Re: [PATCH 1/2] drm/i915/dp: Fix pipe BPP clamping due to HDR

["Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com>]

On 2/6/2026 7:20 PM, Nautiyal, Ankit K wrote:
> 
> On 2/6/2026 4:12 PM, Imre Deak wrote:
>> The pipe BPP value shouldn't be set outside of the source's / sink's
>> valid pipe BPP range, ensure this when increasing the minimum pipe BPP
>> value to 30 due to HDR.
>>
>> Fixes: ba49a4643cf53 ("drm/i915/dp: Set min_bpp limit to 30 in HDR mode")
>> Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
>> Cc: <stable@vger.kernel.org> # v6.18+
>> Signed-off-by: Imre Deak <imre.deak@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 2b8f43e211741..4d8f480cf803f 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
>> @@ -2697,6 +2697,7 @@ intel_dp_compute_config_limits(struct intel_dp 
>> *intel_dp,
>>                      bool dsc,
>>                      struct link_config_limits *limits)
>>   {
>> +    struct intel_display *display = to_intel_display(intel_dp);
>>       bool is_mst = intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST);
>>       struct intel_connector *connector =
>>           to_intel_connector(conn_state->connector);
>> @@ -2709,8 +2710,7 @@ intel_dp_compute_config_limits(struct intel_dp 
>> *intel_dp,
>>       limits->min_lane_count = intel_dp_min_lane_count(intel_dp);
>>       limits->max_lane_count = intel_dp_max_lane_count(intel_dp);
>> -    limits->pipe.min_bpp = intel_dp_in_hdr_mode(conn_state) ? 30 :
>> -                intel_dp_min_bpp(crtc_state->output_format);
>> +    limits->pipe.min_bpp = intel_dp_min_bpp(crtc_state->output_format);
>>       if (is_mst) {
>>           /*
>>            * FIXME: If all the streams can't fit into the link with their
>> @@ -2726,6 +2726,16 @@ intel_dp_compute_config_limits(struct intel_dp 
>> *intel_dp,
>>                               respect_downstream_limits);
>>       }
>> +    if (intel_dp_in_hdr_mode(conn_state)) {
>> +        if (limits->pipe.min_bpp <= 30 && limits->pipe.max_bpp >= 30)
>> +            limits->pipe.min_bpp = 30;
>> +        else
>> +            drm_dbg_kms(display->drm,
>> +                    "[CONNECTOR:%d:%s] HDR min 30 bpp outside of 
>> valid pipe bpp range (%d-%d)\n",
>> +                    connector->base.base.id, connector->base.name,
>> +                    limits->pipe.min_bpp, limits->pipe.max_bpp);
> 
> 
> pipe.max_bpp < 30 will be either due to the max_bpc property set to less 
> than 10, or perhaps when the panel itself does not support 10 bpc 
> (limited by EDID or VBT).
> With these constraints doesn't make sense to enable HDR and send HDR 
> metadata.
> However, as we see in some reported issues [1] [2], in practice some 
> compositor seems to enable HDR by default and with the hard limit set, 
> they report blankout.
> So it does make sense to raise the min bpp limit only if its inside the 
> supported range.
> 
> Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> 
> 
> [1] https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/7052
> [2] https://gitlab.freedesktop.org/drm/i915/kernel/-/ 
> issues/5969#note_3248404
> 

I am not sure if this patch would help with the above gitlabs. For 
example in case of #7052 pipe max bpp is 30 and the commit still fails.
However, I need to look deeper.

I am thinking of relaxing this restriction all together because the 
earlier assumption that a panel advertising HDR will support atleast 
10bpc in all it's mode turns out to be false.

Currently, I am inclined on the following policy.

- If DSC is not available, fall back to normal bandwidth calculations 
and select the highest bpp the link can support. (Also preferred by Kwin)

- If DSC is available, prefer falling back to DSC and attempt the 
highest bpp allowed by bandwidth constraints.

I am working on a patch for this and should be able to float something 
soon. Imre, if you agree with this policy, would you please wait for the 
patch. That should make it easier to send out fix for stable kernels.

==
Chaitanya

>> +    }
>> +
>>       if (dsc && !intel_dp_dsc_compute_pipe_bpp_limits(connector, 
>> limits))
>>           return false;