[PATCH 1/2] drm/amdgpu: Enable scatter gather display support

[Li, Samuel]

Christian,

You misunderstood Alex’s comments,


>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

Here Alex mentioned the scenarios is 1)VRAM, 2)GTT, 3)VRAM/GTT.
His concern is this might cause ping-pong, not about preferred domain. Since preferred domain can solve the ping-pong issue, it shall address his concern here.

Regards,
Samuel Li

From: Christian König [mailto:ckoenig.leichtzumerken at gmail.com]
Sent: Monday, March 19, 2018 3:45 PM
To: Li, Samuel <Samuel.Li at amd.com>; Marek Olšák <maraeo at gmail.com>; Koenig, Christian <Christian.Koenig at amd.com>
Cc: Alex Deucher <alexdeucher at gmail.com>; Michel Dänzer <michel at daenzer.net>; amd-gfx list <amd-gfx at lists.freedesktop.org>
Subject: Re: [PATCH 1/2] drm/amdgpu: Enable scatter gather display support

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:maraeo at gmail.com]
Sent: Monday, March 19, 2018 3:39 PM
To: Koenig, Christian <Christian.Koenig at amd.com><mailto:Christian.Koenig at amd.com>
Cc: Li, Samuel <Samuel.Li at amd.com><mailto:Samuel.Li at amd.com>; Michel Dänzer <michel at daenzer.net><mailto:michel at daenzer.net>; Alex Deucher <alexdeucher at gmail.com><mailto:alexdeucher at gmail.com>; amd-gfx list <amd-gfx at lists.freedesktop.org><mailto:amd-gfx at lists.freedesktop.org>
Subject: Re: [PATCH 1/2] drm/amdgpu: Enable scatter gather display support

On Mon, Mar 19, 2018 at 3:27 PM, Christian König <christian.koenig at amd.com<mailto:christian.koenig at amd.com>> 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 <michel at daenzer.net<mailto:michel at daenzer.net>>; Li, Samuel
<Samuel.Li at amd.com<mailto:Samuel.Li at amd.com>>; Alex Deucher <alexdeucher at gmail.com<mailto:alexdeucher at gmail.com>>
Cc: amd-gfx list <amd-gfx at lists.freedesktop.org<mailto:amd-gfx at lists.freedesktop.org>>
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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