[Nouveau] [PATCH 4/4] drm/nouveau: introduce CPU cache flushing macro
Roy Spliet
seven at nimrod-online.com
Thu Jun 12 11:15:21 PDT 2014
Perhaps a bit late, but here's some random pointers I recall from my
previous experience: on ARM, caching policy of different mappings of the
same physical memory object must always correspond perfectly. If the
userspace mapping is uncached, so should the kernel mapping be. If the
memory is allocated using get_(zeroed_)page(), by default the caching
policy for the pointer returned is WRITE_ALLOC on SMP systems.
One useful tool for debugging cache problems is the pagewalk api's
(mm/pagewalk.c), which will help you find the raw page table entry (and
thus it's caching properties) for a virtual memory address.
Roy
op 12-06-14 15:50, Alexandre Courbot schreef:
> On Mon, Jun 9, 2014 at 7:41 PM, Alexandre Courbot <gnurou at gmail.com> wrote:
>> On Mon, May 19, 2014 at 6:22 PM, Lucas Stach <l.stach at pengutronix.de> wrote:
>>> Am Montag, den 19.05.2014, 11:02 +0200 schrieb Thierry Reding:
>>>> On Mon, May 19, 2014 at 04:10:58PM +0900, Alexandre Courbot wrote:
>>>>> Some architectures (e.g. ARM) need the CPU buffers to be explicitely
>>>>> flushed for a memory write to take effect. Not doing so results in
>>>>> synchronization issues, especially after writing to BOs.
>>>> It seems to me that the above is generally true for all architectures,
>>>> not just ARM.
>>>>
>>> No, on PCI coherent arches, like x86 and some PowerPCs, the GPU will
>>> snoop the CPU caches and therefore an explicit cache flush is not
>>> required.
>>>
>>>> Also: s/explicitely/explicitly/
>>>>
>>>>> This patch introduces a macro that flushes the caches on ARM and
>>>>> translates to a no-op on other architectures, and uses it when
>>>>> writing to in-memory BOs. It will also be useful for implementations of
>>>>> instmem that access shared memory directly instead of going through
>>>>> PRAMIN.
>>>> Presumably instmem can access shared memory on all architectures, so
>>>> this doesn't seem like a property of the architecture but rather of the
>>>> memory pool backing the instmem.
>>>>
>>>> In that case I wonder if this shouldn't be moved into an operation that
>>>> is implemented by the backing memory pool and be a noop where the cache
>>>> doesn't need explicit flushing.
>>>>
>>>>> diff --git a/drivers/gpu/drm/nouveau/core/os.h b/drivers/gpu/drm/nouveau/core/os.h
>>>>> index d0ced94ca54c..274b4460bb03 100644
>>>>> --- a/drivers/gpu/drm/nouveau/core/os.h
>>>>> +++ b/drivers/gpu/drm/nouveau/core/os.h
>>>>> @@ -38,4 +38,21 @@
>>>>> #endif /* def __BIG_ENDIAN else */
>>>>> #endif /* !ioread32_native */
>>>>>
>>>>> +#if defined(__arm__)
>>>>> +
>>>>> +#define nv_cpu_cache_flush_area(va, size) \
>>>>> +do { \
>>>>> + phys_addr_t pa = virt_to_phys(va); \
>>>>> + __cpuc_flush_dcache_area(va, size); \
>>>>> + outer_flush_range(pa, pa + size); \
>>>>> +} while (0)
>>>> Couldn't this be a static inline function?
>>>>
>>>>> diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c
>>>> [...]
>>>>> index 0886f47e5244..b9c9729c5733 100644
>>>>> --- a/drivers/gpu/drm/nouveau/nouveau_bo.c
>>>>> +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c
>>>>> @@ -437,8 +437,10 @@ nouveau_bo_wr16(struct nouveau_bo *nvbo, unsigned index, u16 val)
>>>>> mem = &mem[index];
>>>>> if (is_iomem)
>>>>> iowrite16_native(val, (void __force __iomem *)mem);
>>>>> - else
>>>>> + else {
>>>>> *mem = val;
>>>>> + nv_cpu_cache_flush_area(mem, 2);
>>>>> + }
>>>>> }
>>>>>
>>>>> u32
>>>>> @@ -461,8 +463,10 @@ nouveau_bo_wr32(struct nouveau_bo *nvbo, unsigned index, u32 val)
>>>>> mem = &mem[index];
>>>>> if (is_iomem)
>>>>> iowrite32_native(val, (void __force __iomem *)mem);
>>>>> - else
>>>>> + else {
>>>>> *mem = val;
>>>>> + nv_cpu_cache_flush_area(mem, 4);
>>>>> + }
>>>> This looks rather like a sledgehammer to me. Effectively this turns nvbo
>>>> into an uncached buffer. With additional overhead of constantly flushing
>>>> caches. Wouldn't it make more sense to locate the places where these are
>>>> called and flush the cache after all the writes have completed?
>>>>
>>> I don't think the explicit flushing for those things makes sense. I
>>> think it is a lot more effective to just map the BOs write-combined on
>>> PCI non-coherent arches. This way any writes will be buffered. Reads
>>> will be slow, but I don't think nouveau is reading back a lot from those
>>> buffers.
>>> Using the write-combining buffer doesn't need any additional
>>> synchronization as it will get flushed on pushbuf kickoff anyways.
>> I tried to go that way, and something interesting happened.
>>
>> What I did: remove this patch and instead set the following caching
>> parameters for the TTM_PL_TT case in nouveau_bo_init_mem_type():
>>
>> man->available_caching = TTM_PL_FLAG_UNCACHED | TTM_PL_FLAG_WC;
>> man->default_caching = TTM_PL_FLAG_WC;
>>
>> What happened: no runtime errors as what happened when caching is
>> enabled. However, many of the vertex and texture buffers seem to be
>> partially corrupted. In glmark2 the 3d models had many vertices (but
>> not all) at the wrong position. Note that not all the scenes ended up
>> being corrupted - in particular, when two consecutive scenes used the
>> same model, the second instance would be uncorrupted.
>>
>> Forcing the caching to TTM_PL_FLAG_UNCACHED led to the same result.
>> What is interesting is that while data like vertices and textures got
>> corrupted, pushbuffers and shader programs seem to be just fine, as I
>> could not see any runtime error.
> An interesting fact: if I change ttm_bo_kmap_ttm() such as kernel
> mappings of BOs are always performed write-combined, and leave the
> TTM_PL_TT default caching to TTM_PL_FLAG_CACHED so user-space mappings
> remain cached, the corruptions just vanish. It seems to be the fact of
> setting user-space mappings to anything non-cached that leads to this
> puzzling behavior. Certainly some subtlety of ARM mappings are getting
> over my head here.
>
> If we need to implement different policies for kernel and user-space
> mappings, this might complicate things a bit, especially since support
> needs to be in TTM and not only Nouveau. I will submit a RFC tomorrow
> if I don't hear better ideas by then.
>
> Alex.
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