[PATCH] drm/pl111: Use max memory bandwidth for resolution

[Linus Walleij]

On Thu, Jan 25, 2018 at 4:46 AM, Eric Anholt <eric at anholt.net> wrote:

>> +     pl111_choose_max_resolution(dev, priv->memory_bw,
>> +                                 &mode_config->max_width,
>> +                                 &mode_config->max_height, &bpp);
>> +     dev_info(dev->dev, "cap resolution at %u x %u, %u BPP\n",
>> +              mode_config->max_width, mode_config->max_height, bpp);
>
> I think this is the wrong place in the pipeline to be doing this, but I
> don't have a complete solution so I'm not necessarily saying no.

So currently the driver does this:

mode_config->max_width = 1024;
mode_config->max_height = 768;

And that is because it cannot really handle anything. I guess ideally
the DRM driver should set these to -1 or something so that any widths
and heights negotiated will work.

>  Things I think we should do for bandwidth limits:
>
> A new pl111_mode_valid() rejects modes with width*height*2 > bandwidth
> (if we can't scan it out with our smallest format, don't advertise it).
>
> pl111_display_check() rejects modes with width*height*bpp > bandwidth
> (if we can't scan out this particular configuration, let them know we
> can't set the mode).
>
> Ideally given those two things, fbdev and X11 would notice that the
> preferred mode fails at 24bpp and fall back to 16bpp.  I don't think
> either of those does so today, though.
>
> Interested in tackling any of these?

I tried the pl111_display_check() version. It just made the driver
fail to initialize anything, at least when using the dumb VGA
bridge.

There are .mode_valid() callbacks on the bridges we use
(panel and dumb VGA) but neither uses it at the moment, hm.
I could just assign my own .mode_valid() callback to the bridge,
but it seems a bit fragile. But it's worth a hack, I'll try it.

I sent a sent of lesser controversial patches in the meantime,
and rebased this on top of those so we can deal with the
memory BW issue separately.

Yours,
Linus Walleij