Re: [PATCH] drm/omap: gem: Fix tearing with BO_TILED

[Tony Lindgren <tony@atomide.com>]

* Matthijs van Duin <matthijsvanduin@gmail.com> [200106 10:07]:
> On Sun, Jan 05, 2020 at 12:37:04PM -0800, Tony Lindgren wrote:
> > 4. The issue I'm seeing with stellarium on droid4 may be a stride
> >    issue as about one out of 3 or 4 frames is OK and aligning to
> >    512 also fixes the issue maybe because it happens to make
> >    multiple frames align to 4096
> 
> Yeah if your buffers are 960 pixels wide (assuming the droid4's screen
> is natively portrait) and 32bpp then 512-byte alignment suffices to
> automatically make them 4KB alignment.

Hmm sounds like I need to retest this. But doesn't 512-byte alignment
only make the 960 pixels case 2KB aligned?

> The most obvious thing I can think of that could do wrong is that it
> might contiguously map the pages that cover each line, which is what
> will happen if they use e.g. for_each_sg_page, but subsequently assume
> that the stride in sgx virtual memory is ALIGN( width * cpp, PAGE_SIZE )
> without taking the offset of the buffer inside the mapping into account.

OK

> If each line is at most 4KB (i.e. 1024 pixels @ 32bpp) but each line
> straddles an MMU page boundary, then the result would be that the even
> lines of the frame are written to the top half of the buffer, causing it
> to be scaled to 50% vertically, while the odd lines are "lost" (written
> outside the buffer, either to a different buffer or unmapped tiler
> memory).  This sounds like what you described on irc?

Yes this sounds like what I've been seeing.

Regards,

Tony