Subject: Re: [PATCH] s2io ppc64 fix for readq/writeq

From: Jeff Garzik <[email-masked]>
Date: Mon, 06 Nov 2006 04:34:01 -0500

Benjamin Herrenschmidt wrote:
>> This is why I said "good enough for drivers".  This is _key_.
>>
>> I have run into several [PCI] devices with 64-bit registers, and 
>> __none__ of them had requirements such that the Linux platform code 
>> -must- provide an atomic readq/writeq.  Probably because everybody wants 
>> to support 32-bit platforms with their devices.
>>
>> What you call "fairly bogus" is precisely what drivers need.  These 
>> devices with 64-bit registers just don't need the atomicity that arch 
>> developers harp about :)
> 
> Is there any consistency in that case in which half need to be
> read/written first ? Or none of these ever had side effects ?

Generally the kernel code should write the two 32-bit chunks to the 
memory-mapped region in order (low dword first), and let things take 
care of themselves from there.

That's pretty much the implementation that -every- driver copies, when 
they need readq/writeq to work on a 32-bit platform.

	Jeff