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    SubjectRe: USB mass storage and ARM cache coherency
    On Tue, 2010-02-02 at 09:11 +0000, Sebastian Andrzej Siewior wrote:
    > * Catalin Marinas | 2010-02-01 17:29:14 [+0000]:
    > >> So, let's put this in the HCD drivers and be done with it.
    > That is the correct place. MMC -hcd drivers for instance are doing this
    > way.
    > >The patch below is what fixes the I-D cache incoherency issues on ARM. I
    > >don't particularly like the solution but it seems to be the only one
    > >available.
    > The PIO-MMC drivers walk through a scatter list via sg_miter_start() and
    > friends. Those helpers take care of this automaticly.
    > >IMHO, Linux should have functions similar to the DMA API but for PIO
    > >drivers (e.g. pio_map_single/pio_unmap_single) that non-coherent
    > >architectures can define, otherwise being no-ops. Any thoughts?
    > What is wrong with flush_dcache_page() ?

    In this particular case, it's too many lines to do the virt_to_page for
    the transfer buffer since the HCD driver doesn't have access to the
    individual pages (via something like urb->sg). A better solution would
    be to move such loop in a flush_dcache_range() function to make it
    easier for drivers.

    Apart from that, flush_dcache_page() doesn't have any data flow
    information. Optimisations could be done on ARM if we know that the
    kernel only intends to read from a page (no flushing necessary with a
    non-aliasing D-cache).

    > And I think linux-arch is the appropriate place.

    For changes to the cache flushing API, yes, that's the right place. I'll
    get there with a patch.
    > >diff --git a/drivers/usb/host/isp1760-hcd.c b/drivers/usb/host/isp1760-hcd.c
    > >index 27b8f7c..4d3eeee 100644
    > >--- a/drivers/usb/host/isp1760-hcd.c
    > >+++ b/drivers/usb/host/isp1760-hcd.c
    > >@@ -18,6 +18,8 @@
    > > #include <linux/uaccess.h>
    > > #include <linux/io.h>
    > > #include <asm/unaligned.h>
    > >+#include <asm/cacheflush.h>
    > >+#include <asm/memory.h>
    > I'm fine with the patch generally but I don't like the asm headers.
    > cacheflush.h is available on most architectures as far as I can see it but
    > memory.h is only available on arm. So you break the build on !arm and
    > therefore I NAK this.

    Yes, that was already pointed out. I'll post a revised patch (until we
    maybe get a better API for such things but that's for linux-arch).



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