[PATCH v3 1/3] ARM: Introduce atomic MMIO modify

[gregory.clement@free-electrons.com (Gregory CLEMENT)]

Hi all,

On 30/08/2013 12:03, Catalin Marinas wrote:
> On Fri, Aug 30, 2013 at 10:20:33AM +0100, Will Deacon wrote:
>> On Fri, Aug 30, 2013 at 10:15:36AM +0100, Catalin Marinas wrote:
>>> On Fri, Aug 30, 2013 at 10:08:07AM +0100, Will Deacon wrote:
>>>> On Fri, Aug 23, 2013 at 12:48:05PM +0100, Catalin Marinas wrote:
>>>>> On Fri, Aug 23, 2013 at 12:32:26PM +0100, Ezequiel Garcia wrote:
>>>>>> ... or maybe yes. I'm not seeing {readl,writel}_relaxed as guaranteed
>>>>>> to exist in every architecture. So, indeed, this seems to be ARM-dependent.
>>>>>
>>>>> There was a discussion couple of years ago to make these part of the IO
>>>>> specification since many architectures define them:
>>>>>
>>>>> http://thread.gmane.org/gmane.linux.ports.arm.kernel/117626
>>>>>
>>>>> (and some older threads on linux-arch which I haven't searched)
>>>>>
>>>>> We could have some default implementation pointing to readl/writel while
>>>>> letting the arch code to define more optimised variants.
>>>>
>>>> The main thing I dislike about that is the back-to-back dsbs that you will
>>>> get from the read-(modify)-write. It really makes the non-optimised version
>>>> needlessly expensive.
>>>
>>> Yes, it's pretty bad. But we don't have relaxed (write) accessors on
>>> other architectures and I'm not sure about their semantics either. I
>>> guess here it's a data dependency so you cannot write the value before
>>> reading it, especially since sane architectures should speculate reads
>>> or writes to device memory.
>>>
>>> What about making it always use *_relaxed() accessors if the
>>> architecture provides them? No need for atomic_io_modify_relaxed().
>>
>> The only potential problem there is if somebody uses this function to kick
>> off a DMA. That would require explicit barriers to enforce ordering against
>> population of normal, cacheable buffers, which isn't usually the case in
>> driver code (since we have the dsb/outer_sync in the accessor).
>>
>> Perhaps we should just bit the bullet and define relaxed accessors for all
>> architectures? It's not difficult to default them to the non-relaxed
>> variants if the architecture doesn't provide an optimised implementation.
> 
> Yes, an asm-generic default relaxed would be good (that's what I
> suggested earlier in this thread and it was discussed in the past). But
> no-one volunteered ;).
> 

I would like to make the things move on about this subject. Should it
be possible to merge this version of the patch set? Currently the
only users of this new API are drivers for ARM SoCs.

In the meantime, I am willing to introduce an asm-generic default
relaxed variant of read and write, but as Catalin had already pointed
in the past
http://thread.gmane.org/gmane.linux.ports.arm.kernel/117626
it may take time to get an agreement from all the other architectures.

That's why I propose that this patch set do not depend on the
introduction of a asm-generic default relaxed variant of read and
write. Later when it will be accepted then this new API will be moved
in the asm-generic part.

Thanks,

-- 
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com