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

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

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, 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).

Amicalement,
-- 
Albert.