[U-Boot] i.MX51: FEC: Cache coherency problem?

[J. William Campbell <jwilliamcampbell@comcast.net>]

On 7/19/2011 2:05 AM, Albert ARIBAUD wrote:
> Le 19/07/2011 10:43, Aneesh V a ?crit :
>
>>>> You would have to flush (before sending packets / starting external
>>>> memory-to-device DMA) and invalidate (before reading received packets /
>>>> after external device-to-memory DMA is done); using MMU and mapping
>>>> cached/non-cached areas is IMO overkill, and will hurt CPU accesses to
>>>> the xmit/receive buffers and descriptors.
>>> So, you say actually what I did while exploring the problem would have
>>> been a
>>> correct way of solving this problem?
>>>
>>> Like this:
>>>
>>> 587 flush_cache(&fec->tbd_base[fec->tbd_index], 4);
>> This is what is needed assuming the below is initiating a memory to
>> peripheral DMA. Is your buffer only 4 bytes long?
> Generally:
>
> - for sending data through a device that has its own, external, DMA
> engine, you'll obviously need to flush the data buffer(s) but also any
> DMA descriptors used by the engine, before you start the engine;
>
> - for rceiving, if you have to set up receive descriptors, you must
> flush that before telling the device to enter receive mode (so that the
> device reads the descriptors as you wrote them), and you should
> invalidate the receive buffers at the latest when the device signals
> that data has been received,
Hi All,

>   or preferably long before (at the same time
> you flushed the read descriptor, so that cache-related actions are
> grouped in the same place in the code).
I think this last statement is incorrect. You should invalidate the 
cache for the receive buffers just before you intend to reference them. 
If you do it right after receive mode is entered, subsequent access to 
items NEAR the receive buffer may reload the cache with receive buffer 
data before the dma is done, re-creating the problem you are trying to 
avoid. Also, I don't know if ARM cache is write-back or write -thru, but 
if it is write-back, the only way to avoid problems is to allocate the 
receive buffers on cache line boundaries, so no "nearby" writes can 
cause something in the DMA buffer to be corrupted. If all receive 
buffers are allocated on cache-line boundaries (both start and end of 
each buffer), you can invalidate the cache "early" under the assumption 
that there will be no read accesses to the read buffers themselves until 
after DMA is complete. IMHO it is better, even in this case., to 
invalidate cache after dma is done but before referencing the read data.

Best Regards,
Bill Campbell

>
> Amicalement,