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

[Tony Lindgren]

* Matthijs van Duin <matthijsvanduin at gmail.com> [200104 04:53]:
> On Fri, Dec 20, 2019 at 04:57:11PM -0800, Tony Lindgren wrote:
> > On my droid4 I noticed bad constant tearing on the LCD with stellarium in
> > landscape mode with xorg-video-omap rotated with xrandr --rotate right.
> > Every second or so update gets squeezed down in size to only the top half
> > of the LCD panel.
> 
> Odd, there's not really a good reason for rotation to have any effect on
> whether tearing happens or not.
> 
> BTW, with "top half", I assume you mean the top of the screen (i.e.
> right side of the display), not the top of the display (i.e. left side
> of the screen) ?

Correct right side of the display that appears on top after rotate right.

> > This issue does not happen with xrandr --rotate normal, or when HDMI
> > display is also connected.
> 
> Ehhhh, mirroring onto HDMI fixes the problem?  Strange

Yup just connecting an additional HDMI panel fixes the issue on the LCD :)

> > Looking around what might affect BO_TILED, I noticed Matthijs had this
> > change in his earlier pyra tiler patches. The earlier patch "XXX omapdrm:
> > force tiled buffers to be pinned and page-aligned" has no commit log
> > though, so I'm not sure what other issues this might fix.
> 
> This is just part of a hacky patch series to improve performance for
> userspace access to tiled buffers.  Page alignment has no effect by
> itself, but it's necessary to allow the tiled memory allocated by
> tiled_reserve_2d() to be mapped directly into userspace instead of using
> the really slow "usergart" mechanism.

OK

> You can find the full patch series in github.com/mvduin/linux branch
> 4.15/patch/tiler-fast (based on mainline 4.15-rc6):
> 
> ae664249050b ARM: introduce pgprot_device()
> fc1e8ffd1334 drm: omapdrm: improve choice of memory type for tiled memory
>    these improve performance on omap5/dra7 by mapping tiled buffers as
>    "device" memory by default instead of the pointlessly slow "strongly
>    ordered" which is currently used as a workaround for the
>    incompatibility between TILER and the bizarre way the ARM Cortex-A15
>    implements loads from normal non-cacheable memory.
> 
> 3d4c98cc47dd XXX omapdrm: factor out _omap_gem_(un)pin
> 70593563f531 XXX omapdrm: force tiled buffers to be pinned and page-aligned
> e061e454afd5 XXX omapdrm: fast userspace mapping of tiled buffers
>    these greatly improve performance of userspace access to tiled
>    buffers (on all devices that use tiler) at the expense of using more
>    tiler virtual memory.  note that tiler virtual memory is a less
>    scarce resource on omap5/dra7 where 2d and 1d mappings have separate
>    page tables than on omap4 where they share a page table.
> 
> None of this should have any impact on tearing.

OK so the alignment change just happens to fix the issue then.

Regards,

Tony