[PATCH 03/27] libcamera: dma_buf_allocator: Favour udmabuf over cma heap allocations

[Kieran Bingham]

Quoting Laurent Pinchart (2025-04-22 23:22:31)
> Hi Bryan,
> 
> Thank you for the patch.
> 
> On Tue, Apr 22, 2025 at 10:58:56PM +0100, Bryan O'Donoghue wrote:
> > When /dev/dma_heap/linux,cma or /dev/dma_heap/system exist currently we
> > favour allocation from this type of heap over /dev/udmabuf.
> > 
> > We ought to favour udmabuf though
> > 
> > - udmabuf is the preferred method by various distros for security reasons
> > - Contiguous memory is a scarce resource
> > 
> > Change the ordering of the allocator lookup so that the udmabuf lookup
> > comes first.
> 
> This means that on a system where CMA allocation is possible from
> userspace, the buffers allocated by libcamera for the virtual pipeline
> handler and ISP won't be shareable without copies with a consumer that
> requires contiguous memory (e.g. most KMS devices on Arm platforms).
> Isn't that an issue ? It seems to even count as a regression.
> 

On my X13s, everything works better for me on udmabuf. If we try to use
the cma allocator things fail.

That /could/ be a separate topic/issue - but at least might explain some
of the rationale behind this.

I usually end up with something like this on my system.


> > Fixes: ea4baaacc325 ("libcamera: DmaBufAllocator: Support allocating from /dev/udmabuf")
> > Signed-off-by: Bryan O'Donoghue <bryan.odonoghue at linaro.org>
> > ---
> >  src/libcamera/dma_buf_allocator.cpp | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/libcamera/dma_buf_allocator.cpp b/src/libcamera/dma_buf_allocator.cpp
> > index d8c62dd6..722ffd46 100644
> > --- a/src/libcamera/dma_buf_allocator.cpp
> > +++ b/src/libcamera/dma_buf_allocator.cpp
> > @@ -45,10 +45,10 @@ static constexpr std::array<DmaBufAllocatorInfo, 4> providerInfos = { {
> >        * /dev/dma_heap/linux,cma is the CMA dma-heap. When the cma heap size is
> >        * specified on the kernel command line, this gets renamed to "reserved".
> >        */
> > +     { DmaBufAllocator::DmaBufAllocatorFlag::UDmaBuf, "/dev/udmabuf" },
> >       { DmaBufAllocator::DmaBufAllocatorFlag::CmaHeap, "/dev/dma_heap/linux,cma" },
> >       { DmaBufAllocator::DmaBufAllocatorFlag::CmaHeap, "/dev/dma_heap/reserved" },
> >       { DmaBufAllocator::DmaBufAllocatorFlag::SystemHeap, "/dev/dma_heap/system" },
> > -     { DmaBufAllocator::DmaBufAllocatorFlag::UDmaBuf, "/dev/udmabuf" },
> >  } };
> >  
> >  LOG_DEFINE_CATEGORY(DmaBufAllocator)
> 
> -- 
> Regards,
> 
> Laurent Pinchart