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    SubjectRE: USB mass storage and ARM cache coherency
    > -----Original Message-----
    > From: Oliver Neukum []
    > Sent: Tuesday, February 16, 2010 7:03 PM
    > To: Russell King - ARM Linux
    > Cc: Shilimkar, Santosh; Catalin Marinas; Pavel Machek; Greg KH; Matthew Dharm; Sergei Shtylyov; Ming
    > Lei; Sebastian Siewior;; linux-kernel; linux-arm-kernel; Mankad, Maulik
    > Ojas
    > Subject: Re: USB mass storage and ARM cache coherency
    > Am Dienstag, 16. Februar 2010 10:39:46 schrieb Russell King - ARM Linux:
    > > However, because ARM CPUs can now speculatively prefetch, just leaving it
    > > at that results in corruption of buffers used for DMA. So we have to
    > > invalidate DMA_FROM_DEVICE and DMA_BIDIRECTIONAL buffers on unmap to
    > > ensure coherency with DMA operations.
    > >
    > > If the CPU writes to a DMA_FROM_DEVICE buffer between map and unmap, the
    > > writes can sit in the cache, and on unmap, they will be discarded.
    > >
    > > Cleaning the cache on unmap is not an option; that too can lead to DMA
    > > buffer corruption in the DMA case.
    > I am afraid for these controllers the controller driver must be responsible
    > for all DMA and cache issues. Indicating the exact requirements to the
    > upper layer would be a battle already lost.
    > so the safe choice is not to set has_dma and the generic layer will leave
    > the issue to the lower level.
    This means don't use dma at all which will almost kill the performance.


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