[PATCH RFC 102/111] staging: etnaviv: separate GPU pipes from execution state

[Lucas Stach]

Am Dienstag, den 07.04.2015, 16:51 +0200 schrieb Jon Nettleton:
> 
> 
> On Tue, Apr 7, 2015 at 4:38 PM, Alex Deucher <alexdeucher at gmail.com>
> wrote:
>         On Tue, Apr 7, 2015 at 3:46 AM, Lucas Stach
>         <l.stach at pengutronix.de> wrote:
>         > Am Sonntag, den 05.04.2015, 21:41 +0200 schrieb Christian
>         Gmeiner:
>         >> 2015-04-02 18:37 GMT+02:00 Russell King - ARM Linux
>         <linux at arm.linux.org.uk>:
>         >> > On Thu, Apr 02, 2015 at 05:30:44PM +0200, Lucas Stach
>         wrote:
>         >> >> While this isn't the case on i.MX6 a single GPU pipe can
>         have
>         >> >> multiple rendering backend states, which can be selected
>         by the
>         >> >> pipe switch command, so there is no strict mapping
>         between the
>         >> >> user "pipes" and the PIPE_2D/PIPE_3D execution states.
>         >> >
>         >> > This is good, because on Dove we have a single Vivante
>         core which
>         >> > supports both 2D and 3D together.  It's always bugged me
>         that
>         >> > etnadrm has not treated cores separately from their
>         capabilities.
>         >> >
>         >>
>         >> Today I finally got the idea how this multiple pipe stuff
>         should be
>         >> done the right way - thanks Russell.
>         >> So maybe you/we need to rework how the driver is designed
>         regarding
>         >> cores and pipes.
>         >>
>         >> On the imx6 we should get 3 device nodes each only
>         supporting one pipe
>         >> type. On the dove we
>         >> should get only one device node supporting 2 pipes types.
>         What do you think?
>         >>
>         > Sorry, but I strongly object against the idea of having
>         multiple DRM
>         > device nodes for the different pipes.
>         >
>         > If we need the GPU2D and GPU3D to work together (and I can
>         already see
>         > use-cases where we need to use the GPU2D in MESA to do
>         things the GPU3D
>         > is incapable of) we would then need a lot more DMA-BUFs to
>         get buffers
>         > across the devices. This is a waste of resources and
>         complicates things
>         > a lot as we would then have to deal with DMA-BUF fences just
>         to get the
>         > synchronization right, which is a no-brainer if we are on
>         the same DRM
>         > device.
>         >
>         > Also it does not allow us to make any simplifications to the
>         userspace
>         > API, so I can't really see any benefit.
>         >
>         > Also on Dove I think one would expect to get a single pipe
>         capable of
>         > executing in both 2D and 3D state. If userspace takes
>         advantage of that
>         > one could leave the sync between both engines to the FE,
>         which is a good
>         > thing as this allows the kernel to do less work. I don't see
>         why we
>         > should throw this away.
>         
>         Just about all modern GPUs support varying combinations of
>         independent
>         pipelines and we currently support this just fine via a single
>         device
>         node in other drm drivers.  E.g., modern radeons support one
>         or more
>         gfx, compute, dma, video decode and video encode engines.
>         What
>         combination is present depends on the asic.
>         
>         
> 
> 
> That reminds me.  We should also have in the back of our heads that
> compute is supported by the newer Vivante chips.  We will also need to
> support multiple independent 3d cores as that support has shown up in
> the V5 galcore drivers.
> 
AFAIK compute is just another state of the 3D pipe where instead of
issuing a draw command you would kick the thread walker.

Multicore with a single FE is just a single pipe with chip selects set
to the available backends and mirrored pagetables for the MMUs. With
more than one FE you get more than one pipe which is more like a SLI
setup on the desktop, where userspace has to deal with splitting the
render targets into portions for each GPU.
One more reason to keep things in one DRM device, as I think no one
wants to deal with syncing pagetables across different devices.

Regards,
Lucas

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