Re: [PATCH] asm-generic/io.h: Implement read[bwlq]_relaxed()

Will Deacon
2014-09-09T14:15:22Z

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?

> >> 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.

> >> 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. How
about I dust off the series again?

Will