Re: [PATCH v5 20/22] drm/vc4: vec: Convert to the new TV mode property

[Noralf Trønnes]

Den 16.10.2022 20.52, skrev Mateusz Kwiatkowski:
> Hi Maxime,
> 
>>  static int vc4_vec_connector_get_modes(struct drm_connector *connector)
>>  {
>> -	struct drm_connector_state *state = connector->state;
>>  	struct drm_display_mode *mode;
>>  
>> -	mode = drm_mode_duplicate(connector->dev,
>> -				  vc4_vec_tv_modes[state->tv.legacy_mode].mode);
>> +	mode = drm_mode_analog_ntsc_480i(connector->dev);
>>  	if (!mode) {
>>  		DRM_ERROR("Failed to create a new display mode\n");
>>  		return -ENOMEM;
>>  	}
>>  
>> +	mode->type |= DRM_MODE_TYPE_PREFERRED;
>>  	drm_mode_probed_add(connector, mode);
>>  
>> -	return 1;
>> +	mode = drm_mode_analog_pal_576i(connector->dev);
>> +	if (!mode) {
>> +		DRM_ERROR("Failed to create a new display mode\n");
>> +		return -ENOMEM;
>> +	}
>> +
>> +	drm_mode_probed_add(connector, mode);
>> +
>> +	return 2;
>> +}
> 
> Referencing those previous discussions:
> - https://lore.kernel.org/dri-devel/0255f7c6-0484-6456-350d-cf24f3fee5d6@xxxxxxxxxxx/
> - https://lore.kernel.org/dri-devel/c8f8015a-75da-afa8-ca7f-b2b134cacd16@xxxxxxxxx/
> 
> Unconditionally setting the 480i mode as DRM_MODE_TYPE_PREFERRED causes Xorg
> (at least on current Raspberry Pi OS) to display garbage when
> video=Composite1:PAL is specified on the command line, so I'm afraid this won't
> do.
> 
> As I see it, there are three viable solutions for this issue:
> 
> a) Somehow query the video= command line option from this function, and set
>    DRM_MODE_TYPE_PREFERRED appropriately. This would break the abstraction
>    provided by global DRM code, but should work fine.
> 
> b) Modify drm_helper_probe_add_cmdline_mode() so that it sets
>    DRM_MODE_TYPE_PREFERRED in addition to DRM_MODE_TYPE_USERDEF. This seems
>    pretty robust, but affects the entire DRM subsystem, which may break
>    userspace in different ways.
> 
>    - Maybe this could be mitigated by adding some additional conditions, e.g.
>      setting the PREFERRED flag only if no modes are already flagged as such
>      and/or only if the cmdline mode is a named one (~= analog TV mode)
> 
> c) Forcing userspace (Xorg / Raspberry Pi OS) to get fixed and honor the USERDEF
>    flag.
> 
> Either way, hardcoding 480i as PREFERRED does not seem right.
> 

My solution for this is to look at tv.mode to know which mode to mark as
preferred. Maxime didn't like this since it changes things behind
userspace's back. I don't see how that can cause any problems for userspace.

If userspace uses atomic and sets tv_mode, it has to know which mode to
use before hand, so it doesn't look at the preferreded flag.

If it uses legacy and sets tv_mode, it can end up with a stale preferred
flag, but no worse than not having the flag or that ntsc is always
preferred.

If it doesn't change tv_mode, there's no problem, the preferred flag
doesn't change.

Noralf.