Re: [RFC/PATCH v2] powerpc: add ioremap_early() function for mapping IO regions before MMU_init()

["Grant Likely" <grant.likely@secretlab.ca>]

On Thu, Aug 14, 2008 at 10:24 AM, Kumar Gala <galak@kernel.crashing.org> wrote:
>>> what happens if we run out of bats?
>>
>> Then it returns NULL and the caller must handle it.  The board port
>> maintainer needs understand the board/CPU/SoC and not depend on more
>> BATs than are available.  They also need to understand that there is a
>> tradeoff between BATs for IO and BATs for RAM.  If the board port uses
>> up all the BATs for IO, then RAM above 256MB ends up getting mapped
>> with PTEs and there is a performance hit.  My expectation is that only
>> platform code will use this facility.  Device drivers should continue
>> to use ioremap() and will gain the benefit of the BATs if platform
>> code already set them up.  I can add some text to the documentation to
>> describe this.
>>
>> I'm not going to make any attempt to fallback to PTEs for IO when
>> there isn't enough BATs.  Doing so adds an order of magnitude more
>> complexity.
>
> that's fine.. I just didn't look at setbat() to see it errors out.
>
> Also can we get rid of LOAD_BAT in head_32.S?

Mostly.  It is still needed for BAT0 when setting up RAM, but the rest
of it can be dumped.

g.


-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.