USB mass storage and ARM cache coherency

[benh@kernel.crashing.org (Benjamin Herrenschmidt)]

On Thu, 2010-03-04 at 19:51 +0530, James Bottomley wrote:
> 
> Technically, he is.  In the old days, most VI architectures were high
> end enough not to require PIO transfers.  The only exception was an
> IDE driver used by sparc, which lead to the arch specific ide in/out
> string instructions, in which sparc actually did all the necessary
> flushing.

Actually, Catalin's problem is with newer PIPT ARM :-)

> So no other drivers than old IDE grew up with cache flushing in the
> PIO case (and almost no high end VI hardware had an IDE interface, so
> they rarely got implemented in the arch layer).  However, recently,
> with the transition from old IDE to libata and the prevalence of ARM
> with more commodity hardware, the deficiency is becoming exposed.
> Even the PA8000 workstations now come with an IDE CD, which means
> we're starting to have problems with them as well.

I don't think there's a core or driver problem in this specific case. As
we discussed earlier, I believe the problem is that ARM considers a
fresh page out of the page cache as "clean" instead of "dirty", and
inverting that like we do on powerpc will fix their problem too.

> > Seems like ARM has requirement other architectures do not, that is
> > a) not documented anywhere
> > b) causes problems
> > 
> > You could argue that performance improvement (how big is it,
> anyway?)
> > is worth it, but this should be agreed to by wider community...
> 
> Performance is always worth it provided we don't sacrifice
> correctness.
> The thing which was discovered in this thread is basically that ARM is
> handling deferred flushing (for D/I coherency) in a slightly different
> way from everyone else ... once that's fixed, ARM will likely not have
> the D/I problem, but we'll still have the libata (and other PIO
> systems) D flushing issue. 

You mean older VIVT ARM will grow a new issue there ?

Cheers,
Ben.