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

[Maxime Ripard]

Hi,

On Wed, Jul 08, 2026 at 01:11:49PM +0300, Cristian Ciocaltea wrote:
> 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.

If we're going that road, we can also move the format, bpc and vendor
and product there for example.

> 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.

I'm not sure why we would want to do that?

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

It should be renamed indeed.

Maxime

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