[Freedreno] [DPU PATCH v2 2/2] drm/panel: add backlight control support for truly panel

[abhinavk at codeaurora.org]

Hi Bjorn

Thanks very much for the detailed response.

Yes, we decided that userspace hardcoding this node name is not a strong 
enough
reason to register as another backlight device.

Had one follow up question though.

The QC WLED driver, drivers/leds/leds-qpnp-wled.c is not registering 
itself as a backlight device.

It only registers as a led device.

In that case, we cannot invoke of_find_backlight_by_node to get a handle 
on it.

One question we have is , is it required that every WLED driver register 
itself as a backlight device too?

In this case since it is not doing so, but we need to trigger it for the 
backlight.

Is there any way we can do this?

OR shall we just abandon the backlight control out of this driver 
entirely.

Thanks

Abhinav

On 2018-04-18 21:00, Bjorn Andersson wrote:
> On Tue 17 Apr 17:04 PDT 2018, abhinavk at codeaurora.org wrote:
> 
>> Hi Bjorn
>> 
>> Apologies if the prev reply wasnt clear.
>> 
>> Hope this one is.
>> 
> 
> Much better, now we can discuss the actual issues :)
> 
>> reply inline.
>> 
>> On 2018-04-17 14:29, Bjorn Andersson wrote:
>> > On Tue 17 Apr 11:21 PDT 2018, abhinavk at codeaurora.org wrote:
>> > > On 2018-04-16 23:13, Bjorn Andersson wrote:
>> > [..]
>> > > > If the panel isn't actually a piece of backlight hardware then it should
>> > > > not register a backlight device. Why do you need your own sysfs?
>> > > >
>> > > > Regards,
>> > > > Bjorn
>> > > [Abhinav] This particular panel isnt a piece of backlight hardware.
>> > > But, we want to have our own sysfs to give flexibility to our
>> > > userspace
>> > > to write and read stuff for its proprietary uses.
>> >
>> > Please be more specific in your replies, no one will accept code that
>> > "does stuff" and expecting a reviewer to spend time guessing or pulling
>> > the information out of you is a sure way to get your patches ignored.
>> >
>> > Exactly what kind of stuff are you talking about here and exactly which
>> > problem are you solving.
>> >
>> > > Thats how our downstream has been working so far and hence to maintain
>> > > the compatibility would like to have it.
>> >
>> > Make your proprietary code work with the upstream kernel and you
>> > shouldn't ever have to modify it.
>> >
>> > Regards,
>> > Bjorn
>> 
>> [Abhinav] We have a few userspace clients today for the backlight 
>> sysfs node
>> which read/write directly to
>> "/sys/class/backlight/panel0-backlight/brightness"
>> When I said "stuff" I was only referring to the brightness value.
>> This separate sysfs node allows us to validate those userspace
>> features of ours which directly edit the backlight value on our
>> reference platform.
> 
> That's nice, but you're enforcing that either all panel drivers must
> implement this backlight wrapper or that your customers must modify
> their user space to match their backlight implementation.
> 
>> Since our reference platform uses this panel,hence having our own
>> sysfs alias helps.  Otherwise, whenever we try to use this panel with
>> upstream code we will have to end up changing all those places in our
>> userspace/framework to change the sysfs path.
> 
> The actual problem comes down to "how does user space know the name of
> the backlight instance associated with the panel" and this is a valid
> question to pursue.
> 
> But given your current design you could just scan for the one and only
> backlight device available in the system; as your use of the static 
> name
> "panel0-backlight" doesn't allow multiple backlights anyway.
> 
> 
> If your goal is simply to ship your reference with something that you
> can show work, then just replace the hard coded panel0-backlight with
> the name of the wled backlight device. Customers can change panels as
> they wish, but in the event that they plug in a different backlight
> controller they would need to modify the code.
> 
>> Hence we wanted to preserve that sysfs node name.
>> The wled device is not created under /sys/class/backlight but under
>> /sys/class/leds/wled.
>> So we will have to change the name of this node across all our 
>> userspace.
>> 
> 
> Hard coding /sys/class/backlight/panel0-backlight in your user space
> instead of /sys/class/leds/wled is hardly a gain, in particular since
> the cost is 94 insertions - per panel driver.
> 
> Regards,
> Bjorn
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