uncached user space mapping with mmap() ???

Hi there,

I am trying to access a hardware timer implemented in an FPGA from
user space. I implemented a simple mmap() functionality (taken from
Runbini's Linux Device Driver)

int simple_remap_mmap(struct file *filp, struct vm_area_struct *vma)
{
      unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;

      if (offset >= __pa(high_memory) || (filp->f_flags & O_SYNC)) {
          vma->vm_flags |= VM_IO;
      }
      vma->vm_flags |= VM_RESERVED;

      if (remap_page_range(vma->vm_start, offset,
	vma->vm_end-vma->vm_start, vma->vm_page_prot))
	        return -EAGAIN;

      return 0;
}

I just learned that

 > getting an uncached user space mapping is architecture dependent.
 > On ARM, however, passing pgprot_noncached(vma->vm_page_prot) to
 > remap_page_range() will alter the page protections such that the
 > mapping will be uncached.

How could I do this on an MPC8xx?

Thanks a million!


--
Steven Scholz

imc Measurement & Control               imc Meßsysteme GmbH
Voltastr. 5                             Voltastr. 5
13355 Berlin                            13355 Berlin
Germany                                 Deutschland


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Steven Scholz wrote:

How could I do this on an MPC8xx?

Open /dev/mem and just map the physical address of your
registers.  You don't even need your own driver.  If you
are mapping outside of the range of real memory, the mem
driver will automatically give you uncached access.


	-- Dan


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Hi there,

thanks very much for all your help.
I used the stuff Stéphane suggested. But I have to ask again about
mapping of huge memory areas:

Let's say the peripheral has 32 MB address range staring at BASE. At
address GRAM 1MB of external SDRAM is located.

I use a simple test loop

	p = (unsigned short *)
		ioremap(base + GRAM, 32MB);

	START ();
	while (Retries--) {
		for (i = 0; i < Size; i++) {
			pData[i] = *p;
		}
	}
	STOP ();

When I map only the the GRAM I get a throughput of

IoremapTest...        8.0 s      => 2049.4 kW/s

But when I map the whole address range

	p = (unsigned short *)
		ioremap(BASE, imcdevif_iosize);

and move the pointer

	p += GRAM;

before entering the test loop I only get

IoremapTest...        8.4 s      => 1944.8 kW/s

How is that???

Is it always better to map only the small part I am going to use?

Thanks a million!

--
Steven Scholz

imc Measurement & Control               imc Meßsysteme GmbH
Voltastr. 5                             Voltastr. 5
13355 Berlin                            13355 Berlin
Germany                                 Deutschland


** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/

Steven Scholz wrote:

| Is it always better to map only the small part I am going to use?

Of course it is always best to map only the small range that you are
going to use and there will not necessarily be only one TLB mapping (I
am here referring to your comments on ARM Linux Kernel List) etc. etc.
However here especially I am inclined to repeat what they said on the
other list - these figures are so close that they might differ for other
reasons and so on. Did you repeat this in a controlled way many times?

You will need to answer the questions Russell King posted in order for
us to work through this and figure out where the difference lies.

Cheers,

Jon.


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Jon Masters wrote:

| Is it always better to map only the small part I am going to use?

Of course it is always best to map only the small range that you are
going to use and there will not necessarily be only one TLB mapping (I
am here referring to your comments on ARM Linux Kernel List) etc. etc.
However here especially I am inclined to repeat what they said on the
other list - these figures are so close that they might differ for other
reasons and so on. Did you repeat this in a controlled way many times?

Yes. The thruput is calculated after repeating the loop 1000 times
(Retries = 1000).

You will need to answer the questions Russell King posted in order for
us to work through this and figure out where the difference lies.

I am just about to do it.


--
Steven Scholz

imc Measurement & Control               imc Meßsysteme GmbH
Voltastr. 5                             Voltastr. 5
13355 Berlin                            13355 Berlin
Germany                                 Deutschland


** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/

Steven Scholz wrote:

| Yes. The thruput is calculated after repeating the loop 1000 times
| (Retries = 1000).

That is not an accurate measurement though because you really need to
repeat this a few times on the host from a cold start under similar
conditions for people to actually see much difference in these figures -
within a single run you might well see a small delta between the two.

|> You will need to answer the questions Russell King posted in order for
|> us to work through this and figure out where the difference lies.

| I am just about to do it.

Ok.

Jon.


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