Re: [linux-usb-devel] Patch for UHCI driver (from kernel 2.6.6).

[David Brownell <david-b@pacbell.net>]

Alan Stern wrote:
> On Mon, 14 Jun 2004, David Brownell wrote:
> 
> 
>>Seems like the dma_alloc_coherent() API spec can't be
>>implemented on such machines then, since it's defined
>>to return memory(*) such that:
>>
>>   ... a write by either the device or the processor
>>   can immediately be read by the processor or device
>>   without having to worry about caching effects.
>>
>>...
> 
> That text strikes me as rather ambiguous.  ...
> ...  It doesn't specify what happens to the other data
> bytes in the same cache line which _weren't_ written -- maybe they'll be
> messed up.

Actually I thought it was quite explicit:  "without having
to worry about caching effects".  What you described is
clearly a caching effect:  caused by caching.  And maybe
fixable by appropriate cache-flushing, or other cache-aware
driver logic ... making it "worry about caching effects".
Like the patch from Nicolas.

Maybe what we really need is patches to make USB switch to
dma_alloc_noncoherent(), checking dma_is_consistent() to
see whether a given QH/TD/ED/iTD/sITD/FSTN/... needs to be
explicitly flushed from cpu cache before handover to HC.

- Dave



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