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

[Aneesh V <aneesh@ti.com>]

On Tuesday 19 July 2011 02:07 PM, David Jander wrote:
> On Tue, 19 Jul 2011 10:21:12 +0200
> Albert ARIBAUD<albert.u.boot@aribaud.net>  wrote:
>
>> 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 ;-)

Are you talking about some memory-mapped IO region(register space). If
that is the case, that region won't be cached. ARM mmu implementation
makes only the SDRAM region cached. Rest is non-cached non-buffered.

>>
>> 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?

Also, please check if flush_cache() is correctly supported for your
CPU. The default implementation in in arch/arm/lib/cache.c has support
for only a handful of cpus. AFAIK, only armv7 is over-riding this
default implementation at the moment.

The fact that it's helping you indicates that it may be working for
you. But still worth a check.

> 588         fec_tx_task_enable(fec);
> 589         flush_dcache_all();

This should not be needed.

> 590
> 591         /*
> 592          * wait until frame is sent .
> 593          */
> 594         while (readw(&fec->tbd_base[fec->tbd_index].status)&  FEC_TBD_READY) {
> 595                 udelay(1);
> 596         }
>
> I am still not sure why I need line 587 above.

Did you try keeping 587 and removing 589?

best regards,
Aneesh