Hi,
i've got a working version of an mmap()'ed I/O, though read-only.
A device puts its data on RAM through dma and userspace can read it (and
consequentely dispatch it to other devices).
Now, I need to write some data on the buffer, before the data is sent out.
So, I added PROT_WRITE to the userspace mmap() and <bang>. :)
Data went written on the virtual address, by the userspace, and indeed the
userspace app can read it correcly, but... the physical memory seems
untouched.
Briefly, my driver allocate a page of memory this way:
buffer = __get_free_pages(GFP_KERNEL | GFP_DMA, get_order(4K));
and fill it with a pattern:
for(i=0; i<maxframebuffer; i++)
buffer[i] = (char)(maxframebuffer - i);
That works fine. (I can inspect the physical memory through a JTAG
debugger).
Then I define the device_mmap(), nothing more that the usual mmap stub:
int mmaptest_mmap(struct file *filp, struct vm_area_struct *vma)
{
unsigned int buffersize = 0;
buffersize = (4K >> PAGE_SHIFT)+1;
if ( (vma->vm_end - vma->vm_start)>>PAGE_SHIFT > buffersize )
return -ENOMEM;
vma->vm_flags |= VM_IO | VM_RESERVED;
vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
if (remap_pfn_range(vma, vma->vm_start,
virt_to_phys(buffer) >> PAGE_SHIFT,
vma->vm_end - vma->vm_start, vma->vm_page_prot)){
return -EAGAIN;
}
vma->vm_ops = &simple_remap_vm_ops;
simple_vma_open(vma);
return 0;
}
Finally, the userspace program do simply:
pointer = mmap(blablabla); //with PROT_WRITE and MAP_PRIVATE contained
*pointer = 0xA5; // just a random constant.
After this, the userspace pointer contains actually 0xA5, while both the
virtual and physical address of kernel side still contain the values
initialized before.
( a snapshot of both memories, user side (left) and kernel
(right) could be found here: http://imageshack.us/f/18/screenshot1fo.png/ )
Some ideas of what happen here?
Seems there is some kind of COW on pages used with mmap()... is there a way
to disable it?
Thanks in advance. bye!
--
- Andrea Gasparini -
-----------------------------------------------
-------- https://launchpad.net/~gaspa ---------
----- HomePage: http://gaspa.yattaweb.it ------
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