RE: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings

Marek Szyprowski
2011-08-17T12:49:54Z

Hello,

On Wednesday, August 17, 2011 10:01 AM Marek Szyprowski wrote:
> On Tuesday, August 16, 2011 4:26 PM Arnd Bergmann wrote:
> > On Tuesday 16 August 2011, Russell King - ARM Linux wrote:
> > > On Tue, Aug 16, 2011 at 03:28:48PM +0200, Arnd Bergmann wrote:
> > > > Hmm, I don't remember the point about dynamically sizing the pool for
> > > > ARMv6K, but that can well be an oversight on my part. I do remember the
> > > > part about taking that memory pool from the CMA region as you say.
> > >
> > > If you're setting aside a pool of pages, then you have to dynamically
> > > size it. I did mention during our discussion about this.
> > >
> > > The problem is that a pool of fixed size is two fold: you need it to be
> > > sufficiently large that it can satisfy all allocations which come along
> > > in atomic context. Yet, we don't want the pool to be too large because
> > > then it prevents the memory being used for other purposes.
> > >
> > > Basically, the total number of pages in the pool can be a fixed size,
> > > but as they are depleted through allocation, they need to be
> > > re-populated from CMA to re-build the reserve for future atomic
> > > allocations. If the pool becomes larger via frees, then obviously
> > > we need to give pages back.
> >
> > Ok, thanks for the reminder. I must have completely missed this part
> > of the discussion.
> >
> > When I briefly considered this problem, my own conclusion was that
> > the number of atomic DMA allocations would always be very low
> > because they tend to be short-lived (e.g. incoming network packets),
> > so we could ignore this problem and just use a smaller reservation
> > size. While this seems to be true in general (see "git grep -w -A3
> > dma_alloc_coherent | grep ATOMIC"), there is one very significant
> > case that we cannot ignore, which is pci_alloc_consistent.
> >
> > This function is still called by hundreds of PCI drivers and always
> > does dma_alloc_coherent(..., GFP_ATOMIC), even for long-lived
> > allocations and those that are too large to be ignored.
> >
> > So at least for the case where we have PCI devices, I agree that
> > we need to have the dynamic pool.
>
> Do we really need the dynamic pool for the first version? I would like to
> know how much memory can be allocated in GFP_ATOMIC context. What are the
> typical sizes of such allocations?
>
> Maybe for the first version a static pool with reasonably small size
> (like 128KiB) will be more than enough? This size can be even board
> depended or changed with kernel command line for systems that really
> needs more memory.
>
> I noticed one more problem. The size of the CMA managed area must be
> the multiple of 16MiBs (MAX_ORDER+1). This means that the smallest CMA area
> is 16MiB. These values comes from the internals of the kernel memory
> management design and page blocks are the only entities that can be managed
> with page migration code.

I'm really sorry for the confusion. This 16MiB value worried me too much and
I've checked the code once again and found that this MAX_ORDER+1 value was
a miscalculation, which appeared in v11 of the patches. The true minimal
CMA area size is 8MiB for ARM architecture. I believe this shouldn't be
an issue for the current ARMv6+ based machines.

I've checked it with "mem=16M cma=8M" kernel arguments. System booted fine
and CMA area has been successfully created.

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