[PATCH v8 02/39] drm/connector: Add caps-based HDMI connector init helper

[Cristian Ciocaltea]

Hi Maxime,

On 7/7/26 7:10 PM, Maxime Ripard wrote:
> On Fri, Jul 03, 2026 at 10:31:55PM +0300, Cristian Ciocaltea wrote:
>> Hi Dmitry,
>>
>> Thanks for your quick review!
>>
>> On 7/3/26 5:05 PM, Dmitry Baryshkov wrote:
>>> On Thu, Jul 02, 2026 at 05:46:15PM +0300, Cristian Ciocaltea wrote:
>>>> In preparation for adding HDMI 2.x source capabilities, introduce struct
>>>> drm_connector_hdmi_caps and a new drmm_connector_hdmi_init_with_caps()
>>>> helper.
>>>>
>>>> The existing drmm_connector_hdmi_init() helper currently takes
>>>> individual capability arguments such as supported_formats and max_bpc.
>>>> Adding more HDMI-specific arguments to that function would not scale
>>>> well, so move those values into a dedicated capabilities structure and
>>>> implement the existing helper as a wrapper around the new caps-based
>>>> interface.
>>>
>>> I think, it was an intention of Maxime: make sure that every driver is
>>> forced to provide some values here. With the struct-based init it is
>>> easy to overlook or to ommit a value.
>>
>> Agreed that the struct-based init loses the compile-time guarantee that every
>> argument is explicitly provided - that's a real downside.  
>>
>> I'd argue it's recoverable, though: the init helper validates the mandatory
>> fields, so a driver that omits a required value gets rejected at init time
>> rather than silently misconfigured.  The "you must provide sane values" property
>> is expected to be preserved, just enforced at runtime instead of by the
>> compiler. 
> 
> Yeah, I don't think we can win with C here. Rust might, but we're
> probably a long way from that.
> 
>> The main motivation for the struct is scalability/maintainability as we add HDMI
>> 2.x capabilities: new fields go into the struct rather than growing the helper's
>> argument list, so existing callers don't need churny signature updates on every
>> extension.
>>
>> FWIW, in the previous revision we discussed addressing the concern with a
>> callback instead.  Sadly, I had to discard that approach, as it proved not
>> flexible enough, e.g. drm_bridge_connector_init() computes caps dynamically, and
>> would have required either stateful callbacks, or storing redundant/temporary
>> cap data in driver-private structures just to satisfy the callback.
> 
> I just realized something reviewing your patch: we don't necessarily
> need an extra argument or a callback, we can just put these fields into
> drm_hdmi_connector_funcs directly, and then validate them in init.

If I understand correctly, we should drop the drm_connector_hdmi_caps struct
introduced by this patch and move all its fields into drm_hdmi_connector_funcs.

In that case, how should we proceed with drmm_connector_hdmi_init()? I see the
following options:

1. Continue with drmm_connector_hdmi_init_with_caps() after removing the caps
   parameter, and then drop drmm_connector_hdmi_init() after the migration.

2. Keep the existing (unaltered) drmm_connector_hdmi_init(), which would be
   slightly inconsistent: supported_formats and max_bpc would still be passed 
   as arguments, while the rest would go through drm_hdmi_connector_funcs.

3. A variation of option 1: additionally rename
   drmm_connector_hdmi_init_with_caps() to drmm_connector_hdmi_init() after the
   migration.

Thanks,
Cristian