Subject: Re: How to map memory uncached on PPC.

From: Stephen Williams <[email-masked]>
Date: Sat, 20 Aug 2005 08:59:42 -0700

Benjamin Herrenschmidt wrote:

> On Fri, 2005-08-19 at 08:17 -0700, Stephen Williams wrote:
>>I did some measurements, at it seems that the vast amount of
>>the time is spent in pci_map_single, which calls only the
>>consistent_sync function, which for FROMDEVICE calls only
>>invalidate_dcache_range. So I'm convinced that invalidating
>>the cache for the output buffer (which is large, in case the
>>image that arrives is large) is taking most of the time. So
>>I want to eliminate it.

> A simple experiment you can do is limit the memory used by the kernel
> (booting with mem=xxxx) and then use mmap of /dev/mem to map the
> remaining memory like if it was an IO device, uncached. With that, you
> get a quick hack solution to validate the performance benefit at least.

I did an even simpler experiment: I commented out the pci_map_single,
which on a PPC only has the effect of calling invalidate_dcache_range
and returning the virt_to_bus of the address. Obviously, the cache
is still enabled for the processor, and the image data may get
corrupted, but this was a performance test, not a solution.

Your "test" is a not implausible solution, although it has for me
some administrative problems.

-- 
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."