Re: RFC on writel and writel_relaxed

[Sinan Kaya <okaya@codeaurora.org>]

On 3/26/2018 9:43 AM, Arnd Bergmann wrote:
> On Wed, Mar 21, 2018 at 2:58 PM, Sinan Kaya <okaya@codeaurora.org> wrote:
>> On 3/21/2018 8:53 AM, Sinan Kaya wrote:
>>> BTW, I have no idea what compiler barrier does on PPC and if
>>>
>>> wrltel() == compiler barrier() + wrltel_relaxed()
>>>
>>> can be said.
>>
>> this should have been
>>
>> writel_relaxed() == compiler barrier() + __raw_writel()
> 
> I don't think anyone clarified this so far, but there are additional differences
> between the two, writel_relaxed() assumes we are talking to a 32-bit
> little-endian
> MMIO register, while __raw_writel() is primarily used for writing into
> memory-type
> regions with no particular byte order. This means:
> 
> - writel_relaxed() must perform a byte swap when running on big-endian kernels
> - when used with __packed MMIO pointers, __raw_writel() may turn into a series
>   of byte writes, while writel_relaxed() must result in a single 32-bit access.
> - A set if consecutive writel_relaxed() on the same device is issued in program
>   order, while __raw_writel() is not ordered. This typically requires
> only a compiler
>   barrier, but may also need a CPU barrier (in addition to the
> barriers we use to
>   serialize with spinlocks and DMA in writel() but not writel_relaxed()).

Thanks for the great summary. I didn't know that __raw_writel() could get converted
to byte writes.

> 
>         Arnd
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.