PXA270 overlay problem

[Vasily Khoruzhick]

On Monday 31 January 2011 19:39:29 Russell King - ARM Linux wrote:
> On Mon, Jan 31, 2011 at 07:08:48PM +0200, Vasily Khoruzhick wrote:
> > On Monday 31 January 2011 15:04:14 Russell King - ARM Linux wrote:
> > > On Wed, Jan 26, 2011 at 10:46:00PM +0200, Vasily Khoruzhick wrote:
> > > > Hi, I'm experiencing problems with overlay1/overlay2 on PXA270 using
> > > > pxafb driver. Main problem is overlays just don't work for some
> > > > reason, and even more - after enabling any overlay something weird
> > > > happens (LCD blinks for a 0.5 second, and then main plane comes
> > > > back, no overlay plane is visible), I'm getting following messages
> > > > on dmesg:
> > > > 
> > > > [   93.679574] overlay1fb_disable: timeout disabling overlay1
> > > > [   95.601537] BUG: Bad page state in process sh  pfn:a1b60
> > > > [   95.601645] page:c0456c00 count:0 mapcount:0 mapping:  (null)
> > > > index:0x0 [   95.601698] page flags: 0x200(arch_1)
> > > 
> > > Ouch.  PG_arch_1 is our 'dcache clean' bit, which we set to indicate
> > > that the page is clean.  This should never be set on a newly allocated
> > > page.
> > > 
> > > It's cleared by generic code whenever a page enters the free lists, so
> > > newly allocated pages should never have the bit set.
> > > 
> > > What your report means is that someone did DMA cache maintainence
> > > (specifically, unmapping the page), copied the page as a result of
> > > a COW fault, or called flush_dcache_page() on an already free'd page.
> > > 
> > > Maybe the pages were mapped into userspace, meanwhile someone free'd
> > > the pages.
> > > 
> > > And yes, I can see one way that this could happen:
> > > 
> > > - open overlay
> > > - map buffer
> > > - set framebuffer parameters
> > > 
> > >    (free's mapped buffer, leaving the mapped one in place, creates new
> > > 
> > > buffer) - close overlay
> > 
> > But I map framebuffer only after FBIOPUT_VSCREENINFO ioctl.
> > 
> > > Maybe another way:
> > > 
> > > static int overlayfb_release(struct fb_info *info, int user)
> > > {
> > > 
> > >         struct pxafb_layer *ofb = (struct pxafb_layer*) info;
> > >         
> > >         atomic_dec(&ofb->usage);
> > >         ofb->ops->disable(ofb);
> > >         
> > >         free_pages_exact(ofb->video_mem, ofb->video_mem_size);
> > > 
> > > So if two users open the overlay, both map it, and then one closes, the
> > > memory backing the overlay gets freed - meanwhile the other user still
> > > has it mapped etc.
> > 
> > Again, there's only one user - my app.
> 
> I didn't look any deeper so I can't say - but it feels very much like this
> kind of thing is responsible for your problem.
> 
> Things actually get worse if I look at the driver:
> 
> static int overlayfb_open(struct fb_info *info, int user)
> {
>         /* allow only one user at a time */
>         if (atomic_inc_and_test(&ofb->usage))
>                 return -EBUSY;
> 
> This is rubbish.  atomic_inc_and_test(v) does:
> 
> 	val = *v;
> 	val += 1;
> 	*v = val;
> 
> 	return val == 0;
> 
> So this doesn't stop multiple opens (and arguably you _can't_ prevent
> multiple opens anyway.)
> 
> Anyway, I think it would be worth fixing this, and seeing what the effect
> is.  Note that one of the side effects of one of this changes is that you
> only get one attempt at increasing the memory size in FBIOPUT_VSCREENINFO.
> Once the buffer has been allocated, we never change it - as there is no
> way of knowing whether it's mapped or not.
> 
> The other change is that we properly remove all references to the
> allocated memory when closing the device - which ensures that an open()
> followed by mmap() with no FBIOPUT_VSCREENINFO call will always fail.
> 
> Lastly, it does allow concurrent opens, but makes sure that we have the
> necessary number of closes before freeing the buffer.
> 
> Please give this a try and see whether it makes any difference for you.

Overlay still does not work, but now it does not crash system. Driver 
complains:

[   36.062235] overlay1fb_disable: timeout disabling overlay1

Regards
Vasily