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

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

On 7/15/26 11:50 AM, Maxime Ripard wrote:
> On Fri, Jul 10, 2026 at 01:27:37PM +0300, Cristian Ciocaltea wrote:
>> On 7/8/26 1:11 PM, Cristian Ciocaltea wrote:
>>> 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()? 
>>
>> Actually, this brings us to the callback issue: we cannot compute caps
>> dynamically, as it only works with static data, since funcs is supposed to be
>> immutable.
> 
> Does it? The core and helpers must consider it immutable but it doesn't
> have to. drm_bridge_connector for example could totally allocate it and
> dynamically create it based on the bridge capabilities.

If we take the VC4 case, is it fine to drop the const from the static
drm_connector_hdmi_funcs to allow dynamically setting up supported_hdmi_ver and
max_bpc in vc4_hdmi_connector_init()?

static struct drm_connector_hdmi_funcs vc4_hdmi_hdmi_connector_funcs = {
	.tmds_char_rate_valid	 = vc4_hdmi_connector_clock_valid,
	...
}

static int vc4_hdmi_connector_init() 
{
	...
    
	if (vc4_hdmi->variant->supports_hdr)
		vc4_hdmi_hdmi_connector_funcs.max_bpc = 12;

	if (vc4_hdmi->variant->max_pixel_clock >= HDMI_2_0_TMDS_CHAR_RATE_MAX_HZ)
		vc4_hdmi_hdmi_connector_funcs.supported_hdmi_ver = HDMI_VERSION_2_0;
	else if (vc4_hdmi->variant->max_pixel_clock >= HDMI_1_3_TMDS_CHAR_RATE_MAX_HZ)
		vc4_hdmi_hdmi_connector_funcs.supported_hdmi_ver = HDMI_VERSION_1_3;
	...
}

>> Moreover, we would end up with some redundancy, as most of these input fields,
>> after validation, would be stored (altered or not) directly in
>> drm_connector/drm_connector_hdmi structs.
> 
> I mean, part of the conversion would obviously be to remove the
> redundant fields.

Ack.