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

Maarten Lankhorst
2019-08-26T11:33:05Z

Op 23-08-2019 om 12:17 schreef Jani Nikula:
> On Wed, 21 Aug 2019, Manasi Navare <manasi.d.navare@intel.com> wrote:
>> 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?
> I would use int for *all* of the parameters and local vars. Don't try to
> save space or limit the range by unsigned or fixed size. Only use the
> fixed size types for when the size really matters, i.e. serialization
> and lots of data. IMO almost everything else is trouble in C.

Hmm, looking at the source drm_mode_modeinfo it's intended to be unsigned.

We will definitely need a 64-bits because 1000 * 1 Ghz clock overflows u32.

I will see if I can make this to work with mul_u32_u32 for the nominator and denominator.

> BR,
> Jani.
>
>
>>> 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
>>>
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