[PATCH 0/2] Lima DRM driver

[Rob Herring]

On Thu, Feb 7, 2019 at 9:51 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Thu, Feb 07, 2019 at 11:21:52PM +0800, Qiang Yu wrote:
> > On Thu, Feb 7, 2019 at 5:09 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> > >
> > > On Wed, Feb 06, 2019 at 09:14:55PM +0800, Qiang Yu wrote:
> > > > Kernel DRM driver for ARM Mali 400/450 GPUs.
> > > >
> > > > Since last RFC, all feedback has been addressed. Most Mali DTS
> > > > changes are already upstreamed by SoC maintainers. The kernel
> > > > driver and user-kernel interface are quite stable for several
> > > > months, so I think it's ready to be upstreamed.
> > > >
> > > > This implementation mainly take amdgpu DRM driver as reference.
> > > >
> > > > - Mali 4xx GPUs have two kinds of processors GP and PP. GP is for
> > > >   OpenGL vertex shader processing and PP is for fragment shader
> > > >   processing. Each processor has its own MMU so prcessors work in
> > > >   virtual address space.
> > > > - There's only one GP but multiple PP (max 4 for mali 400 and 8
> > > >   for mali 450) in the same mali 4xx GPU. All PPs are grouped
> > > >   togather to handle a single fragment shader task divided by
> > > >   FB output tiled pixels. Mali 400 user space driver is
> > > >   responsible for assign target tiled pixels to each PP, but mali
> > > >   450 has a HW module called DLBU to dynamically balance each
> > > >   PP's load.
> > > > - User space driver allocate buffer object and map into GPU
> > > >   virtual address space, upload command stream and draw data with
> > > >   CPU mmap of the buffer object, then submit task to GP/PP with
> > > >   a register frame indicating where is the command stream and misc
> > > >   settings.
> > > > - There's no command stream validation/relocation due to each user
> > > >   process has its own GPU virtual address space. GP/PP's MMU switch
> > > >   virtual address space before running two tasks from different
> > > >   user process. Error or evil user space code just get MMU fault
> > > >   or GP/PP error IRQ, then the HW/SW will be recovered.
> > > > - Use TTM as MM. TTM_PL_TT type memory is used as the content of
> > > >   lima buffer object which is allocated from TTM page pool. all
> > > >   lima buffer object gets pinned with TTM_PL_FLAG_NO_EVICT when
> > > >   allocation, so there's no buffer eviction and swap for now.
> > >
> > > All other render gpu drivers that have unified memory (aka is on the SoC)
> > > use GEM directly, with some of the helpers we have. So msm, etnaviv, vc4
> > > (and i915 is kinda the same too really). TTM makes sense if you have some
> > > discrete memory to manage, but imo not in any other place really.
> > >
> > > What's the design choice behind this?
> > To be honest, it's just because TTM offers more helpers. I did implement
> > a GEM way with cma alloc at the beginning. But when implement paged mem,
> > I found TTM has mem pool alloc, sync and mmap related helpers which covers
> > much of my existing code. It's totally possible with GEM, but not as easy as
> > TTM to me. And virtio-gpu seems an example to use TTM without discrete
> > mem. Shouldn't TTM a super set of both unified mem and discrete mem?
>
> virtio does have fake vram and migration afaiui. And sure, you can use TTM
> without the vram migration, it's just that most of the complexity of TTM
> is due to buffer placement and migration and all that stuff. If you never
> need to move buffers, then you don't need that ever.
>
> Wrt lack of helpers, what exactly are you looking for? A big part of these
> for TTM is that TTM is a bid a midlayer, so reinvents a bunch of things
> provided by e.g. dma-api. It's cleaner to use the dma-api directly. Basing
> the lima kernel driver on vc4, freedreno or etnaviv (last one is probably
> closest, since it doesn't have a display block either) would be better I
> think.

FWIW, I'm working on the panfrost driver and am using the shmem
helpers from Noralf. It's the early stages though. I started a patch
for etnaviv to use it too, but found I need to rework it to sub-class
the shmem GEM object.

Rob