Re: No more new fbdev drivers, please

[Rob Clark]

On Sat, Sep 26, 2015 at 2:46 PM, Geert Uytterhoeven
<geert@xxxxxxxxxxxxxx> wrote:
> Hi David,
>
> On Sat, Sep 26, 2015 at 8:13 PM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
>> On Sat, Sep 26, 2015 at 8:01 PM, Geert Uytterhoeven
>> <geert@xxxxxxxxxxxxxx> wrote:
>>> On Sat, Sep 26, 2015 at 7:07 PM, Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
>>>> On Sat, Sep 26, 2015 at 4:28 AM, Geert Uytterhoeven
>>>> <geert@xxxxxxxxxxxxxx> wrote:
>>>>> For the (mailing list) record, can you please provide some explicit pointers
>>>>> to these existing really simple drivers?
>>>>
>>>> See the tilcdc, ast, mgag200, and udl drivers for example.
>>>
>>> Thanks for the list!
>>>
>>> The smallest of these (udl) still counts in at ca. 2800 LoC, while there are
>>> several fbdev drivers that have less than 200 LoC.
>>> Granted, these really small ones support a single fixed video mode only, but
>>> you can write a simple fbdev driver with mode setting in less than 1000 LoC.
>>>
>>> I'm sure DRM can do better?
>>
>> Is counting lines really the level of the discussion to go here?
>
> LoC is not the most important. But if the smallest DRM driver needs an order
> of magnitude more LoC than the smallest fbdev driver, I start to wonder.

I think most of the drm/kms drivers are bigger due to more features..
iirc original tilcdc was ~2k loc (compared to ~1.6kloc for da8xx-fb),
but it already supported multiple modes, page flipping, vblank
notification, etc.  It has grown since then.  Although still probably
smaller than downstream da8xx-fb + tda998x hdmi bridge (and re-using
the same tda998x bridge code with several other drivers too, compared
to downstream solution for the same)..

Probably there is room for more helpers for even more restrictive hw.

BR,
-R

> E.g. if I want to write a new simple driver for my new shiny hardware, it
> can make a big difference if I have to write (and test/debug) 800 LoC, or
> 3000 LoC.
>
>> DRM is a big set of helpers, nothing else. If many trivial, small
>> drivers share common code, developers are more than welcome to
>> contribute them to drm-core and help making drivers less complex.
>
> Good. But from the figures above, I don't think we're at that point yet that
> writing a new DRM driver is less/equal amount of work than writing a new
> fbdev driver, at least for some classes of hardware. So it may be a bit
> premature to put a moratorium on new fbdev drivers.
> I may be mistaken, I'm still not sufficiently familiar with the DRM subsystem
> as I'd like to be.
>
>> As Daniel mentioned, the connector+encoder+crtc combination is one of
>> those simplifications that would make sense if more such drivers are
>> added. Furthermore, the not-yet-merged SimpleDRM driver is one example
>> how to implement multiple of those dumb-fb drivers with a shared
>> code-base.
>
> Thanks, looking forward to SimpleDRM!
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
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