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

* Tony Lindgren [off-list ref] [240226 13:26]:

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* Thomas Zimmermann [off-list ref] [240226 09:10]:

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Am 26.02.24 um 10:01 schrieb Tomi Valkeinen:

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On 26/02/2024 10:26, Tomi Valkeinen wrote:

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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.

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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));