[Intel-gfx] [PATCH] drm/i915: Allow user to override PWM backlight frequency and duty cycle

[Alex Ivanov]

17.01.2018, 16:13, "Jani Nikula" <jani.nikula at linux.intel.com>:
> I'm divided. Clearly, the patch at hand is a technically better solution
> than what the blog post has.
>
> But I also think the folks over at the blog know they're directly
> hacking on graphics registers, and whatever it is they're doing is not
> supported. There's also warranted speculation this might be harmful to
> their displays.

But those using such calculators and guides don't know what precisely
they are doing, because authors only provided setup logic for *their own*
hardware, and not for other chips. This leads to more harm than it should
would be there an official way for doing overrides.

> The modulation frequency and the minimum duty cycle have been chosen by
> the OEM to work with their board design and display specs. 

Unfortunately some vendors tend to use "whatever defaults" that are not good
for humans. That's the reason why people poke with PWM registers.

> We don't have enough data to validate the values given by the user are within spec. If
> we add this, it'll get used, it'll be expected to be supported, the
> "unsafe" there will go unnoticed by folks copy-pasting the parameters
> from blogs and forums

I can rename the knobs into e.g. 
"debug_backlight_freq" and "debug_backlight_min_level" and add warnings to
descriptions so there will be FAT warnings.

> and if it breaks displays for folks, they'll be
> angry at us, not at some random blog poster.

The license on i915 driver says that its provided "as is" without any warranty.
With added warnings on kernel parameters people MUST know what they're
doing and take all responsibility.

>
> I guess damned if you do, damned if you don't.
>
> BR,
> Jani.
>