[PATCH v12 1/6] drm: Add arch arm64 for drm_clflush_virt_range

[Robin Murphy]

On 2022-02-25 19:27, Michael Cheng wrote:
> Hi Robin,
> 
> [ +arm64 maintainers for their awareness, which would have been a good 
> thing to do from the start ]
> 
>   * Thanks for adding the arm64 maintainer and sorry I didn't rope them
>     in sooner.
> 
> Why does i915 need to ensure the CPU's instruction cache is coherent 
> with its data cache? Is it a self-modifying driver?
> 
>   * Also thanks for pointing this out. Initially I was using
>     dcache_clean_inval_poc, which seem to be the equivalently to what
>     x86 is doing for dcache flushing, but it was giving me build errors
>     since its not on the global list of kernel symbols. And after
>     revisiting the documentation for caches_clean_inval_pou, it won't
>     fly for what we are trying to do. Moving forward, what would you (or
>     someone in the ARM community) suggest we do? Could it be possible to
>     export dcache_clean_inval_poc as a global symbol?

Unlikely, unless something with a legitimate need for CPU-centric cache 
maintenance like kexec or CPU hotplug ever becomes modular.

In the case of a device driver, it's not even the basic issues of 
assuming to find direct equivalents to x86 semantics in other CPU 
architectures, or effectively reinventing parts of the DMA API, it's 
even bigger than that. Once you move from being integrated in a single 
vendor's system architecture to being on a discrete card, you 
fundamentally *no longer have any control over cache coherency*. Whether 
the host CPU architecture happens to be AArch64, RISC-V, or whatever 
doesn't really matter, you're at the mercy of 3rd-party PCIe and 
interconnect IP vendors, and SoC integrators. You'll find yourself in 
systems where PCIe simply cannot snoop any caches, where you'd better 
have the correct DMA API calls in place to have any hope of even the 
most basic functionality working properly; you'll find yourself in 
systems where even if the PCIe root complex claims to support No Snoop, 
your uncached traffic will still end up snooping stale data that got 
prefetched back into caches you thought you'd invalidated; you'll find 
yourself in systems where your memory attributes may or may not get 
forcibly rewritten by an IOMMU depending on the kernel config and/or 
command line.

It's not about simply finding a substitute for clflush, it's that the 
reasons you have for using clflush in the first place can no longer be 
assumed to be valid.

Robin.

> On 2022-02-25 10:24 a.m., Robin Murphy wrote:
>> [ +arm64 maintainers for their awareness, which would have been a good 
>> thing to do from the start ]
>>
>> On 2022-02-25 03:24, Michael Cheng wrote:
>>> Add arm64 support for drm_clflush_virt_range. caches_clean_inval_pou
>>> performs a flush by first performing a clean, follow by an invalidation
>>> operation.
>>>
>>> v2 (Michael Cheng): Use correct macro for cleaning and invalidation the
>>>             dcache. Thanks Tvrtko for the suggestion.
>>>
>>> v3 (Michael Cheng): Replace asm/cacheflush.h with linux/cacheflush.h
>>>
>>> v4 (Michael Cheng): Arm64 does not export dcache_clean_inval_poc as a
>>>             symbol that could be use by other modules, thus use
>>>             caches_clean_inval_pou instead. Also this version
>>>                 removes include for cacheflush, since its already
>>>             included base on architecture type.
>>>
>>> Signed-off-by: Michael Cheng <michael.cheng at intel.com>
>>> Reviewed-by: Matt Roper <matthew.d.roper at intel.com>
>>> ---
>>>   drivers/gpu/drm/drm_cache.c | 5 +++++
>>>   1 file changed, 5 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
>>> index c3e6e615bf09..81c28714f930 100644
>>> --- a/drivers/gpu/drm/drm_cache.c
>>> +++ b/drivers/gpu/drm/drm_cache.c
>>> @@ -174,6 +174,11 @@ drm_clflush_virt_range(void *addr, unsigned long 
>>> length)
>>>         if (wbinvd_on_all_cpus())
>>>           pr_err("Timed out waiting for cache flush\n");
>>> +
>>> +#elif defined(CONFIG_ARM64)
>>> +    void *end = addr + length;
>>> +    caches_clean_inval_pou((unsigned long)addr, (unsigned long)end);
>>
>> Why does i915 need to ensure the CPU's instruction cache is coherent 
>> with its data cache? Is it a self-modifying driver?
>>
>> Robin.
>>
>> (Note that the above is somewhat of a loaded question, and I do 
>> actually have half an idea of what you're trying to do here and why it 
>> won't fly, but I'd like to at least assume you've read the 
>> documentation of the function you decided was OK to use)
>>
>>> +
>>>   #else
>>>       WARN_ONCE(1, "Architecture has no drm_cache.c support\n");
>>>   #endif