[PATCH v6 13/13] video: backlight: mt6370: Add MediaTek MT6370 support

[ChiaEn Wu]

On Tue, Jul 26, 2022 at 7:59 PM Daniel Thompson
<daniel.thompson at linaro.org> wrote:

 ...

> > > > > Does the MT6372 support more steps than this? In other words does it use
> > > > > a fourteen bit scale or does it use an 11-bit scale at a different
> > > > > register location?
> > > >
> > > > Hi Daniel,
> > > >
> > > > Thanks for your reply.
> > > > Yes, MT6372 can support 16384 steps and uses a 14-bit scale register
> > > > location. But the maximum current of each
> > > > channel of MT6372 is the same as MT6370 and MT6371, both 30mA.
> > > > The main reason why MT6372 is designed this way is that one of the
> > > > customers asked for a more delicate
> > > > adjustment of the backlight brightness. But other customers actually
> > > > do not have such requirements.
> > > > Therefore, we designed it this way for maximum compatibility in software.
> >
> > Sorry for I used of the wrong word, I mean is 'driver', not
> > higher-level software
> >
> > >
> > > I don't think that is an acceptable approach for the upstream kernel.
> > >
> > > To be "compatible" with (broken) software this driver ends up reducing
> > > the capability of the upstream kernel to the point it becomes unable to
> > > meet requirements for delicate adjustment (requirements that were
> > > sufficiently important to change the hardware design so you could meet
> > > them).
> >
> > Originally, we just wanted to use one version of the driver to cover
> > all the SubPMIC of the 6370 series(6370~6372).
> > And, the users who use this series SubPMIC can directly apply this
> > driver to drive the backlight device without knowing the underlying
> > hardware.
> > To achieve this goal, we have designed it to look like this.
>
> You don't need a second driver to support two different values for
> max-brightness. The same driver can support both ranges with nothing but
> a small tweak during the driver probe function.
>
>
> > ...
> > > > > > +
> > > > > > +     if (brightness) {
> > > > > > +             brightness_val[0] = (brightness - 1) & MT6370_BL_DIM2_MASK;
> > > > > > +             brightness_val[1] = (brightness - 1) >> fls(MT6370_BL_DIM2_MASK);
> > > > > > +
> > > > > > +             /*
> > > > > > +              * To make MT6372 using 14 bits to control the brightness
> > > > > > +              * backward compatible with 11 bits brightness control
> > > > > > +              * (like MT6370 and MT6371 do), we left shift the value
> > > > > > +              * and pad with 1 to remaining bits. Hence, the MT6372's
> > > > > > +              * backlight brightness will be almost the same as MT6370's
> > > > > > +              * and MT6371's.
> > > > > > +              */
> > > > > > +             if (priv->vid_type == MT6370_VID_6372) {
> > > > > > +                     brightness_val[0] <<= MT6370_BL_DIM2_6372_SHIFT;
> > > > > > +                     brightness_val[0] |= MT6370_BL_DUMMY_6372_MASK;
> > > > > > +             }
> > > > >
> > > > > This somewhat depends on the answer to the first question above, but
> > > > > what is the point of this shifting? If the range is 14-bit then the
> > > > > driver should set max_brightness to 16384 and present the full range of
> > > > > the MT6372 to the user.
> > > >
> > > > So should we make all 16384 steps of MT6372 available to users?
> > >
> > > Yes.
> > >
> > >
> > > > Does that mean the DTS needs to be modified as well?
> > >
> > > Yes... the property to set initial brightness needs a 14-bit range.
> > >
> > > It would also be a good idea to discuss with the DT maintainers whether
> > > you should introduce a second compatible string (ending 6372) in order
> > > to allow the DT validation checks to detect accidental use of MT6372
> > > ranges on MT6370 hardware.
> >
> > hmmm... I have just thought about it,
> > maybe I can just modify the maximum value of default-brightness and
> > max-brightness in DT to 16384,
> > modify the description and add some comments.
>
> What for?
>
> All the other backlight drivers (there are >130 of them) expose the hardware
> range[1]. Most userspaces will already know how to handle that (by reading
> the max_brightness and, if it is recent enough, also the scale
> properties in sysfs).
>
> I'm still don't understand why one should fix a bug in the userspace by
> implementing a hack in the driver.
>
>
> [1] Well almost. The PWM backlight driver does contain support for
>     dead-spot avoidance and to allow the adoption of exponential scale.
>     However this  purpose of these is based on how PWM backlights work
>
> > And then on the driver side,
> > we can use mt6370_check_vendor_info() to determine whether it is MT6372.
> > If no, then in mt6370_bl_update_status(), first brightness_val / 8 and then set.
> > In mt6370_bl_get_brightness(), first brightness_val * 8 and then return;
> >
> > If I do this change, does this meet your requirements?
>
> Not really.
>
> It's still misleading a sophisticated userspace, which may make bad
> rounding decisions for backlight animation, in order to support a broken
> one.
>
>
> > > > Or, for the reasons, I have just explained (just one customer has this
> > > > requirement), then we do not make any changes for compatibility
> > > > reasons?
> > >
> > > I'd be curious what the compatiblity reasons are. In other words what
> > > software breaks?
> >
> > The reason is as above. We just hope the users who use this series SubPMIC can
> > directly apply this driver to drive the backlight device without
> > knowing the underlying hardware.
> > Not software breaks.
>
> As above, ignoring the max_brightness property is a bug in the
> userspace. I'm still unclear why sending faked ranges to userspace
> it a better solution than fixing the userspace.

Ok, I got it!
If I add a second compatible string (like 'mediatek,mt6372-backlight')
in the DT section,
and append 'if-then-else' to determine the correct maximum value of
'default-brightness' and 'max-brightness',
Also, I will append 'bled exponential mode' to make user control using
linear or exponential mode.
These changes I will explain to DT's maintainer again.

Back to the driver section,
do I still need to use the register to confirm again whether this
SubPMIC used now is MT6372 and record this information? (using
'mt6370_check_vendor_info()')
I am afraid that the user who uses the MT6370 hardware, but the DT
compatible string is set to 'mediatek,mt6372-backlight'.
This may cause errors when update/get brightness values.
So I hope the driver here can check again to make sure the
'default-brightness', 'max-brightness', can be updated/got correctly.
I don't know if this will make you feel redundant if I do this??

Thank you so much!

-- 
Best Regards,
ChiaEn Wu