Re: [Xen-devel] Fbdev graphics broken in xen/next dom0

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Mar 12, 2010 at 04:51:30PM -0800, Jeremy Fitzhardinge wrote:
> On 03/12/2010 04:44 PM, Eamon Walsh wrote:
>> I have narrowed the problem down: it has something to do with mmap of
>> /dev/fb0 not syncing.  The attached C code mmaps /dev/fb0 and writes

Is the machine spinning? Meaning if you start writting to the mmap
region the machine looks to be stuck?

>> some random bits.  On a configuration that does work (2.6.31.4 on
>> 4.0-rc6, or xen/next on bare metal) the random bits are visible on the
>> screen.  With xen/next on 4.0-rc6, nothing is visible.  Calling msync()
>> before the sleep has no effect.  Also, using write() on /dev/fb0 always
>> works so it appears to be mmap related.
>>    
>
> Yes.  I suspect there's a missing VM_IO in there, and so the mmap is  
> mapping the wrong pages (if you're lucky you might be able to crash the  
> machine to see something juicy).

<scratches his head>

The nvidia framebuffer (drivers/video/nvidia/nvidia.c) does this:

1369         info->screen_base = ioremap(nvidiafb_fix.smem_start, 
par->FbMapSize);

where the start of memory is obtained via
1328         nvidiafb_fix.smem_start = pci_resource_start(pd, 1);

I believe the 'ioremap' works pretty good, otherwise we would have other
PCI devices having trouble.

... and in another code (fbmem.c):

1321 static int
1322 fb_mmap(struct file *file, struct vm_area_struct * vma)
..
1345         start = info->fix.smem_start;
..
1363         /* This is an IO map - tell maydump to skip this VMA */
1364         vma->vm_flags |= VM_IO | VM_RESERVED;

.. it _does_ set the VM_IO, but that is OK since the memory is actually
backed by the PCI device.

Eamon, can you provide a more detail serial output? That could shed some
light on this. Another thing you could try to make sure you are actually
hitting the right mmap, is to instrument fb_mmap. I would recommend
printing out the vma->vm_start, vm_end, and start to see if the look
reasonable.



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