On Tue, Mar 04, 2003 at 09:18:40PM +0100, Benjamin Herrenschmidt wrote:
>
> The purpose of this barrier isn not to ensure sequential access on
> either the MMIO mapping (card registers) nor the actual ring buffer
> memory mapping (could be IOs, or just real memory with PCI GART).
>
> It's here to actually sync those 2 between each other. That is make
> sure the writes to the ring are completed before the chip is actually
> kicked. This is a nasty corner case often overlooked in drivers, see
> a mail I just posted to lkml '[RFC] IO vs. DMA and barriers'
Ooo. Quite different.
So that sounds like I'll want DRM_READMEMORYBARRIER() to call
ddi_dma_sync()
[ you've probably never seen that function before in your life, but
I think it's a heck of a lot more self-documenting than
"DRM_READMEMORYBARRIER()" , "mb()", or even "bus_space_barrier()"
;-) ]
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