USB mass storage and ARM cache coherency

[Benjamin Herrenschmidt]

On Tue, 2010-03-02 at 17:05 +0000, Catalin Marinas wrote:

> 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.

I don't see the point of a "PIO" API. I would thus vote for 2 :-) Note
that flushing the D-cache isn't enough, you also need to invalidate the
I-cache as we discussed earlier, though you mostly get away if you don't
by luck.

There's also a question as to whether clearing PG_arch_1 is
flush_dcache_page() is really necessary or not.

> Option 2 above looks pretty appealing to me since it can be done in the
> ARM code exclusively. I've done some tests and it indeed solves the
> cache coherency with a rootfs on a USB stick. As Russell suggested, it
> can be optimised to mark a page as clean when the DMA API is involved to
> avoid duplicate flushing.

That wouldn't solve the need for invalidating the I-cache... Unless we
use another bit.

> It was also suggested to add a PG_arch_2 flag which would keep track of
> the I-cache status as well.
> 
> I can post a proposal to modify the cachetlb.txt document to reflect the
> issues we currently have on ARM.

Cheers,
Ben.