Re: drm fb helpers hotplug/resize

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi Zack

Am 05.10.22 um 21:49 schrieb Zack Rusin:
> Hi, Thomas.
> 
> Because you've been the one who's been working on drm_fb_helper.c the most the last
> few years I wanted to pick your brain a bit.
> 
> I was porting vmwgfx to drm_fb_helper code which is largely trivial, just removing
> all of vmwgfx_fb.c and replacing it with a call to drm_fbdev_generic_setup. But

Thanks a lot for this work. I have been looking into doing this 
conversion myself at some point, but never found the time to actually do 
it.

> drm_fb_helper.c code never deals with resizes which is a bit of a problem.
> 
> e.g. replacing the drm_sysfs_hotplug_event() call from
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vmwgfx/vmwgfx_kms.c#L2255
> with drm_kms_helper_hotplug_event will call drm_fbdev_client_hotplug and end up in
> drm_fb_helper_hotplug_event:
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_fb_helper.c#L2003
> 
> Now drm_fb_helper_hotplug_event does drm_client_modeset_probe but it never resizes
> drm_fb_helper::buffer and drm_fb_helper::fb so they're both incorrectly sized.
> 
> In general I don't see drm_fb_helper code ever being able to deal with resizes. In
> particular because the fbdev's xres_virtual/yres_virtual are sized exactly to the
> initial xres/yres.
> 
> It's definitely a lot bigger issue on virtualized environments where at boot we'll
> have some very conservative size (800x600) on vmwgfx which is then usually resized
> to the size of the window. drm_fb_helper breaks pretty bad in that case because it
> can't deal with those resizes at all.
> 
> Is this scenario something that drm_fb_helper should be able to handle or is it not
> worth pursuing it? I don't think there's a trivial way of handling it so my guess is
> that it would make drm_fb_helper quite a bit more complicated.

I'm aware that resizing is missing. It's one of the few things I'd like 
to see being added to generic fbdev emulation. But as you say, it's not 
easy. The generic fbdev emulation has all kinds of code paths for the 
various drivers' memory managers. That makes it complicated.

The problem is that fbdev's mmap'ed memory cannot be reallocated. It is 
expected to behave like 'real video memory.' So either reserve a chunk 
of the video ram for fbdev's GEM objects or use deferred I/O, which 
provides mmaped pages from a shadow buffer in system memory. vmwgfx uses 
the latter IIRC.

But ideally, we'd get rid of most of the shadow buffering and try to 
mmap pages directly from GEM objects. For modesetting, this means that 
the new mode's framebuffer has to inherit the old framebuffer's buffer 
objects. Probably the easiest solution is to allocate a framebuffer once 
and reconfigure its parameters (width, height, pitch) on each modeset 
operation.

Switching to a higher resolution would require more video memory. 
Although we cannot reallocate, this problem can be solved with the 
drm_fbdev_overalloc parameter. It gives the percentage of allocated 
video memory. If you start with 800x600 with overalloc at 400, you'd get 
enough video memory for 2400 scanlines. This allows for fbdev panning 
(i.e., pageflipping). With that extra memory fbdev could switch to 
another display mode with a higher resolution. For example, changing to 
1024x786 would result in 1875 scanlines at the given overalloc of 400.

To implement this, I guess that some of fbdev's memory allocation needs 
to be changed. The check_var and set_par code needs an update to handle 
the modeset. And I suspect that there are other dark corners that need 
to be reworked as well.

Best regards
Thomas

> 
> z

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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