Re: [PATCH 04/12] drm/i915/display: Extract helpers to set dsc/scaler prefill latencies

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 8/22/2025 4:53 PM, Jani Nikula wrote:
> On Wed, 20 Aug 2025, Ankit Nautiyal <ankit.k.nautiyal@intel.com> wrote:
>> Currently dsc/scaler prefill latencies are handled during watermark
>> calculations. With the optimized guardband, we need to compute the
>> latencies to find the minimum guardband that works for most cases.
>> Extract the helpers to compute these latencies, so that they can be used
>> while computing vrr guardband.
>>
>> While at it, put declarations in reverse xmas tree order for better
>> redability.
>>
>> v2: Initialize {h,v}scale_k to 0, and simplify the check in
>> intel_display_scaler_prefill_latency(). (Mitul)
>>
>> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
>> Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
>> ---
>>   drivers/gpu/drm/i915/display/intel_display.c | 33 ++++++++++++++
>>   drivers/gpu/drm/i915/display/intel_display.h |  8 ++++
> Side note, basically adding anything to intel_display.c becomes a new
> todo item of things to move out of intel_display.c.
>
> It has long been a dumping ground, and continues to be so. :(

Thanks, Jani. Understood about intel_display.c being overloaded.

I can keep the latency computation helpers in skl_watermark.c for now to 
avoid adding more to intel_display.c.

Let me know if you'd prefer a different location.


Regards,

Ankit

>
> BR,
> Jani.
>
>
>>   drivers/gpu/drm/i915/display/skl_watermark.c | 46 +++++++++-----------
>>   3 files changed, 62 insertions(+), 25 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
>> index c1a3a95c65f0..62ec95a75154 100644
>> --- a/drivers/gpu/drm/i915/display/intel_display.c
>> +++ b/drivers/gpu/drm/i915/display/intel_display.c
>> @@ -8328,3 +8328,36 @@ bool intel_scanout_needs_vtd_wa(struct intel_display *display)
>>   
>>   	return IS_DISPLAY_VER(display, 6, 11) && i915_vtd_active(i915);
>>   }
>> +
>> +int intel_display_scaler_prefill_latency(int num_scaler_users, u64 hscale, u64 vscale,
>> +					 int chroma_downscaling_factor,
>> +					 int cdclk_prefill_adjustment,
>> +					 int linetime)
>> +{
>> +	int scaler_prefill_latency;
>> +
>> +	scaler_prefill_latency = 4 * linetime +
>> +				 DIV_ROUND_UP_ULL((4 * linetime * hscale * vscale *
>> +						   chroma_downscaling_factor), 1000000);
>> +
>> +	scaler_prefill_latency *= cdclk_prefill_adjustment;
>> +
>> +	return scaler_prefill_latency;
>> +}
>> +
>> +int intel_display_dsc_prefill_latency(int num_scaler_users, u64 *hscale, u64 *vscale,
>> +				      int chroma_downscaling_factor,
>> +				      int cdclk_prefill_adjustment,
>> +				      int linetime)
>> +{
>> +	int dsc_prefill_latency;
>> +
>> +	dsc_prefill_latency = DIV_ROUND_UP(15 * linetime * chroma_downscaling_factor, 10);
>> +
>> +	for (int i = 0; i < num_scaler_users; i++)
>> +		dsc_prefill_latency = DIV_ROUND_UP_ULL(dsc_prefill_latency * hscale[i] * vscale[i],
>> +						       1000000);
>> +	dsc_prefill_latency *= cdclk_prefill_adjustment;
>> +
>> +	return dsc_prefill_latency;
>> +}
>> diff --git a/drivers/gpu/drm/i915/display/intel_display.h b/drivers/gpu/drm/i915/display/intel_display.h
>> index 37e2ab301a80..8d094b0a8c6b 100644
>> --- a/drivers/gpu/drm/i915/display/intel_display.h
>> +++ b/drivers/gpu/drm/i915/display/intel_display.h
>> @@ -559,5 +559,13 @@ bool assert_port_valid(struct intel_display *display, enum port port);
>>   
>>   bool intel_scanout_needs_vtd_wa(struct intel_display *display);
>>   int intel_crtc_num_joined_pipes(const struct intel_crtc_state *crtc_state);
>> +int intel_display_scaler_prefill_latency(int num_scaler_users, u64 hscale, u64 vscale,
>> +					 int chroma_downscaling_factor,
>> +					 int cdclk_prefill_adjustment,
>> +					 int linetime);
>> +int intel_display_dsc_prefill_latency(int num_scaler_users, u64 *hscale, u64 *vscale,
>> +				      int chroma_downscaling_factor,
>> +				      int cdclk_prefill_adjustment,
>> +				      int linetime);
>>   
>>   #endif
>> diff --git a/drivers/gpu/drm/i915/display/skl_watermark.c b/drivers/gpu/drm/i915/display/skl_watermark.c
>> index 97b42bbf5642..f0213785e9fc 100644
>> --- a/drivers/gpu/drm/i915/display/skl_watermark.c
>> +++ b/drivers/gpu/drm/i915/display/skl_watermark.c
>> @@ -2179,11 +2179,12 @@ cdclk_prefill_adjustment(const struct intel_crtc_state *crtc_state)
>>   static int
>>   dsc_prefill_latency(const struct intel_crtc_state *crtc_state, int linetime)
>>   {
>> +	const struct intel_crtc_scaler_state *scaler_state = &crtc_state->scaler_state;
>> +	int chroma_downscaling_factor = skl_scaler_chroma_downscale_factor(crtc_state);
>>   	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
>> -	const struct intel_crtc_scaler_state *scaler_state =
>> -					&crtc_state->scaler_state;
>>   	int num_scaler_users = hweight32(scaler_state->scaler_users);
>> -	int chroma_downscaling_factor = skl_scaler_chroma_downscale_factor(crtc_state);
>> +	u64 hscale_k[ARRAY_SIZE(scaler_state->scalers)];
>> +	u64 vscale_k[ARRAY_SIZE(scaler_state->scalers)];
>>   	u32 dsc_prefill_latency = 0;
>>   
>>   	if (!crtc_state->dsc.compression_enable ||
>> @@ -2191,18 +2192,16 @@ dsc_prefill_latency(const struct intel_crtc_state *crtc_state, int linetime)
>>   	    num_scaler_users > crtc->num_scalers)
>>   		return dsc_prefill_latency;
>>   
>> -	dsc_prefill_latency = DIV_ROUND_UP(15 * linetime * chroma_downscaling_factor, 10);
>> -
>>   	for (int i = 0; i < num_scaler_users; i++) {
>> -		u64 hscale_k, vscale_k;
>> -
>> -		hscale_k = max(1000, mul_u32_u32(scaler_state->scalers[i].hscale, 1000) >> 16);
>> -		vscale_k = max(1000, mul_u32_u32(scaler_state->scalers[i].vscale, 1000) >> 16);
>> -		dsc_prefill_latency = DIV_ROUND_UP_ULL(dsc_prefill_latency * hscale_k * vscale_k,
>> -						       1000000);
>> +		hscale_k[i] = max(1000, mul_u32_u32(scaler_state->scalers[i].hscale, 1000) >> 16);
>> +		vscale_k[i] = max(1000, mul_u32_u32(scaler_state->scalers[i].vscale, 1000) >> 16);
>>   	}
>>   
>> -	dsc_prefill_latency *= cdclk_prefill_adjustment(crtc_state);
>> +	dsc_prefill_latency =
>> +		intel_display_dsc_prefill_latency(num_scaler_users, hscale_k, vscale_k,
>> +						  chroma_downscaling_factor,
>> +						  cdclk_prefill_adjustment(crtc_state),
>> +						  linetime);
>>   
>>   	return dsc_prefill_latency;
>>   }
>> @@ -2210,28 +2209,25 @@ dsc_prefill_latency(const struct intel_crtc_state *crtc_state, int linetime)
>>   static int
>>   scaler_prefill_latency(const struct intel_crtc_state *crtc_state, int linetime)
>>   {
>> -	const struct intel_crtc_scaler_state *scaler_state =
>> -					&crtc_state->scaler_state;
>> +	const struct intel_crtc_scaler_state *scaler_state = &crtc_state->scaler_state;
>> +	int chroma_downscaling_factor = skl_scaler_chroma_downscale_factor(crtc_state);
>>   	int num_scaler_users = hweight32(scaler_state->scaler_users);
>> +	u64 hscale_k = 0, vscale_k = 0;
>>   	int scaler_prefill_latency = 0;
>>   
>>   	if (!num_scaler_users)
>>   		return scaler_prefill_latency;
>>   
>> -	scaler_prefill_latency = 4 * linetime;
>> -
>>   	if (num_scaler_users > 1) {
>> -		u64 hscale_k = max(1000, mul_u32_u32(scaler_state->scalers[0].hscale, 1000) >> 16);
>> -		u64 vscale_k = max(1000, mul_u32_u32(scaler_state->scalers[0].vscale, 1000) >> 16);
>> -		int chroma_downscaling_factor = skl_scaler_chroma_downscale_factor(crtc_state);
>> -		int latency;
>> -
>> -		latency = DIV_ROUND_UP_ULL((4 * linetime * hscale_k * vscale_k *
>> -					    chroma_downscaling_factor), 1000000);
>> -		scaler_prefill_latency += latency;
>> +		hscale_k = max(1000, mul_u32_u32(scaler_state->scalers[0].hscale, 1000) >> 16);
>> +		vscale_k = max(1000, mul_u32_u32(scaler_state->scalers[0].vscale, 1000) >> 16);
>>   	}
>>   
>> -	scaler_prefill_latency *= cdclk_prefill_adjustment(crtc_state);
>> +	scaler_prefill_latency =
>> +		intel_display_scaler_prefill_latency(num_scaler_users, hscale_k, vscale_k,
>> +						     chroma_downscaling_factor,
>> +						     cdclk_prefill_adjustment(crtc_state),
>> +						     linetime);
>>   
>>   	return scaler_prefill_latency;
>>   }