Quoting Alex:
Regardless of which scenarios we need to support, I think we also need
to really plumb this through to mesa however since user space is who
ultimately requests the location. Overriding it in the kernel gets
tricky and can lead to ping-ponging as others have noted. Better to
have user space know what chips support it or not and request display
buffers in GTT or VRAM from the start.
And I completely agree with Alex here. So overriding the domain in the
kernel is a serious NAK from my side as well.
Please implement the necessary bits in Mesa, shouldn't be more than a
few lines of code anyway.
Regards,
Christian.
Am 19.03.2018 um 20:42 schrieb Li, Samuel:
Agreed.
>I think that the consensus with Alex and me is that we should avoid
exactly that.
Christian, Alex’s concern is about ping-pong, not about the preferred
domain.
Regards,
Samuel Li
*From:*Marek Olšák [mailto:[email protected]]
*Sent:* Monday, March 19, 2018 3:39 PM
*To:* Koenig, Christian <[email protected]>
*Cc:* Li, Samuel <[email protected]>; Michel Dänzer
<[email protected]>; Alex Deucher <[email protected]>; amd-gfx
list <[email protected]>
*Subject:* Re: [PATCH 1/2] drm/amdgpu: Enable scatter gather display
support
On Mon, Mar 19, 2018 at 3:27 PM, Christian König
<[email protected] <mailto:[email protected]>> wrote:
I think that the consensus with Alex and me is that we should
avoid exactly that.
Overriding the preferred domain in the kernel is a no-go for that
patch set, so please implement the discussed changes in Mesa.
I don't see how Mesa can make a smarter decision than the kernel. If
you overwrite the preferred domain of the buffer in the kernel, there
will be no ping-ponging between domains. Mesa never changes the
initial preferred domain.
Marek
Regards,
Christian.
Am 19.03.2018 um 20:22 schrieb Li, Samuel:
I agree with Marek/Michel: since kernel sets the domain before
scanning out, it shall update the preferred domain here too.
Regards,
Samuel Li
-----Original Message-----
From: Koenig, Christian
Sent: Thursday, March 08, 2018 4:07 AM
To: Michel Dänzer <[email protected]
<mailto:[email protected]>>; Li, Samuel
<[email protected] <mailto:[email protected]>>; Alex
Deucher <[email protected] <mailto:[email protected]>>
Cc: amd-gfx list <[email protected]
<mailto:[email protected]>>
Subject: Re: [PATCH 1/2] drm/amdgpu: Enable scatter gather
display support
Am 08.03.2018 um 09:35 schrieb Michel Dänzer:
On 2018-03-07 10:47 AM, Christian König wrote:
Am 07.03.2018 um 09:42 schrieb Michel Dänzer:
On 2018-03-06 07:23 PM, Christian König wrote:
E.g. the last time I tested it placing
things into GTT still
resulted in quite a performance penalty
for rendering.
FWIW, I think the penalty is most likely IOMMU
related. Last time I
tested, I couldn't measure a big difference
with IOMMU disabled.
No, the penalty I'm talking about came from the
ping/pong we did with
the scanout buffers.
See when I tested this the DDX and Mesa where
unmodified, so both
still assumed VRAM as placement for scanout BOs,
but the kernel
forced scanout BOs into GTT for testing.
So what happened was that on scanout we moved the
VRAM BO to GTT
and
after unpinning it on the first command submission
which used the BO
we moved it back to VRAM again.
In the meantime, I've had the same idea as Marek:
Can't the kernel
driver simply change the BO's preferred domain to GTT
when scanning
out from it? Then it won't move back to VRAM.
Yes, I've considered this as well.
But I think making the decision in Mesa is the cleaner
approach.
E.g. so far we only override the placement decision of
userspace for two
reasons:
1. We where running out of memory in VRAM.
2. We have a hardware restriction which makes VRAM usage
mandatory.
And even then we never adjust the placement permanently,
we just
temporary moved the buffer where it was needed and moved
it back after
the operation completed.
Additional to that Mesa might want to set even more flags
and/or changes
it's behavior. E.g. use a tilling mode which both importer
and export in an
A+A laptop understands etc...
Regards,
Christian.
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