drm fb helpers hotplug/resize

[Zack Rusin]

On Thu, 2022-10-06 at 10:01 +0200, Thomas Zimmermann wrote:
> 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.

That sounds good. In a similar fashion to drm_fbdev_overalloc another, rather hacky
but vastly simpler approach, would be to basically allow the drivers to specify the
maximum size of fb to support in drm_fbdev_generic_setup. This would just directly
set the drm_fb_helper_surface_size::surface_width and surface_height with the end
result being that drm_client_framebuffer_create would be called with those values
and xres_virtual/yres_virtual would be set to them. Resizing would basically just
work then, right? Of course at the cost of possibly large allocation, e.g. 4k fb
even when only 800x600 is actually used.

Either way I'll send out the patch that ports vmwgfx to drm_fb_helpers because even
without resizing support, it removes ~1000loc from vmwgfx and I think is well worth
it and we can figure out how to handle drm_fb_helpers later.

z