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

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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, Alexâ??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.
>
>
>     _______________________________________________
>     amd-gfx mailing list
>     amd-gfx at lists.freedesktop.org <mailto:amd-gfx at lists.freedesktop.org>
>     https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>
>
>
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