Re: [PATCH v2 2/2] io: prevent compiler reordering on the default readX() implementation

Sinan Kaya
2018-04-05T00:06:46Z

On 4/4/2018 3:50 PM, Arnd Bergmann wrote:
> On Wed, Apr 4, 2018 at 7:48 PM, Sinan Kaya <okaya@codeaurora.org> wrote:
>> On 4/4/2018 11:55 AM, Arnd Bergmann wrote:
>>> Yes, exactly, plus the same for write and in/out of course.
>>
>> I was looking at this...
>>
>> inb() and outb() seem to be calling writeb(). It gets the wmb/barrier automatically
>> when we fix writeb().
>>
>> Did I miss something?
>
> At least outb() needs stricter barriers than writeb() in theory, what
> we want here
> is that outb() has not just made it out to the device but that the
> write has been
> confirmed completed by the device. Some architectures can't do it, but those
> that can should have an easy way to hook into that using a separate set of
> barriers.
>
> Using the riscv barrier names, we could do this like
>
> #ifndef __io_bw()
> #define __io_bw() wmb()
> #endif
>
> #ifndef __io_aw
> #define __io_aw() barrier()
> #endif
>
> #ifndef __io_pbw
> #define __io_pbw() __io_bw()
> #endif
>
> #ifndef __io_paw
> #define __io_paw() __io_aw()
> #endif
>
> and the same thing for reads. This way, an architecture could override
> any of those, but still get reasonable defaults for the others.
> For __io_bw(), I picked barrier() instead of do {} while (0), no idea
> if that's any better, I just play safe here.

I posted V3. I hope I captured what you mean above correctly.

>
> 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.