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

[Maxime Ripard <mripard@kernel.org>]

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On Wed, Jul 15, 2026 at 01:34:19PM +0300, Cristian Ciocaltea wrote:
> 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;
> 	...
> }

No, but you don't have to do that either.

You have three cases:
 - Rpi < 4 has max_bpc = 8, HDMI 1.4
 - RPI 4 HDMI 0 and RPI 5 has max_bpc = 12, HDMI 2.0
 - RPI 4 HDMI 1 has max_bpc = 12, HDMI 1.4

Just create three different structures, and put a pointer to the right
one in the vc4_hdmi_variant structure. There's no need to be dynamic
there.

Maxime

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