Re: [PATCH v2 0/2] Fixes for omapdrm console

[Tony Lindgren <tony@atomide.com>]

* Tony Lindgren <tony@atomide.com> [240226 13:26]:
> * Thomas Zimmermann <tzimmermann@suse.de> [240226 09:10]:
> > Am 26.02.24 um 10:01 schrieb Tomi Valkeinen:
> > > On 26/02/2024 10:26, Tomi Valkeinen wrote:
> > > > How is it broken? I don't usually use the console (or fbdev) but
> > > > enabling it now, it seems to work fine for me, on DRA76 EVM with
> > > > HDMI output.
> > 
> > Omapdrm implements drm_framebuffer_funcs.dirty withomap_framebuffer_dirty().
> > AFAIK DRM semantics requires to run the dirty helper after writing to the
> > framebuffer's memory. Userspace does this via the DIRTYFB ioctl. [1] But (at
> > least) for correctness the console needs to do the same.
> > 
> > [1] https://elixir.bootlin.com/linux/v6.7.6/source/drivers/gpu/drm/drm_ioctl.c#L679
> 
> Yes I noticed console not updating and bisected it down to the two
> commits listed. I did the bisect on a droid4 though with command mode
> LCD. I did not test with HDMI, will give that a try too.

I can reproduce the cache issue with Tomi's omapfb-tests [2] below:

while true;
      do dd if=/dev/urandom of=/dev/fb0
      ~/src/omapfb-tests/test
      sleep 1
done

That produces short random data stripes on the test image.

> > > After applying your patches, I see a lot of cache-related artifacts on
> > > the screen when updating the fb.
> > 
> > I guess we might need a dma-specific mmap helper to make this work
> > correctly.

Comparing the difference between drm_gem_mmap_obj() and
fb_deferred_io_mmap(), the following test patch makes the cache issue
go away for me. Not sure if this can be set based on some flag, or if
we need a separate fb_deferred_io_wc_mmap() or something like that?

Regards,

Tony

[2] https://github.com/tomba/omapfb-tests

8< --------------------
diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -224,6 +224,7 @@ static const struct address_space_operations fb_deferred_io_aops = {
 int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
+	vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
 
 	vma->vm_ops = &fb_deferred_io_vm_ops;
 	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP);