Re: [PATCH] drm: return false in drm_arch_can_wc_memory() for ARM

[Eric Anholt <eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>]

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Alex Deucher <alexdeucher-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:

> On Fri, Dec 21, 2018 at 9:16 AM Liviu Dudau <Liviu.Dudau-5wv7dgnIgG8@public.gmane.org> wrote:
>>
>> On Thu, Dec 20, 2018 at 04:36:19PM +0100, Daniel Vetter wrote:
>> > On Thu, Dec 20, 2018 at 09:56:57AM -0500, Alex Deucher wrote:
>> > > I'm not familiar enough with ARM to know if write combining
>> > > is actually an architectural limitation or if it's an issue
>> > > with the PCIe IPs used on various platforms, but so far
>> > > everyone that has tried to run radeon hardware on
>> > > ARM has had to disable it.  So let's just make it official.
>> >
>> > wc on arm is Really Complicated (tm) afaiui. There's issues with aliasing
>> > mappings and stuff, so you need to allocate your wc memory from special
>> > pools. So probably best to just disable it until we figure this out.
>>
>> I believe both of you are conflating different issues under the wrong
>> name. Write combining happens all the time with Arm, the ARMv8
>> architecture is a weakly-ordered model of memory so hardware is allowed
>> to re-order or combine memory access as they seem fit.
>>
>> A while ago I did run an AMD GPU card on my Juno dev board and it worked
>> (for a very limited definition of worked, I've only validated the fact
>> that I could get an fbcon and could run un-accelerated X11). So I would
>> be interested if Alex could share some of the scenarios where people are
>> seeing failures.
>
> Here's an example:
> https://bugs.freedesktop.org/show_bug.cgi?id=108625
> But there are probably 5 or 6 other cases where people have emailed me
> or our team directly with issues on ARM resolved by disabling WC.
> Generally the driver seems to load ok, but then hangs as soon as you
> try and use acceleration from userspace or we end up with page
> flipping timeouts.  Not really sure what the issue is.  Michel
> suggested maybe ARM has a cacheable kernel mapping of all "normal"
> system memory, and having
> both that mapping and another non-cacheable mapping of the same page
> can result in bad behaviour.
>
>>
>> As for aliasing, yeah, having multiple aliases to the same piece of
>> memory is a bad thing. The problem arises when devices on the PCI bus
>> have memory allocated as device memory (which on Arm is non-cacheable
>> and non-reorderable), but the PCI bus effectively acts as a write-combiner
>> which changes the order of transactions. Therefore, for devices that
>> have local memory associated with them (i.e. more than just register
>> accesses) one should allocate memory in the first place that is
>> Device-GRE (gathering, reordering and early-access). Otherwise, problems
>> will surface that are not visible on x86 as that is a strongly ordered
>> architecture.
>
> PCI framebuffer BARs are mapped on the CPU with WC.  We also use
> uncached WC mappings for system memory in cases where it's not likely
> we will be doing any CPU reads.  When accessing system memory, the GPU
> can either do a CPU cache snooped transaction or a non-snooped
> transaction.  The non-snooped transaction has lower latency and better
> throughput since it doesn't have to snoop the CPU cache.
>
>>
>> >
>> > > Signed-off-by: Alex Deucher <alexander.deucher-5C7GfCeVMHo@public.gmane.org>
>> >
>> > Reviewed-by: Daniel Vetter <daniel.vetter-/w4YWyX8dFk@public.gmane.org>
>>
>> Given that this API is only used by AMD I'm OK for now with the change,
>> but I think in general it is misleading and we should work towards
>> fixing radeon and amd drivers.
>
> Alternatively, we could just disable WC in the amdgpu driver on ARM.
> I'm not sure to what extent other drivers are using WC in general or
> have been tested on ARM.

FWIW, I use WC mappings of BOs on V3D (shmem) and VC4 (cma).  V3D is
totally stable.  VC4 I've heard reports of stability issues long-term
but I don't think it's related.  I don't do any cached mappings of my
BOs, though.

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