On Tue, Jul 26, 2022 at 07:28:48PM +0800, ChiaEn Wu wrote: > On Tue, Jul 26, 2022 at 5:31 PM Daniel Thompson > <daniel.thompson@xxxxxxxxxx> 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. Daniel.