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

[Christian König]

Am 19.03.2018 um 20:39 schrieb Marek Olšák:
> 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.

Yeah, but it can set the initial domain based on what it knows how the 
buffer will be used.

E.g. when scanout from GTT is supported we would like to always set the 
initial domain as GTT instead of VRAM.

Christian.

>
> 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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