Nouveau - Nov 2022 - [PATCH v6 16/23] drm/probe-helper: Provide a TV get_modes helper

Hi Noralf,

I'll leave aside your comments on the code, since we'll use your
implementation.

On Sun, Nov 06, 2022 at 05:33:48PM +0100, Noralf Tr?nnes
wrote:
> Den 26.10.2022 17.33, skrev maxime at cerno.tech:
> > +
> > +	if (cmdline->tv_mode_specified)
> > +		default_mode = cmdline->tv_mode;
> 
> I realised that we don't verify tv_mode coming from the command line,
> not here and not in the reset helper. Should we do that? A driver should
> be programmed defensively to handle an illegal/unsupported value, but it
> doesn't feel right to allow an illegal enum value coming through the
> core/helpers.
I don't think we can end up with an invalid value here if it's been
specified.

We parse the command line through drm_mode_parse_tv_mode() (introduced
in patch 13 "drm/modes: Introduce the tv_mode property as a command-line
option") that will pick the tv mode part of the command line, and call
drm_get_tv_mode_from_name() using it.

drm_get_tv_mode_from_name() will return a EINVAL if it's not a value we
expect, and mode->tv_mode is only set on success. And AFAIK, there's no
other path that will set tv_mode.

Maxime
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Den 07.11.2022 11.21, skrev Maxime Ripard:
> Hi Noralf,
> 
> I'll leave aside your comments on the code, since we'll use your
implementation.
> 
> On Sun, Nov 06, 2022 at 05:33:48PM +0100, Noralf Tr?nnes wrote:
>> Den 26.10.2022 17.33, skrev maxime at cerno.tech:
>>> +
>>> +	if (cmdline->tv_mode_specified)
>>> +		default_mode = cmdline->tv_mode;
>>
>> I realised that we don't verify tv_mode coming from the command
line,
>> not here and not in the reset helper. Should we do that? A driver
should
>> be programmed defensively to handle an illegal/unsupported value, but
it
>> doesn't feel right to allow an illegal enum value coming through
the
>> core/helpers.
> 
> I don't think we can end up with an invalid value here if it's been
> specified.
> 
> We parse the command line through drm_mode_parse_tv_mode() (introduced
> in patch 13 "drm/modes: Introduce the tv_mode property as a
command-line
> option") that will pick the tv mode part of the command line, and call
> drm_get_tv_mode_from_name() using it.
> 
> drm_get_tv_mode_from_name() will return a EINVAL if it's not a value we
> expect, and mode->tv_mode is only set on success. And AFAIK, there's
no
> other path that will set tv_mode.
> 
I see now that illegal was the wrong word, but if the driver only
supports ntsc, the user can still set tv_mode=PAL right? And that's an
unsupported value that the driver can't fulfill, so it errors out. But
then again maybe that's just how it is, we can also set a display mode
that the driver can't handle, so this is no different in that respect.
Yeah, my argument lost some of its strength here :)

Noralf.