linux-kernel - Re: [PATCH] Add io_sync stubs to generic IO memcpy/memset

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Message-ID: <8f22066a-a2cc-4e6d-91aa-a2bdd0e53b79@outer-limits.org>
Date: Tue, 28 Jan 2025 09:32:55 +0100
From: Julian Vetter <julian@...er-limits.org>
To: Arnd Bergmann <arnd@...db.de>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add io_sync stubs to generic IO memcpy/memset

On 1/27/25 16:48, Arnd Bergmann wrote:
> On Mon, Jan 27, 2025, at 15:11, Julian Vetter wrote:
>> On 1/27/25 11:27, Arnd Bergmann wrote:
>>> On Mon, Jan 27, 2025, at 11:04, Julian Vetter wrote:
>>
>> Thank you for your quick reply. You're right, I was just going with the
>> naming used in the powerpc arch which has an io_sync define. I'm now
>> wondering if we can't simply use the read{l,q}/write{l,q} functions
>> (instead of the __raw_xxx version), there are already calls to __io_br
>> before and__io_ar after each read (and write). But this might have
>> performance implications on some architectures, depending what it
>> resolves to.
>>
>> Otherwise I propose renaming the __pre_io_sync and __post_io_sync into a
>> single __io_mbr which is called before and after each loop. Looking at
>> PowerPC and SuperH, both of them could be consolidated into the generic
>> IO memcpy code when adding this. What do you think?
> 
> Having barriers between the accesses would be very expensive, and
> prevent the write-combining and prefetching that can otherwise happen
> (depending on mapping flags).

Yes, ok. I see.

> 
> I suspect that the powerpc variant got this wrong for historic
> reasons, but that's hard to tell now. The ppc32 variant didn't
> have barriers at all originally, it was just memcpy/memset
> before it got combined with ppc64 into arch/powerpc.
> 

hmmm... ok. I'm not sure what do make of this. But maybe I can just send 
a patch to the PowerPC mailinglist, without those "sync" calls and see 
what they have to say.

>> The existing ones, especially __io_br unfortunately don't resolve to the
>> right define on these architectures. The __io_ar and __io_br resolve to
>> the right mb() on SuperH and PowerPC as well, but this would again have
>> implications on other architectures.
> 
> The barriers in the sh functions seem arbitrary, and I would
> expect them to be wrong.
> 
Ok, same for SuperH, I will send a patch to the mailinglist without the 
'mb()' before and after and see what they say. If they don't like it, I 
will come back to this.

Julian

> 
>        ARnd