ARM, MMU and IO space mapping

[Marc Kleine-Budde]

On 11/24/2011 01:04 PM, Sascha Hauer wrote:
> On Thu, Nov 24, 2011 at 12:24:55PM +0100, Robert Jarzmik wrote:
>> Hi,
>>
>> I was wandering in the MMU arm implementation and I'm wondering if anybody is
>> using it and accessing some IO mapped space.
>>
>> My trouble is that mmu_init() only maps the RAM space (and with cacheable memory
>> type). For the PXA, I would need a non-cacheable non-bufferable memory type,
>> which could be allocated either :
>>  - through dev_request_mem_region(), which would be amended for the MMU case
>>  - or through a dedicated ioremap() function, which will be empty in no MMU case
>>
>> The current issue I'm facing is that when I launch barebox with MMU enabled, the
>> UDC device doesn't work (I presume because the IO space is not mapped into the
>> MMU). This prevents me from using barebox in MMU context (and therefore I have
>> no D-Cache).
>>
>> Would the evolution of dev_request_mem_region() be acceptable, or should I
>> consider another way ?
> 
> The generic arm code sets up a flat 1:1 noncached mapping. Then it
> remaps the SDRAM cacheable/bufferable. So a normal driver can just
> access the registers without worrying about ioremap() since they are
> outside of the SDRAM. What you need in your udc driver is either
> dma_alloc_coherent which allocates a chunk of SDRAM and remaps it
> uncacheable/unbufferable or you have to sync manually using
> dma_clean_range/dma_flush_range. See drivers/usb/gadget/fsl_udc.c or
> drivers/net/fec_imx.c for examples.
> 
> I remember Marc had some trouble with the mmu/cache implementation on
> PXA. I don't know the details though. Maybe you have to dig through
> the manuals/kernel code to find out the differences between the normal
> arm v5 and pxa.

I had some troubles with the cache setup, but it was before Sascha fixed
a problem with the v5 chache handling. IIRC loading the environment form
NOR sometimes failed with enabled caches.

cheers, Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
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