Re: PCI DMA to small buffers on cache-incoherent arch

[David S. Miller]

   From: Oliver Neukum <[e-mail]>
   Date: Sun, 9 Jun 2002 08:45:49 +0200

   Am Sonntag, 9. Juni 2002 07:29 schrieb David S. Miller:
   >    From: Roland Dreier <[e-mail]>
   >    Date: 08 Jun 2002 18:26:12 -0700
   >
   >    Just to make sure I'm reading this correctly, you're saying that as
   >    long as a buffer is OK for DMA, it should be OK to use a
   >    sub-cache-line chunk as a DMA buffer via pci_map_single(), and
   >    accessing the rest of the cache line should be OK at any time before,
   >    during and after the DMA.
   >
   > Yes.
   
   Does this mean that this piece of memory does have to be declared
   uncacheable until DMA is finished ?
   How else do you solve th problem of validity during DMA and
   especially after DMA ?

You flush either before/after depending upon whether the cpu caches
are writeback in nature or not, and the cpu is not allowed to touch
those addresses while the device is doing the DMA.