[Intel-gfx] [PATCH] drm/i915/display: Return correct err code for bpc < 0

[Nautiyal, Ankit K]

On 4/19/2023 4:12 AM, Manasi Navare wrote:
> Hi Ville and Ankit,
>
> I actually do not think this is a problem with the DSC logic, but it
> is a problem with the intel_dp_link_compute_config() where we should
> do something if max_bpp is 0 instead of just returning a -EINVAL
> directly.
> My question here:
> - In case of PCON, yuv format, is it a valid case to have max bpp set to 0?
> - This is where I am seeing it as set to 0
> - If it isnt then the problem is probably where it computes max bpp
> for hdmi case for yuv format

I got your point. if limits.max_bpp is set to 0 (as given by 
intel_dp_max_bpp)

and we cannot discard it as mentioned by Ville, then even with DSC we 
cant do anything.

In such a case perhaps it makes sense to check if limit.max_bpp is 0 and 
return -EINVAL from intel_dp_compute_link_config.

Regards,

Ankit

>
> Manasi
>
> On Tue, Apr 18, 2023 at 7:54 AM Ville Syrjälä
> <ville.syrjala at linux.intel.com> wrote:
>> On Tue, Apr 18, 2023 at 08:09:16PM +0530, Nautiyal, Ankit K wrote:
>>> On 4/18/2023 6:16 PM, Ville Syrjälä wrote:
>>>> On Mon, Apr 17, 2023 at 03:48:12PM -0700, Manasi Navare wrote:
>>>>> Hi Ville,
>>>>>
>>>>> Could you suggest how to handle the intel_dp_link_compute_config()
>>>>> when the max_bpp is returned as 0, currently
>>>>> it just exits the loop and returns a -EINVAL and this triggers the DSC path.
>>>>> While we should be completely failing the modeset and encoder_config
>>>>> in this case instead of trying DSC, correct?
>>>> The DSC path needs to handle the bpp limits correctly:
>>>> 1. Take the baseline limits already computed
>>>> 2. Further restrict them based on sink/source DSC capabilities/etc.
>>>> 3. Make sure the uncompressed bpp value chosen is between the min/max
>>> I have some older patch to try similar thing [1]. We try to iterate over
>>> bpc to find pipe_bpp in the limits, then try to find out compressed_bpp.
>>>
>>> But if intel_dp_max_bpp returns 0, limits.max_bpp is set to 0, so we
>>> discard this for dsc case and re-calculate the limits.max_bpp?
>> You shouldn't discard anything. DSC should take the already computed
>> limits and potentially just shrink them further based on DSC specific
>> constraints.
>>
>> Or is there some weird case where DSC would allow lower/higher bpp
>> than what our uncompressed bpp limits declare?
>>
>>>
>>> [1] https://patchwork.freedesktop.org/patch/519346/?series=111391&rev=3
>>>
>>>>> Manasi
>>>>>
>>>>> On Thu, Apr 13, 2023 at 8:23 AM Manasi Navare <navaremanasi at chromium.org> wrote:
>>>>>> On Tue, Apr 11, 2023 at 10:22 PM Ville Syrjälä
>>>>>> <ville.syrjala at linux.intel.com> wrote:
>>>>>>> On Tue, Apr 11, 2023 at 05:07:01PM -0700, Manasi Navare wrote:
>>>>>>>> On Tue, Apr 11, 2023 at 10:42 AM Ville Syrjälä
>>>>>>>> <ville.syrjala at linux.intel.com> wrote:
>>>>>>>>> On Tue, Apr 11, 2023 at 05:34:08PM +0000, Manasi Navare wrote:
>>>>>>>>>> In the function intel_dp_max_bpp(), currently if bpc < 0 in case of error,
>>>>>>>>>> we return 0 instead of returning an err code of -EINVAL.
>>>>>>>>>> This throws off the logic in the calling function.
>>>>>>>>> What logic? The caller doesn't expect to get an error.
>>>>>>>> If this returns a 0, we end up using limits.max_bpp = 0 and in
>>>>>>>> intel_dp_compute_link_config_wide(),
>>>>>>>> since max_bpp is 0, it exits this for loop:
>>>>>>>>
>>>>>>>> for (bpp = limits->max_bpp; bpp >= limits->min_bpp; bpp -= 2 * 3) and returns
>>>>>>>> -EINVAL which then wrongly goes to enable DSC even when link BW is
>>>>>>>> sufficient without DSC.
>>>>>>> And how woud max_bpp<0 prevent that?
>>>>>>>
>>>>>>> The real problem seems to be that the DSC code totally
>>>>>>> ignores bpp limits.
>>>>>> Hi Ville,
>>>>>>
>>>>>> So I see a few concerns/questions:
>>>>>> - Why is the Max bpp value 0 in intel_dp_max_bpp, is that a valid case
>>>>>> and how should our link configurations handle that case when max_bpp
>>>>>> is 0?
>>>>>> - This is happening in a bug I am looking at with HDMI PCON, @Ankit
>>>>>> Nautiyal  have we ever seen something similar where max_bpp for HDMi
>>>>>> PCON
>>>>>> is returned 0?
>>>>>> - I dont think its a problem with DSC code, but rather
>>>>>> intel_dp_compute_link_config() outer for loop where we vary
>>>>>> from max_bpp to min_bpp and see if any bpp works with available link
>>>>>> bw, how should we handle this when max_bpp = 0 if you are saying thats
>>>>>> a valid case?
>>>>>> - In this patch if I return -EINVAL instead of 0, then atleast the
>>>>>> entire encoder_config will fail and that will fail the modeset, since
>>>>>> it assumes max_bpp cannot be 0
>>>>>>
>>>>>> Could you please help answer above concerns and how to handle max bpp
>>>>>> = 0 case if that is valid? This patch is simply making that invalid
>>>>>> resulting into modeset failure
>>>>>>
>>>>>> Manasi
>>>>>>> --
>>>>>>> Ville Syrjälä
>>>>>>> Intel
>> --
>> Ville Syrjälä
>> Intel