Re: [PATCH 01/10] drm/i915/dp: Fix dsc bpp calculations.

Manasi Navare
2019-08-21T17:05:48Z

On Wed, Aug 21, 2019 at 03:32:12PM +0200, Maarten Lankhorst wrote:
> There was a integer wraparound when mode_clock became too high,
> and we didn't correct for the FEC overhead factor when dividing,
> also the calculations would break at HBR3.

But the mode_clock is obtained from the adusted_mode->crtc_clock which is
defined as an int in drm_mode_config struct, does that need to change also?

>
> As a result our calculated bpp was way too high, and the link width
> bpp limitation never came into effect.
>
> Print out the resulting bpp calcululations as a sanity check, just
> in case we ever have to debug it later on again.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Fixes: d9218c8f6cf4 ("drm/i915/dp: Add helpers for Compressed BPP and Slice Count for DSC")
> Cc: <stable@vger.kernel.org> # v5.0+
> Cc: Manasi Navare <manasi.d.navare@intel.com>
> ---
> drivers/gpu/drm/i915/display/intel_dp.c | 16 +++++++++-------
> drivers/gpu/drm/i915/display/intel_dp.h | 4 ++--
> 2 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 921ad0a2f7ba..614a25911f07 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -4323,10 +4323,10 @@ intel_dp_get_sink_irq_esi(struct intel_dp *intel_dp, u8 *sink_irq_vector)
> DP_DPRX_ESI_LEN;
> }
>
> -u16 intel_dp_dsc_get_output_bpp(int link_clock, u8 lane_count,
> - int mode_clock, int mode_hdisplay)
> +u16 intel_dp_dsc_get_output_bpp(u32 link_clock, u8 lane_count,
> + u32 mode_clock, u32 mode_hdisplay)
> {
> - u16 bits_per_pixel, max_bpp_small_joiner_ram;
> + u32 bits_per_pixel, max_bpp_small_joiner_ram;

But bits_per_pixel is a 16 bit value for DSC PPS, why does this need to be u32?

> int i;
>
> /*
> @@ -4335,13 +4335,14 @@ u16 intel_dp_dsc_get_output_bpp(int link_clock, u8 lane_count,
> * FECOverhead = 2.4%, for SST -> TimeSlotsPerMTP is 1,
> * for MST -> TimeSlotsPerMTP has to be calculated
> */
> - bits_per_pixel = (link_clock * lane_count * 8 *
> - DP_DSC_FEC_OVERHEAD_FACTOR) /
> - mode_clock;
> + bits_per_pixel = div_u64((u64)link_clock * lane_count * 8 *
> + DP_DSC_FEC_OVERHEAD_FACTOR, 1000ULL * mode_clock);

Thanks for catching this, this had the 1000 in the denominator in my original patches :https://patchwork.freedesktop.org/series/47514/#rev3
And then the nex rev lost it somehow, so thanks for catching this

Manasi

> + DRM_DEBUG_KMS("Max link bpp: %u\n", bits_per_pixel);
>
> /* Small Joiner Check: output bpp <= joiner RAM (bits) / Horiz. width */
> max_bpp_small_joiner_ram = DP_DSC_MAX_SMALL_JOINER_RAM_BUFFER /
> mode_hdisplay;
> + DRM_DEBUG_KMS("Max small joiner bpp: %u\n", max_bpp_small_joiner_ram);
>
> /*
> * Greatest allowed DSC BPP = MIN (output BPP from avaialble Link BW
> @@ -4351,7 +4352,8 @@ u16 intel_dp_dsc_get_output_bpp(int link_clock, u8 lane_count,
>
> /* Error out if the max bpp is less than smallest allowed valid bpp */
> if (bits_per_pixel < valid_dsc_bpp[0]) {
> - DRM_DEBUG_KMS("Unsupported BPP %d\n", bits_per_pixel);
> + DRM_DEBUG_KMS("Unsupported BPP %u, min %u\n",
> + bits_per_pixel, valid_dsc_bpp[0]);
> return 0;
> }
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.h b/drivers/gpu/drm/i915/display/intel_dp.h
> index 657bbb1f5ed0..007d1981a33b 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.h
> +++ b/drivers/gpu/drm/i915/display/intel_dp.h
> @@ -102,8 +102,8 @@ bool intel_dp_source_supports_hbr2(struct intel_dp *intel_dp);
> bool intel_dp_source_supports_hbr3(struct intel_dp *intel_dp);
> bool
> intel_dp_get_link_status(struct intel_dp *intel_dp, u8 *link_status);
> -u16 intel_dp_dsc_get_output_bpp(int link_clock, u8 lane_count,
> - int mode_clock, int mode_hdisplay);
> +u16 intel_dp_dsc_get_output_bpp(u32 link_clock, u8 lane_count,
> + u32 mode_clock, u32 mode_hdisplay);
> u8 intel_dp_dsc_get_slice_count(struct intel_dp *intel_dp, int mode_clock,
> int mode_hdisplay);
>
> --
> 2.20.1
>