = *__csi2_get_format(csi2, sd_state, which,
-						  CSI2_PAD_SINK);
-
+	/*
+	 * The source pad must expose the uncompressed format even when the sink
+	 * pad receives DPCM-compressed data. The hardware decompresses DPCM
+	 * (e.g. SGRBG10_DPCM8_1X8 -> SGRBG10_1X10) between sink and source,
+	 * so propagate the sink format but map the mbus code to its uncompressed
+	 * equivalent. atomisp_subdev_uncompressed_code() returns the code
+	 * unchanged for formats that are not DPCM-compressed.
+	 */
+	*actual_ffmt = *__csi2_get_format(csi2, sd_state, which, CSI2_PAD_SINK);
+	actual_ffmt->code = atomisp_subdev_uncompressed_code(actual_ffmt->code);
+	*ffmt = *actual_ffmt;
 	return 0;
 }
 
-- 
2.53.0