Re: [PATCH v7 03/22] drm: Add enum conversions between DRM_COLOR_FORMAT and HDMI_COLORSPACE

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Friday, 6 February 2026 15:08:46 Central European Standard Time Maxime Ripard wrote:
> On Wed, Jan 21, 2026 at 03:45:10PM +0100, Nicolas Frattaroli wrote:
> > While the two enums have similar values, they're not identical, and
> > HDMI's enum is defined as per the HDMI standard.
> > 
> > Add a simple conversion function from DRM to HDMI. Unexpected inputs
> > aren't handled in any clever way, DRM_COLOR_FORMAT_AUTO and any other
> > value that doesn't cleanly map to HDMI just gets returned as
> > HDMI_COLORSPACE_RGB.
> > 
> > Add a second conversion function that gets a DRM_COLOR_FORMAT from an
> > HDMI_COLORSPACE as well. In this case, reserved HDMI values that can't
> > be converted will result in an -EINVAL return value.
> > 
> > Co-developed-by: Marius Vlad <marius.vlad@collabora.com>
> > Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
> > Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> > ---
> >  include/drm/drm_connector.h | 54 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 54 insertions(+)
> > 
> > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> > index b5604dca728a..ffeb42f3b4a3 100644
> > --- a/include/drm/drm_connector.h
> > +++ b/include/drm/drm_connector.h
> > @@ -2612,6 +2612,60 @@ int drm_connector_attach_color_format_property(struct drm_connector *connector);
> >  
> >  const char *drm_get_color_format_name(enum drm_color_format color_fmt);
> >  
> > +/**
> > + * drm_color_format_to_hdmi_colorspace - convert DRM color format to HDMI
> > + * @fmt: the &enum drm_color_format to convert
> > + *
> > + * Convert a given &enum drm_color_format to an equivalent
> > + * &enum hdmi_colorspace. For non-representable values and
> > + * %DRM_COLOR_FORMAT_AUTO, the value %HDMI_COLORSPACE_RGB is returned.
> > + *
> > + * Returns: the corresponding &enum hdmi_colorspace value
> > + */
> > +static inline enum hdmi_colorspace __pure
> > +drm_color_format_to_hdmi_colorspace(enum drm_color_format fmt)
> > +{
> > +	switch (fmt) {
> > +	default:
> > +	case DRM_COLOR_FORMAT_AUTO:
> > +	case DRM_COLOR_FORMAT_RGB444:
> > +		return HDMI_COLORSPACE_RGB;
> 
> I don't think that's correct. What auto ends up as totally depends on
> the atomic state it comes with.
> 
> At the very least, you should output a warning there, because that case
> should never happen.

Yeah, my hope was to keep this function __pure so that the compiler
has maximum freedom to do whatever. With a WARN, it's got side-effects
now, and we're no longer pure. With a status return value and an output
parameter, it's no longer pure either, because the output parameter is
not local memory.

The limiting factor here is that as I understand correctly, I can't
really extend the hdmi_colorspace enum, as it's basically 1:1 from
the standard. Doing this would be the ideal solution, because we'd
keep the function pure and without surprise conversions happening.

Looking at hdmi_colorspace_get_name in drivers/video/hdmi.c, it returns
"Invalid" for any value not in the enum itself. Would it be allowable
to tack an HDMI_COLORSPACE_INVALID at the end of the enum with perhaps
a negative value, or is there a different approach you'd prefer?

I agree that the AUTO-to-RGB conversion shouldn't happen here, that's
a recipe for things implicitly relying on this behaviour, which isn't
great. (And I think I even do this in "hdmi-state-helper: Act on color
format DRM property", where thinking about it again I agree this isn't
super obvious and should be done explicitly instead.)

> > +	case DRM_COLOR_FORMAT_YCBCR444:
> > +		return HDMI_COLORSPACE_YUV444;
> > +	case DRM_COLOR_FORMAT_YCBCR422:
> > +		return HDMI_COLORSPACE_YUV422;
> > +	case DRM_COLOR_FORMAT_YCBCR420:
> > +		return HDMI_COLORSPACE_YUV420;
> > +	}
> > +}
> > +
> > +/**
> > + * drm_color_format_from_hdmi_colorspace - convert HDMI color format to DRM
> > + * @fmt: the &enum hdmi_colorspace to convert
> > + *
> > + * Convert a given &enum hdmi_colorspace to an equivalent
> > + * &enum drm_color_format. For non-representable values,
> > + * %-EINVAL is returned.
> > + *
> > + * Returns: the corresponding &enum drm_color_format value, or %-EINVAL
> > + */
> > +static inline enum drm_color_format __pure
> > +drm_color_format_from_hdmi_colorspace(enum hdmi_colorspace fmt)
> > +{
> > +	switch (fmt) {
> > +	default:
> > +		return -EINVAL;
> 
> Wait, what?
> 
> -EINVAL is not a valid value for your enum.

Not the only part of the kernel where we rely on the int-ness of
enums, but your complaint has been noted :) I guess this means
this approach won't fly for the opposite direction. Thankfully,
in this direction, we can extend the drm_color_format enum to
have an error value.

Kind regards,
Nicolas Frattaroli

> 
> Maxime
>