Re: DMA memory mapping

["Usman S. Ansari" <uansari@yahoo.com>]

I have a driver (this is continuation of preserving memory accross reboot) whic has some memory
allocated and my application wants to write to it. Therefore in the application I (after open
call) do

 if ((ptr = (char *) mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == NULL)
    {
     perror("error ...");
    }

Now I have access to memory starting at virtual location ptr for next 4096 bytes. In my driver
static int ocr_mmap(struct file *file, struct vm_area_struct *vma)
{
 unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
 int minor = MINOR(file->f_dentry->d_inode->i_rdev);

 vma->vm_flags |= VM_RESERVED;

 if (offset >= __pa(high_memory) || (file->f_flags & O_SYNC))
    vma->vm_flags |= VM_IO;                    /* since this driver maps different kind of segs */

if (remap_page_range(vma->vm_start, __pa(dT[minor].addr), vma->vm_end - vma->vm_start,
                     vma->vm_page_prot) < 0)
     return -EAGAIN;
 
 return(0);
}

Following 3 are filled by kernel
vma->vm_start
vma->vm_end
vma->vm_start

__pa(dT[minor].addr) ---> I have a kernel virtual address and __pa() gives me physical address.

I hope this is helpfull.

Usman
--- David Stroupe <dstroupe@keyed-upsoftware.com> wrote:
> Usman S. Ansari wrote:
> 
> >No kernel is.
> >
> >--- David Stroupe <dstroupe@keyed-upsoftware.com> wrote:
> >
> >>Usman S. Ansari wrote:
> >>
> >>>When user process calls mmap system call, this address is setup by the kernel along with
> size,
> >>>this I figure is setup in some suitable place in the user process virtual space.
> >>>
> >>>Usman
> >>>
> >>But since I am the device driver writer, I am responsible for 
> >>this...correct?
> >>
> So, within my user program I do something like this: void* addy = 
> mmap(.....) system call.  The returned variable addy can now be used in 
> a call to my driver as the virtual address in a remap_page_range.  Of 
> course, this remap call would use the same size buffer etc.  Once this 
> is completed,  accesses to the buffer pointed to by addy will access the 
> physical memory used in my remap call?
> 
> Thanks  again
> 
> -- 
> Best regards,
> David Stroupe
> Keyed-Up Software
> 
> 


=====
Usman S. Ansari
Linux OS & Device Drivers

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