sorry for being a busy body....:-)....but hope i can contribute my 2cts...
On Fri, Sep 25, 2009 at 12:54 PM, Mulyadi Santosa
<mulyadi.santosa@xxxxxxxxx> wrote:
> On 9/25/09, Jason Nymble <jason.nymble@xxxxxxxxx> wrote:
>> For posterity, I've finally solved this issue. It ended up having
>> nothing to do with the interrupts/tasklets themselves. The driver uses
>> ioremap() to get hold of some reserved memory, and it seems from about
>> 2.6.25 onwards or so this defaults to ioremap_nocache(), so our driver
>> was doing memory operations in the tasklet on uncacheable pages.
>> Calling ioremap_cache() explicitly in the driver solved the issue
>> (nice when you can fix a perf regression of 50-100x with a single line
>> fix!). Oprofile was of tremendous help in solving this issue.
>
> Hi Jason
>
> Nice to hear you finally fix your problem. BTW, may I ask a bit about
> ioremap_cache? By using this function, it is like saying "map and
> cache these pages so next time we access them, we don't need to fetch
> them from device memory again"? CMIIW
not really about reading.....but writing. and it is not memory
caching, but hardware caching (sounds like the same???? :-))
Read the source:
279 void __iomem *ioremap_cache(resource_size_t phys_addr,
unsigned long size)
280 {
281 return __ioremap_caller(phys_addr, size, _PAGE_CACHE_WB,
282 __builtin_return_address(0));
283 }
it is set with WRITEBACK attribute, and write-back is a hardware
caching feature (search write-back here):
http://en.wikipedia.org/wiki/Cache
http://en.wikipedia.org/wiki/CPU_cache
write-through is synchronous (meaning the CPU will not execute the
next instruction, until the data on the memory bus has updated the
device in a hardware handshake protocol). but write-back is
asynchronous, for performance reason, it try to accumulate changes
before batching them in a burst of write-flush. but the delay is
usually quite small.
so i supposed if u write to it, and want to use its feature almost
immediately, then no-caching mode is needed.
>
> If it is so, then logically it is the answer as it was proven by your
> experiments...
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer
> blog: the-hydra.blogspot.com
>
--
Regards,
Peter Teoh
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