On (20/02/19 09:56), Hans Verkuil wrote:
> > +Memory Consistency Flags
> > +========================
> > +
> > +.. tabularcolumns:: |p{7.0cm}|p{2.2cm}|p{8.3cm}|
> > +
> > +.. cssclass:: longtable
> > +
> > +.. flat-table::
> > + :header-rows: 0
> > + :stub-columns: 0
> > + :widths: 3 1 4
> > +
> > + * .. _`V4L2_FLAG_MEMORY_NON_CONSISTENT`:
> > +
> > + - ``V4L2_FLAG_MEMORY_NON_CONSISTENT``
> > + - 0x00000001
> > + - vb2 buffer is allocated either in consistent (it will be automatically
> > + coherent between CPU and bus) or non-consistent memory. The latter
> > + can provide performance gains, for instance CPU cache sync/flush
> > + operations can be avoided if the buffer is accesed by the corresponding
> > + device only and CPU does not read/write to/from that buffer. However,
> > + this requires extra care from the driver -- it must guarantee memory
> > + consistency by issuing cache flush/sync when consistency is needed.
> > + If this flag is set V4L2 will attempt to allocate vb2 buffer in
> > + non-consistent memory. This flag is ignored if queue does not report
> > + :ret:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
>
> This flag only makes sense for the MMAP memory model, right? That should be
> documented and checked in the code.
Not all buffer allocators respect DMA attrs (V4L2_FLAG_MEMORY_NON_CONSISTENT
is a DMA attribute) even if we use MMAP memory model. Right? E.g. dma-cont
does, dma-sg - does not.
So the list is:
a) buffer allocated for MMAP I/O
b) buffer allocator which does not allocate pages from kernel page
allocator
c) queue that supports user space cache hints
If the driver does not set vb2_dma_contig_memops as q->mem_ops then
that queue should not have ->allow_cache_hints set. But even if it does,
the flag is ignored by the allocator.
So maybe the text can be:
+ ................... The flag takes effect only if the buffer is
+ used for :ref:`memory mapping <mmap>` I/O and the queue reports
+ :ref:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
-ss
