USB mass storage and ARM cache coherency

[Pavel Machek]

Hi!

> > I'm not sure that there are some problems in the mm or common code. Is
> > this ARM's implementation issue? (Of course, the usb stack and the
> > driver's misuse of the DMA API needs to be fixed too).
> 
> Just to summarise - on ARM (PIPT / non-aliasing VIPT) there is I-cache
> invalidation for user pages in update_mmu_cache() (it could actually be
> in set_pte_at on SMP to avoid a race but that's for another thread). The
> D-cache is flushed by this function only if the PG_arch_1 bit is set.
> This bit is set in the ARM case by flush_dcache_page(), following the
> advice in Documentation/cachetlb.txt.
> 
> With some drivers (those doing PIO) or subsystems (SCSI mass storage
> over USB HCD), there is no call to flush_dcache_page() for page cache
> pages, hence the ARM implementation of update_mmu_cache() doesn't flush
> the D-cache (and only invalidating the I-cache doesn't help).
> 
> The viable solutions so far:
> 
>      1. Implement a PIO mapping API similar to the DMA API which takes
>         care of the D-cache flushing. This means that PIO drivers would
>         need to be modified to use an API like pio_kmap()/pio_kunmap()
>         before writing to a page cache page.
>      2. Invert the meaning of PG_arch_1 to denote a clean page. This
>         means that by default newly allocated page cache pages are
>         considered dirty and even if there isn't a call to
>         flush_dcache_page(), update_mmu_cache() would flush the D-cache.
>         This is the PowerPC approach.

What about option

3. Forget about PG_arch_1 and always do the flush?

How big is the performance impact? Note that current code does not
even *work* so working, 10% slower code will be an improvement.

								Pavel

(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html