Re: [PATCH i-g-t] tests/kms_plane_alpha_blend: Skip 7efc alpha-swap CRC check on AMD at >8bpc/DSC

Reviewed-by: Tom Chung <[email protected]>

On 7/16/2026 2:35 PM, James Lin wrote:
> [Why]
> alpha-7efc and coverage-7efc assert that swapping the plane global alpha
> (the KMS "alpha" property) with the per-pixel fb alpha (0x7e <-> 0xfc) yields
> a bit-identical pipe CRC. That identity only holds when the whole blend
> datapath collapses to exact 8-bit arithmetic. On AMD DCN the alpha/gain
> values are 8-bit, but the blend is carried at higher internal precision and
> only quantized down to the output bpc at the very end. A swap that is exact
> in idealized 8-bit math can therefore leave a sub-LSB residue: at 8 bpc the
> output quantization discards it and the CRCs match, but at >8 bpc, or when the
> stream is DSC-compressed (carried at its native >8 bpc), the residue survives
> and the CRCs differ. The strict compare then turns a legitimate precision
> artifact into a test failure - observed as a CRC mismatch on an eDP link
> brought up with DSC, while the same test passes on an 8-bit non-DSC link.
>
> [How]
> Add skip_if_alpha_swap_not_exact() and call it at the start of alpha_7efc and
> coverage_7efc. It is a no-op on non-AMD devices, so every other driver keeps
> the strict check unchanged. On amdgpu it reads the CRTC's current bpc and the
> connector's DSC status, and skips only when bpc > 8 or DSC is active - the
> conditions under which the alpha swap is legitimately not bit-exact. 8 bpc AMD
> configurations still run the strict compare, preserving the regression
> coverage the test was written for. Track the output in data_t so the DSC state
> of the current connector can be queried.
>
> Signed-off-by: James Lin <[email protected]>
> ---
>   tests/kms_plane_alpha_blend.c | 44 +++++++++++++++++++++++++++++++++++
>   1 file changed, 44 insertions(+)
>
> diff --git a/tests/kms_plane_alpha_blend.c b/tests/kms_plane_alpha_blend.c
> index 3a2649833..e040d5bec 100644
> --- a/tests/kms_plane_alpha_blend.c
> +++ b/tests/kms_plane_alpha_blend.c
> @@ -33,6 +33,7 @@
>    */
>   
>   #include "igt.h"
> +#include "igt_amd.h"
>   
>   /**
>    * SUBTEST: alpha-%s
> @@ -77,6 +78,7 @@ typedef struct {
>   	struct igt_fb xrgb_fb, argb_fb_0, argb_fb_cov_0, argb_fb_7e, argb_fb_cov_7e, argb_fb_fc, argb_fb_cov_fc, argb_fb_100, black_fb, gray_fb;
>   	igt_crc_t ref_crc;
>   	igt_pipe_crc_t *pipe_crc;
> +	igt_output_t *output;
>   } data_t;
>   
>   static bool in_simulation;
> @@ -442,11 +444,49 @@ static void constant_alpha_max(data_t *data, igt_crtc_t *crtc,
>   	igt_plane_set_fb(plane, NULL);
>   }
>   
> +/*
> + * The 7e/fc subtests (alpha-7efc, coverage-7efc) assert that swapping the
> + * plane global alpha with the per-pixel fb alpha (0x7e <-> 0xfc) yields a
> + * bit-identical pipe CRC. That identity only holds when the whole blend
> + * datapath collapses to exact 8-bit arithmetic.
> + *
> + * On AMD DCN the alpha/gain values are 8-bit, but the blend itself is carried
> + * at higher internal precision and only quantized down to the output bpc at
> + * the very end, so the swap can leave a sub-LSB residue. At 8 bpc the output
> + * quantization discards it and the two CRCs match; at >8 bpc, or when the
> + * stream is DSC-compressed (carried at its native >8 bpc), the residue is
> + * retained and the CRCs differ. This matches observation: the same test
> + * PASSes on an 8 bpc eDP link without DSC and FAILs on an eDP link brought
> + * up with DSC.
> + *
> + * So keep the strict bit-exact check everywhere except AMD at >8 bpc or with
> + * DSC active, where the swap is legitimately not bit-exact.
> + */
> +static void skip_if_alpha_swap_not_exact(data_t *data, igt_crtc_t *crtc)
> +{
> +	unsigned int bpc;
> +	bool dsc;
> +
> +	if (!is_amdgpu_device(data->gfx_fd))
> +		return;
> +
> +	bpc = igt_get_crtc_current_bpc(crtc);
> +	dsc = data->output &&
> +	      is_dp_dsc_supported(data->gfx_fd, data->output->name) &&
> +	      igt_amd_read_dsc_clock_status(data->gfx_fd, data->output->name) > 0;
> +
> +	igt_skip_on_f(bpc > 8 || dsc,
> +		      "plane-alpha/fb-alpha swap is not bit-exact on AMD DCN "
> +		      "at >8bpc/DSC (bpc=%u, dsc=%d)\n", bpc, dsc);
> +}
> +
>   static void alpha_7efc(data_t *data, igt_crtc_t *crtc, igt_plane_t *plane)
>   {
>   	igt_display_t *display = &data->display;
>   	igt_crc_t ref_crc = {}, crc = {};
>   
> +	skip_if_alpha_swap_not_exact(data, crtc);
> +
>   	if (plane->type != DRM_PLANE_TYPE_PRIMARY)
>   		igt_plane_set_fb(igt_crtc_get_plane_type(crtc, DRM_PLANE_TYPE_PRIMARY),
>   				 &data->gray_fb);
> @@ -475,6 +515,8 @@ static void coverage_7efc(data_t *data, igt_crtc_t *crtc, igt_plane_t *plane)
>   	igt_display_t *display = &data->display;
>   	igt_crc_t ref_crc = {}, crc = {};
>   
> +	skip_if_alpha_swap_not_exact(data, crtc);
> +
>   	igt_require(igt_plane_try_prop_enum(plane, IGT_PLANE_PIXEL_BLEND_MODE, "Coverage"));
>   	igt_display_commit2(display, COMMIT_ATOMIC);
>   	igt_pipe_crc_start(data->pipe_crc);
> @@ -539,6 +581,8 @@ static void run_test_on_crtc_planes(data_t *data, igt_crtc_t *crtc,
>   	int first_plane = -1;
>   	int last_plane = -1;
>   
> +	data->output = output;
> +
>   	for_each_plane_on_crtc(crtc, plane) {
>   		if (!igt_plane_has_prop(plane, IGT_PLANE_ALPHA))
>   			continue;