RE: [PATCHv2 2/4] mm: cma: Contiguous Memory Allocator added

[Marek Szyprowski]

Hello,

On Tuesday, July 27, 2010 2:09 PM Russell King - ARM Linux wrote:

> On Mon, Jul 26, 2010 at 04:40:30PM +0200, Michal Nazarewicz wrote:
> > +** Why is it needed?
> > +
> > +    Various devices on embedded systems have no scatter-getter and/or
> > +    IO map support and as such require contiguous blocks of memory to
> > +    operate.  They include devices such as cameras, hardware video
> > +    decoders and encoders, etc.
> 
> Yes, this is becoming quite a big problem - and many ARM SoCs suffer
> from the existing memory allocators being extremely inadequate for
> their use.
> 
> One of the areas I've been working on is sorting out the DMA coherent
> allocator so we don't violate the architecture requirements for ARMv6
> and ARMv7 CPUs (which basically prohibits multiple mappings of memory
> with different attributes.)
> 
> One of the ideas that I've thought about for this is to reserve an
> amount of contiguous memory at boot time to fill the entire DMA coherent
> mapping, marking the memory in the main kernel memory map as 'no access',
> and allocate directly from the DMA coherent region.
> 
> However, discussing this with people who have the problem you're trying
> to solve indicates that they do not want to set aside an amount of
> memory as they perceive this to be a waste of resources.

Assuming your board have only 128MB of physical memory (quite common case
for some embedded boards), leaving 16MB unused just for DMA coherent
area is a huge waste imho.

> This concern also applies to 'cma'.

Yes, we know. We plan to recover some of that 'wasted' memory by providing
a way to allocate some kind of virtual swap device on it. This is just an
idea, no related works has been started yet.

> 
> > +/*
> > + * Don't call it directly, use cma_alloc(), cma_alloc_from() or
> > + * cma_alloc_from_region().
> > + */
> > +dma_addr_t __must_check
> > +__cma_alloc(const struct device *dev, const char *kind,
> > +	    size_t size, dma_addr_t alignment);
> 
> Does this really always return DMA-able memory (memory which can be
> DMA'd to/from without DMA-mapping etc?)
> 
> As it returns a dma_addr_t, it's returning a cookie for the memory which
> will be suitable for writing directly to the device 'dev' doing the DMA.
> (NB: DMA addresses may not be the same as physical addresses, especially
> if the device is on a downstream bus.  We have ARM platforms which have
> different bus offsets.)
> 
> How does one obtain the CPU address of this memory in order for the CPU
> to access it?

Right, we did not cover such case. In CMA approach we tried to separate
memory allocation from the memory mapping into user/kernel space. Mapping
a buffer is much more complicated process that cannot be handled in a
generic way, so we decided to leave this for the device drivers. Usually
video processing devices also don't need in-kernel mapping for such
buffers at all.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center


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