Re: [Discuss-gnuradio] shmat issue

[Philip Balister]

On 10/20/2010 01:49 PM, Eric Blossom wrote:
> On Wed, Oct 20, 2010 at 01:02:15PM -0400, Philip Balister wrote:
> > On 10/19/2010 10:51 PM, Eric Blossom wrote:
> > OK, it looks like x86 sets SHMLBA to PAGE_SIZE and arm uses 4 *
> > PAGE_SIZE. Need to puzzle through this a little more. This is the
> > failing check in the kernel.
>
> OK.  What's PAGE_SIZE on arm?

PAGE_SIZE is still 4096, there is an additional restriction on the 
address passed to shmat().

> I wonder how we can determine the actual value of SHMLBA at runtime?
> We currently use the result of gr_pagesize() as the required alignment value.

It looks to me like the there are several shm sections created, with 
varying access rights. Then shmat is used to reserve a chunk of memory. 
Then the sections created first are attached to with addresses based on 
the result of the shmat used to get the address space.

The weird thing on arm, the shmat returns a page aligned section, but if 
you pass that address back to shmat it fails, because that address needs 
to be aligned to SHMLBA (found in arch/arm|x86/include/asm/shmparam.h.

What does it mean?

Well gnuradio falls back to a mmap approach to get the circular buffer. 
It is a bigger problem for kalibrate, which just fails.

I'm not sure who is to blame :) Is there another way to get reserve the 
memory on you process to use when you attach the shared memory segments?

Philip