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

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

Hi David,

Le 19/07/2011 09:44, David Jander a ?crit :
>
> Hi Stefano,
>
> On Mon, 18 Jul 2011 18:55:05 +0200
> Stefano Babic<sbabic@denx.de>  wrote:
>
>> On 07/18/2011 05:18 PM, David Jander wrote:
>>>
>>> Hi all,
>>
>> Hi David,
>>
>>> What is going on here? Why did this work with caches enabled before??
>>
>> I think cache was always disabled..
>
> I had even L2-caches enabled in u-boot (copied/adapted some code from OMAP
> cache.S), and called i/dcache_enable() from board code like this:
>
> int board_late_init(void)
> {
>          power_init();
>          probe_board_type();
>          icache_enable();
>          dcache_enable();
>
>          return 0;
> }
>
> Is there a reason this wouldn't have worked before?
>
> Suppose it didn't. Does that mean we need to use the MMU to properly mark
> regions of register space and specially FEC BD's as not-cached? Or do we need
> to flash caches manually each time such a memory region is accessed?
> I am kind of a CPU-speed-junkie, so I am not sure I want to live without
> caches enabled in u-boot ;-)

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.

> Best regards,

Amicalement,
-- 
Albert.