[Nouveau] [RFC] drm/nouveau: disable caching for VRAM BOs on ARM

[Lucas Stach]

Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström:
> On 23.05.2014 17:40, Alex Courbot wrote:
> > On 05/23/2014 06:59 PM, Lucas Stach wrote:
> > So after checking with more knowledgeable people, it turns out this is 
> > the expected behavior on ARM and BAR regions should be mapped uncached 
> > on GK20A. All the more reasons to avoid using the BAR at all.
> 
> This is actually specific to Tegra.
> 
> >> You may want to make yourself aware of all the quirks required for
> >> sharing memory between the GPU and CPU on an ARM host. I think there are
> >> far more involved than what you see now and writing an replacement for
> >> TTM will not be an easy task.
> >>
> >> Doing away with the concept of two memory areas will not get you to a
> >> single unified address space. You would have to deal with things like
> >> not being able to change the caching state of pages in the systems
> >> lowmem yourself. You will still have to deal with remapping pages that
> >> aren't currently visible to the CPU (ok this is not an issue on Jetson
> >> right now as it only has 2GB of RAM), because it's in systems highmem,
> >> or even in a different LPAE area.
> >>
> >> You really want to be sure you are aware of all the consequences of
> >> this, before considering this task.
> > 
> > Yep, that's why I am seeking advice here. My first hope is that with a 
> > few tweaks we will be able to keep using TTM and the current nouveau_bo 
> > implementation. But unless I missed something this is not going to be easy.
> > 
> > We can also use something like the patch I originally sent to make it 
> > work, although not with good performance, on GK20A. Not very graceful, 
> > but it will allow applications to run.
> > 
> > In the long run though, we will want to achieve better performance, and 
> > it seems like a BO implementation targeted at UMA devices would also be 
> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm 
> > interested in gathering thoughts and why not giving it a first try with 
> > GK20A, even if it imposes some limitations like having buffers in lowmem 
> > in a first time (we can probably live with this one for a short while, 
> > and 64 bits will also be coming to the rescue :))
> 
> I don't think lowmem or LPAE is any problem, if the memory manager is
> designed with that in mind. Vast majority of the buffers kernel
> allocates do not need to be touched in kernel space.
> 
> Actually I can't think of any buffers that we allocate on behalf of user
> space that would need to be permanently mapped also to kernel. In case
> or relocs only push buffer needs to be temporarily mapped to kernel.
> 
> Ultimately even relocs are not necessary if we expose GPU virtual
> addresses directly to user space. But that's another topic.
> 
Nouveau already exposes constant virtual addresses to userspace and
skips the pushbuf patching when the presumed offset from userspace is
the same as what the kernel thinks it should be.

The problem with lowmem on ARM is that you can't unmap those pages from
the kernel cached mapping. So if you alloc a page, give it to userspace
and userspace decides to map the page WC you just produced a conflicting
mapping, which may yield undefined results on ARMv7. You may think this
is not a problem as you are not touching the kernel cached mapping, but
in fact it is. The CPUs prefetcher can still access this mapping.

Although it won't wander over a page boundary for automatic prefetching
it may still do this when explicitly instructed to do so by code, which
may happen for example if your page happens to be near a kernel list,
where the list traversal code includes explicit prefetch instructions.

Regards,
Lucas

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