[PATCH 1/2] drm/amdgpu: Enable scatter gather display support
Christian König
ckoenig.leichtzumerken at gmail.com
Mon Mar 19 19:44:49 UTC 2018
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>
> *Cc:* Li, Samuel <Samuel.Li at amd.com>; Michel Dänzer
> <michel at daenzer.net>; Alex Deucher <alexdeucher at gmail.com>; amd-gfx
> list <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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