[PATCH 1/2] drm/shmem: Use cached mappings by default

[Daniel Vetter]

On Fri, May 15, 2020 at 08:58:02AM +0200, Thomas Zimmermann wrote:
> Hi
> 
> Am 14.05.20 um 22:36 schrieb Rob Herring:
> > On Thu, May 14, 2020 at 7:40 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> >>
> >> On Wed, May 13, 2020 at 05:03:11PM +0200, Thomas Zimmermann wrote:
> >>> SHMEM-buffer backing storage is allocated from system memory; which is
> >>> typically cachable. Currently, only virtio uses cachable mappings; udl
> >>> uses its own vmap/mmap implementation for cachable mappings. Other
> >>> drivers default to writecombine mappings.
> >>
> >> I'm pretty sure this breaks all these drivers. quick grep on a few
> >> functions says this is used by lima, panfrost, v3d. And they definitely
> >> need uncached/wc stuff afaiui. Or I'm completely missing something?
> > 
> 
> OK. I think I'll just make a patch that adds a .gem_create_object helper
> that sets the map_cached flag. So drivers can opt-in.
> 
> > Yes, that would be my guess. DMA is usually non-coherent on Arm.
> 
> Can one of you give me some pointer to what you're looking at? For
> example, I grepped for use of vmap within lima and only found [1].
> That's a for memcpy with no DMA involved. There's got to be something
> I'm missing.
> 
> Best regards
> Thomas
> 
> [1]
> https://cgit.freedesktop.org/drm/drm-tip/tree/drivers/gpu/drm/lima/lima_sched.c#n391

$ git grep drm_gem_shmem_mmap

We also need correct access from userspace, otherwise the gpu is going to
be sad.
-Daniel
> 
> > 
> > Rob
> > 
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
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> 




-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch