Re: [Intel-gfx] [PATCH v3 5/5] drm/i915: Clarify probing order in intel_dp_aux_init_backlight_funcs()

Ville Syrjälä
2021-10-19T18:15:10Z

On Tue, Oct 05, 2021 at 10:40:18PM -0400, Lyude Paul wrote:
> Hooray! We've managed to hit enough bugs upstream that I've been able to
> come up with a pretty solid explanation for how backlight controls are
> actually supposed to be detected and used these days. As well, having the
> rest of the PWM bits in VESA's backlight interface implemented seems to
> have fixed all of the problematic brightness controls laptop panels that
> we've hit so far.
>
> So, let's actually document this instead of just calling the laptop panels
> liars. As well, I would like to formally apologize to all of the laptop
> panels I called liars. I'm sorry laptop panels, hopefully you can all
> forgive me and we can move past this~
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> .../drm/i915/display/intel_dp_aux_backlight.c | 16 +++++++++++-----
> 1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> index 91daf9ab50e8..04a52d6a74ed 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -455,11 +455,17 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector *connector)
> }
>
> /*
> - * A lot of eDP panels in the wild will report supporting both the
> - * Intel proprietary backlight control interface, and the VESA
> - * backlight control interface. Many of these panels are liars though,
> - * and will only work with the Intel interface. So, always probe for
> - * that first.
> + * Since Intel has their own backlight control interface, the majority of machines out there
> + * using DPCD backlight controls with Intel GPUs will be using this interface as opposed to
> + * the VESA interface. However, other GPUs (such as Nvidia's) will always use the VESA
> + * interface. This means that there's quite a number of panels out there that will advertise
> + * support for both interfaces, primarily systems with Intel/Nvidia hybrid GPU setups.
> + *
> + * There's a catch to this though: on many panels that advertise support for both
> + * interfaces, the VESA backlight interface will stop working once we've programmed the
> + * panel with Intel's OUI - which is also required for us to be able to detect Intel's
> + * backlight interface at all. This means that the only sensible way for us to detect both
> + * interfaces is to probe for Intel's first, and VESA's second.
> */

You know a lot more about this than I do.

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> if (try_intel_interface && intel_dp_aux_supports_hdr_backlight(connector)) {
> drm_dbg_kms(dev, "Using Intel proprietary eDP backlight controls\n");
> --
> 2.31.1

--
Ville Syrjälä
Intel