Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Feb 03, 2015 at 12:28:14PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 03, 2015 at 08:48:56AM +0100, Daniel Vetter wrote:
> > On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote:
> > > On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> > > >> My initial thought is for dma-buf to not try to prevent something than
> > > >> an exporter can actually do.. I think the scenario you describe could
> > > >> be handled by two sg-lists, if the exporter was clever enough.
> > > >
> > > > That's already needed, each attachment has it's own sg-list. After all
> > > > there's no array of dma_addr_t in the sg tables, so you can't use one sg
> > > > for more than one mapping. And due to different iommu different devices
> > > > can easily end up with different addresses.
> > > 
> > > 
> > > Well, to be fair it may not be explicitly stated, but currently one
> > > should assume the dma_addr_t's in the dmabuf sglist are bogus.  With
> > > gpu's that implement per-process/context page tables, I'm not really
> > > sure that there is a sane way to actually do anything else..
> > 
> > Hm, what does per-process/context page tables have to do here? At least on
> > i915 we have a two levels of page tables:
> > - first level for vm/device isolation, used through dma api
> > - 2nd level for per-gpu-context isolation and context switching, handled
> >   internally.
> > 
> > Since atm the dma api doesn't have any context of contexts or different
> > pagetables, I don't see who you could use that at all.
> 
> What I've found with *my* etnaviv drm implementation (not Christian's - I
> found it impossible to work with Christian, especially with the endless
> "msm doesn't do it that way, so we shouldn't" responses and his attitude
> towards cherry-picking my development work [*]) is that it's much easier to
> keep the GPU MMU local to the GPU and under the control of the DRM MM code,
> rather than attaching the IOMMU to the DMA API and handling it that way.
> 
> There are several reasons for that:
> 
> 1. DRM has a better idea about when the memory needs to be mapped to the
>    GPU, and it can more effectively manage the GPU MMU.
> 
> 2. The GPU MMU may have TLBs which can only be flushed via a command in
>    the GPU command stream, so it's fundamentally necessary for the MMU to
>    be managed by the GPU driver so that it knows when (and how) to insert
>    the flushes.

3. Switching between different address spaces (for per gpu context
isolation) often requires intricate knowledge about the gpu and close
coordination. Well maybe just a part of 2 really, but an important one.

Fully agree and tbh I'm not sure whether the current push in arm to expose
all gpu mmus as iommus is solid. Even for pasid (per-context iommu tables)
which is a big official standard there's still a lot of open questions
about how to do it properly. And it requires strict hw support so that the
hw always knows which pasid it should use for a given dma access.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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