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

[Bjorn Andersson]

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