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

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Rob Herring <robh@kernel.org>, Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>, Sam Ravnborg <sam@ravnborg.org>,
	Emil Velikov <emil.l.velikov@gmail.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Gerd Hoffmann <kraxel@redhat.com>, Sean Paul <sean@poorly.run>
Subject: Re: [PATCH 1/2] drm/shmem: Use cached mappings by default
Date: Fri, 15 May 2020 08:58:02 +0200	[thread overview]
Message-ID: <6cec8a8a-d642-cd93-906a-eafab49a12d7@suse.de> (raw)
In-Reply-To: <CAL_JsqK5J78PS_miF8QmfcLJOiESV-f=hXB_CanFqK9yBgJ=6Q@mail.gmail.com>

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Hi

Am 14.05.20 um 22:36 schrieb Rob Herring:
> On Thu, May 14, 2020 at 7:40 AM Daniel Vetter <daniel@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

> 
> Rob
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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