Messages in this thread | | | Date | Tue, 09 Sep 2014 15:51:07 +0100 | From | Daniel Thompson <> | Subject | Re: [PATCH] asm-generic/io.h: Implement read[bwlq]_relaxed() |
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On 09/09/14 15:15, Will Deacon wrote: > On Tue, Sep 09, 2014 at 02:14:54PM +0100, Daniel Thompson wrote: >> On 09/09/14 14:03, Daniel Thompson wrote: >>> On 09/09/14 13:28, Will Deacon wrote: >>>> I have a larger series adding these (and the write equivalents) to all >>>> architectures that I periodically post and then fail to get on top of. >>> >>> That's why you're on Cc:... > > Ok, so why not just pick the asm-generic patch out of my series?
Only really that your patch introducing the writeX_relaxed() family at the same time.
However it would be fine to subset your patch rather than use mine. You did post it first...
>>>> The key part you're missing is defining some generic semantics for these >>>> accessors. Without those, I don't think it makes sense to put them into >>>> asm-generic, because drivers can't safely infer any meaning from the relaxed >>>> definition. >>> >>> Currently the semantics are described as: >>> --- cut here --- >>> PCI ordering rules also guarantee that PIO read responses arrive after >>> any outstanding DMA writes from that bus, since for some devices the >>> result of a readb call may signal to the driver that a DMA transaction >>> is complete. In many cases, however, the driver may want to indicate >>> that the next readb call has no relation to any previous DMA writes >>> performed by the device. The driver can use readb_relaxed for these >>> cases, although only some platforms will honor the relaxed semantics. >>> Using the relaxed read functions will provide significant performance >>> benefits on platforms that support it. The qla2xxx driver provides >>> examples of how to use readX_relaxed . In many cases, a majority of the >>> driver’s readX calls can safely be converted to readX_relaxed calls, >>> since only a few will indicate or depend on DMA completion. >>> --- cut here --- >>> >>> The implementation provided in the patch trivially meets this definition >>> (by not honouring the relaxedness). > > I still think we need to mention ordering of relaxed reads against each > other and also against spinlocks.
I don't disagree.
I just think the documentation being sub-optimal is not a good reason to avoid implementing the read functions.
>>>> Ben and I agreed on something back in May: >>>> >>>> https://lkml.org/lkml/2014/5/22/468 >>> >>> ... and didn't you also conclude with hpa that the very relaxed x86 >>> implementation of readl_relaxed() already meets this definition (as do >>> these changes to asm-generic/io.h). >> >> Sorry. "very relaxed" is always a very stupid thing to say about x86 >> (especially to an arm guy). >> >> More exactly I was referring to the absence of memory clobber in x86 >> readl_relaxed(). > > Yeah, my series just adds the relaxed write accessors for x86. > >>> Thus allowing its use to perculate more widely really shouldn't do an harm. >>> >>> >>>> but I need to send a new version including: >>>> >>>> - ioreadX_relaxed and iowriteX_relaxed >>>> - Strengthening non-relaxed I/O accessors on architectures with non-empty >>>> mmiowb() >>>> >>>> I'll bump it up the list. In the meantime, you can have a look at my io >>>> branch on kernel.org >>> >>> I'd really like to see your work included (which I spotted after I wrote >>> the patch and when it occured to me to visit >>> https://www.google.com/search?q=asm-generic+readl_relaxed to see if >>> there was a well known reason not to make this change). >>> >>> However... I really can't see why we should delay introducing an already >>> documented function to the remaining architectures. > > I'd just rather fix the interface once instead of churning it about
Churn? 12 lines of code where two people independently produce the same thing (apart from ordering within the file)?
> How about I dust off the series again?
Dusting off the series again would be great.
Would you consider putting readX_relaxed() and its documentation at the front of the patchset? That way if the writel_relaxed() side log jams again we can still get some of it delivered.
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