[Intel-gfx] [PATCH] drm/i915/dp: Don't roundup max bpp, while computing compressed bpp

[Nautiyal, Ankit K]

Thanks Stan for the review.

On 3/9/2023 4:14 PM, Lisovskiy, Stanislav wrote:
> On Thu, Feb 23, 2023 at 05:25:09PM +0530, Ankit Nautiyal wrote:
>> While computing compressed bpp, maximum value of bits_per_pixel is
>> calculated that can be supported with the given link configuration
>> for a given mode. Avoid rounding up of this max bits_per_pixel.
>> Also improve documentation for computing max bits_per_pixel.
>>
>> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
> Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
>
>> ---
>>   drivers/gpu/drm/i915/display/intel_dp.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
>> index b77bd4565864..51e9d0b2d8b3 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
>> @@ -716,9 +716,19 @@ u16 intel_dp_dsc_get_output_bpp(struct drm_i915_private *i915,
>>   	 * (LinkSymbolClock)* 8 * (TimeSlots / 64)
>>   	 * for SST -> TimeSlots is 64(i.e all TimeSlots that are available)
>>   	 * for MST -> TimeSlots has to be calculated, based on mode requirements
>> +	 *
>> +	 * Due to FEC overhead, the available bw is reduced to 97.2261%.
>> +	 * To support the given mode:
>> +	 * Bandwidth required should be <= Available link Bandwidth * FEC Overhead
>> +	 * =>ModeClock * bits_per_pixel <= Available Link Bandwidth * FEC Overhead
>> +	 * =>bits_per_pixel <= Available link Bandwidth * FEC Overhead / ModeClock
>> +	 * =>bits_per_pixel <= (NumberOfLanes * LinkSymbolClock) * 8 (TimeSlots / 64) /
>> +	 *		       (ModeClock / FEC Overhead)
>> +	 * =>bits_per_pixel <= (NumberOfLanes * LinkSymbolClock * TimeSlots) /
>> +	 *		       (ModeClock / FEC Overhead * 8)
>>   	 */
>> -	bits_per_pixel = DIV_ROUND_UP((link_clock * lane_count) * timeslots,
>> -				      intel_dp_mode_to_fec_clock(mode_clock) * 8);
>> +	bits_per_pixel = ((link_clock * lane_count) * timeslots) /
>> +			 (intel_dp_mode_to_fec_clock(mode_clock) * 8);
>>   
>>   	drm_dbg_kms(&i915->drm, "Max link bpp is %u for %u timeslots "
>>   				"total bw %u pixel clock %u\n",
>> -- 
>> 2.25.1
>>