This solution works fantastically for me! I couldn't figure out why my
write speeds were so slow.
Thanks for posting this!
On Monday, October 7, 2013 1:15:45 AM UTC+8, Nils Zottmann wrote:
>
> I think I found a better solution now.
>
> Opening /dev/mem was done by
> info->mem_fd = open("/dev/mem", O_RDWR);
> before, i changed it to
> info->mem_fd = open("/dev/mem", O_RDWR | O_SYNC);
>
> Now it works with the speed I expected and I don't have to drop the cache.
> I'm not really sure if I should also use the O_DIRECT flag, but when doing
> so (and defining #define _GNU_SOURCE (see
> http://www.titov.net/2006/01/02/using-o_largefile-or-o_direct-on-linux/)),
> I get a segfault when reading from the mapped buffer, I don't know why.
> With only O_SYNC it works as it should.
>
> What do you think about this solution?
>
>
> Am Sonntag, 6. Oktober 2013 16:44:59 UTC+2 schrieb Nils Zottmann:
>>
>> Okay, my problem seems to be this:
>> http://en.wikipedia.org/wiki/Direct_memory_access#Cache_coherency
>>
>> Is there a way to do this without programming kernel-level code? I've
>> never done this before and don't really know what to do exactly.
>> I found the option to drop the cache by writing a "1"
>> to /proc/sys/vm/drop_caches. I used this is my program with this few lines
>> of code:
>>
>> FILE * fd;
>> fd = open("/proc/sys/vm/drop_caches", O_WRONLY);
>> write(fd, "1", sizeof(char));
>> close(fd);
>>
>> It works most of the time, but sometimes I have to write the "1" a few
>> more times until the cache is really flushed. Perhaps this is only a
>> problem with another cache when writing to this file.
>>
>> I have some more questions now:
>>
>> This is a workaround, but I think no good solution. Is there any nicer
>> way to drop the cache in C?
>> Could I get performance problems at another point if I drop the cache too
>> often (about every sec. later)?
>> Is there a possibility to drop only the cache for my memory section? Or
>> to disable caching for this part of memory?
>> Is there a function like memcpy which is able to read directly from the
>> memory without looking in the cache?
>>
>>
>> Thank you for the help!
>>
>> Nils
>>
>>
>> Am Freitag, 4. Oktober 2013 23:00:34 UTC+2 schrieb Charles Steinkuehler:
>>>
>>> On 10/4/2013 12:13 PM, Nils Zottmann wrote:
>>> > Has anyone an explanation for this behaviour? Perhaps it is a problem
>>> with
>>> > some kind of cache, how could I get around this? I'd like to implement
>>> a
>>> > ring buffer but it would have to be very large if it has to take data
>>> of
>>> > several seconds.
>>>
>>> You are encountering caching effects on the ARM side. The proper way to
>>> deal with this is to set the memory flags appropriately in the MMU,
>>> typically with kernel-level code that's setting up to do DMA. The PRU
>>> in this use case basically looks like a "smart" peripheral that can
>>> modify the contents of memory directly. That is the same as any other
>>> DMA enabled I/O device, and requires the same care with handling of the
>>> memory regions.
>>>
>>> --
>>> Charles Steinkuehler
>>> cha...@steinkuehler.net
>>>
>>>
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