PXA270 overlay problem

[Vasily Khoruzhick]

On Monday 31 January 2011 20:35:05 Vasily Khoruzhick wrote:
> 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

I got it working and now understand why this bug happens. pxafb driver tries 
to enable overlay (and set its params, allocates memory) in fb_set_par 
callback, but it will be called only if vscreeninfo is changed. But it 
disables overlay (and frees memory) on release. I'll will send a patch as soon 
as I fix it.

Regards
Vasily