[Letux-kernel] drm/omap: Remove panel-dpi driver

H. Nikolaus Schaller hns at goldelico.com
Thu Apr 11 08:57:10 CEST 2019


> Am 11.04.2019 um 08:32 schrieb Tomi Valkeinen <tomi.valkeinen at ti.com>:
> 
> On 11/04/2019 09:14, H. Nikolaus Schaller wrote:
> 
>>> You will need the panel data to be added to the simple panel driver, as
>>> it won't read the timings from the DT (unfortunately, in my opinion).
>> 
>> Hm. How does this work? Is there a description? I have no experience with
>> DRM simple panel.
> 
> See drivers/gpu/drm/panel/panel-simple.c, it just has a list of
> compatibles, and pointers to the (more or less) same data as is in the
> DT. And see a commit that adds a new panel, e.g. "drm/panel: simple: Add
> OSD070T1718-19TS panel support".

Ok, I see. So the task is converting the nice and tested DTS properties to
magic numbers in code... Let's hope without introducing new bugs.

> 
>> So with this DTB stability policy in mind it seems too early to remove the
>> panel-dpi driver unless there is a compatible solution which does not break
>> existing DTB.
> 
> Well, maybe just reverting "drm/omap: Remove panel-dpi driver" would be
> enough. I didn't try, but I think all the plumbing is still there to
> keep the legacy omapdrm panels working.
> 
> If I'm not mistaken, Laurent did try to get the simple-panel to get the
> timings from the DT, but it was rejected.

What has been the reason?

I can only imagine one: if several boards use the same panel it is easier
to define the timings once for all. And the only location where this can
happen is the (common) driver. Unless there will be some dts/panels/$panelname.dtsi
which every DTS can/must include.

> Perhaps backward-compatibility
> with out-of-tree dtbs would be a valid reason to get it accepted?
> Laurent, what do you think?

And a second argument may be: with all these tables in simple-panel we carry
along a big set (ca. 60?) of unused code and struct drm_display_mode tables.
With timings from DT we just have the timings a specific device needs.

My assumption would be that the DTB approach needs a lot less code/data
(unless someone adds a big set of CONFIG options to the simple panel driver).

> 
> That said, in my opinion, we should not care too much about out-of-tree
> stuff. It's a nightmare to support things that you're not even aware of.

I basically agree, but learned from the gpiolib/spi-cs-high discussion
that this is not the general view.

> In this particular case, adding the out-of-tree panels to simple-panel
> should be a very straightforward task, and doesn't need a change in the
> dtbs themselves.

Yes, but it is unnecessary work which distracts us from doing significant
things...

Best regards,
Nikolaus



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