Re: [PATCH v2] media: davinci: vpif: align the buffers size to page page size boundary

[Laurent Pinchart]

On Thursday 18 April 2013 11:08:28 Mauro Carvalho Chehab wrote:
> Em Thu, 18 Apr 2013 15:22:16 +0200 Laurent Pinchart escreveu:
> > On Thursday 18 April 2013 08:35:47 Mauro Carvalho Chehab wrote:
> > > Em Thu, 18 Apr 2013 08:21:21 -0300 Mauro Carvalho Chehab escreveu:
> > > > Em Thu, 18 Apr 2013 10:17:14 +0530 Prabhakar Lad escreveu:
> > > > > On Tue, Apr 16, 2013 at 4:48 PM, Laurent Pinchart wrote:
> > > > > > Hi Prabhakar,
> > > > 
> > > > ...
> > > > 
> > > > > >> *nbuffers = config_params.min_numbuffers;
> > > > > >> 
> > > > > >>       *nplanes = 1;
> > > > > >> 
> > > > > >> +     size = PAGE_ALIGN(size);
> > > > > > 
> > > > > > I wonder if that's the best fix.
> > > > > > The queue_setup operation is supposed to return the size required
> > > > > > by the driver for each plane. Depending on the hardware
> > > > > > requirements, that size might not be a multiple of the page size.
> > > > > > 
> > > > > > As we can't mmap() a fraction of a page, the allocated plane size
> > > > > > needs to be rounded up to the next page boundary to allow mmap()
> > > > > > support. The dma-contig and dma-sg allocators already do so in
> > > > > > their alloc operation, but the vmalloc allocator doesn't.
> > > > > > 
> > > > > > The recent "media: vb2: add length check for mmap" patch verifies
> > > > > > that the mmap() size requested by userspace doesn't exceed the
> > > > > > buffer size. As the mmap() size is rounded up to the next page
> > > > > > boundary the check will fail for buffer sizes that are not
> > > > > > multiple of the page size.
> > > > > > 
> > > > > > Your fix will not result in overallocation (as the allocator
> > > > > > already rounds the size up), but will prevent the driver from
> > > > > > importing a buffer large enough for the hardware but not rounded
> > > > > > up to the page size.
> > > > > > 
> > > > > > A better fix might be to round up the buffer size in the buffer
> > > > > > size check at mmap() time, and fix the vmalloc allocator to round
> > > > > > up the size. That the allocator, not drivers, is responsible for
> > > > > > buffer size alignment should be documented in videobuf2-core.h.
> > > > > 
> > > > > Do you plan to post a patch fixing it as per Laurent's suggestion ?
> > > > 
> > > > I agree with Laurent: page size roundup should be done at VB2 core
> > > > code, for memory allocated there, and not at driver's level. Yet,
> > > > looking at VB2 code, it already does page size align at
> > > > __setup_offsets(), but it doesn't do if for the size field; just for
> > > > the offset.
> > > > 
> > > > The adjusted size should be stored at the VB2 size field, and the
> > > > check for buffer overflow, added on changeset
> > > > 068a0df76023926af958a336a78bef60468d2033 should be kept.
> > > > 
> > > > IMO, it also makes sense to enforce that the USERPTR memory is
> > > > multiple of the page size, as otherwise the DMA transfer may overwrite
> > > > some area that is outside the allocated range. So, the size from
> > > > USERPTR should be round down.
> > 
> > I don't think that's needed. You can transfer a number of bytes not
> > multiple of the page size using DMA. This is true for DMABUF as well, an
> > imported buffer might have a size not aligned on a page boundary.
> 
> Are you sure that, on all supported archs/buses, the DMA transfers are
> byte-aligned?

They're most probably not byte-aligned, but they're not page-aligned either. 
That's something the driver should know, and that information will be passed 
throught the plane sizes by the queue_setup operation.

> > > > That change, however, will break userspace, as it uses the picture
> > > > sizeimage to allocate the buffers. So, sizeimage needs to be PAGE_SIZE
> > > > roundup before passing it to userspace.
> > > > 
> > > > Instead of modifying all drivers, the better seems to patch
> > > > v4l_g_fmt() and v4l_try_fmt() to return a roundup value for sizeimage.
> > > > As usual, uvcvideo requires a separate patch, because it doesn't use
> > > > vidio_ioctl2.
> > > 
> > > Hmm... PAGE_SIZE alignment is not needed on all places. It is needed
> > > only when DMA is done directly into the buffer, e. g. videobuf2-dma-
> > > contig and videobuf2-dma-sg.
> > > 
> > > It means that we'll need an extra function for the VB2 memory allocation
> > > drivers to do do the memory-dependent roundups, and a new ancillary
> > > function at VB2 core for the VB2 clients to call to round sizeimage if
> > > needed.
> > 
> > Can't we just round the size up at allocation time and when checking the
> > size in mmap() ? That's a simple fix, local to vb2, and won't require new
> > vb2 memops.
> 
> That's not needed for videobuf2-vmalloc.

Yes it is, as mmap() works on a page basis. Every buffer that will be mmap()ed 
to userspace needs to be aligned to the page size.

> We shouldn't bloat the core VB2 with memops specific stuff. Ok, in this
> specific case, this is a simple trivial patch, so perhaps we could do it
> there.

-- 
Regards,

Laurent Pinchart

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