[bug report] drm/i915/dp: Get optimal link config to have best compressed bpp

[Dan Carpenter]

Hello Ankit Nautiyal,

Commit 1c56e9a39833 ("drm/i915/dp: Get optimal link config to have
best compressed bpp") from Aug 17, 2023 (linux-next), leads to the
following Smatch static checker warning:

	drivers/gpu/drm/i915/display/intel_dp.c:2481 intel_dp_dsc_compute_pipe_bpp_limits()
	warn: always clamps to 24

drivers/gpu/drm/i915/display/intel_dp.c
    2472 static void
    2473 intel_dp_dsc_compute_pipe_bpp_limits(struct intel_dp *intel_dp,
    2474                                      struct link_config_limits *limits)
    2475 {
    2476         struct intel_display *display = to_intel_display(intel_dp);
    2477         int dsc_min_bpc = intel_dp_dsc_min_src_input_bpc();
    2478         int dsc_max_bpc = intel_dp_dsc_max_src_input_bpc(display);
    2479 
    2480         limits->pipe.max_bpp = clamp(limits->pipe.max_bpp, dsc_min_bpc * 3, dsc_max_bpc * 3);
--> 2481         limits->pipe.min_bpp = clamp(limits->pipe.min_bpp, dsc_min_bpc * 3, dsc_max_bpc * 3);
    2482 }

This is an unpublished static checker warning that complains about weird
clamps() so it only just started showing up now.  The problem is a
mismatch between intel_dp_min_bpp() and intel_dp_dsc_min_src_input_bpc().

In intel_dp_min_bpp() it uses "6 * 3" but then that gets overwriten with
"8 * 3" eventually.  The warning is that "6 * 3" might be pointless?
I haven't followed the code totally, but it seems like potentially the
checker is correct.

drivers/gpu/drm/i915/display/intel_dp.c
  1175  int intel_dp_min_bpp(enum intel_output_format output_format)
  1176  {
  1177          if (output_format == INTEL_OUTPUT_FORMAT_RGB)
  1178                  return 6 * 3;
                               ^^^^^
Is this pointless?  Should we just always return "8 * 3" since
that's what it's clamped to in the end?

  1179          else
  1180                  return 8 * 3;
  1181  }

drivers/gpu/drm/i915/display/intel_dp.c
  2161  int intel_dp_dsc_min_src_input_bpc(void)
  2162  {
  2163          /* Min DSC Input BPC for ICL+ is 8 */
  2164          return 8;

This 8 becomes "8 * 3" in the caller.

  2165  }

regards,
dan carpenter