[PATCH] drm: Fix HDMI_STATIC_METADATA_TYPE1 constant.

[Mario Kleiner]

On Sun, Jan 24, 2021 at 9:24 PM Simon Ser <contact at emersion.fr> wrote:

> On Sunday, January 24th, 2021 at 9:10 PM, Mario Kleiner <
> mario.kleiner.de at gmail.com> wrote:
>
> > But it still needs to be fixed if we want working HDR. I thought
> > libdrm copies the definitions from the kernel periodically, so the
> > fix should propagate?
>
> There will always be user-space that sends 1 instead of 0. This
> shouldn't fail on more recent kernels or it will be a regression.
>

Yes, i know, regressing user-space is bad, but in this very specific case a
"good" one, if ever. At the moment, it wouldn't regress userspace simply
because the kernel doesn't actually check for the correct value in its HDR
metadata handling. But the value itself is sent as HDMI HDR metadata to the
attached HDR display monitor, so if the monitors firmware checks, it will
classify the wrong value of 1 as invalid and disable HDR mode on the
display, which is certainly not what a HDR client application wants. And
future HDR standards which will actually allocate the value 1 to a
different mode of HDR operation will switch to the wrong mode /
misinterpret the sent HDR metadata with hillarious results, which is also
not in the interest of a HDR client application, or a HDR capable
compositor.

Iow. if clients continue to use the wrong value 1 then HDR display will
break in various ways on correctly implemented HDR displays, but in a
mystifying and hard to debug way. The kernel rejecting a wrong setting
explicitly and forcing a bug fix in the client would be a blessing in this
case.

I spent weeks last year, going in circles and hunting ghost bugs related to
HDR because much of the HDR stuff, both drivers and monitor firmware seems
to be in not a great shape. "Less wrong" would be a big step forward.
Especially with the cheaper HDR monitors it is difficult to see when things
go wrong, because we don't have good expectations on how proper HDR should
look and the lower end HDR displays don't help.

-mario
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