Re: [Linaro-mm-sig] [PATCH 08/10] mm: cma: Contiguous Memory Allocator added

[Larry Bassel <lbassel@codeaurora.org>]

On 15 Jun 11 10:36, Marek Szyprowski wrote:
> Hello,
> 
> On Tuesday, June 14, 2011 10:42 PM Arnd Bergmann wrote:
> 
> > On Tuesday 14 June 2011 20:58:25 Zach Pfeffer wrote:
> > > I've seen this split bank allocation in Qualcomm and TI SoCs, with
> > > Samsung, that makes 3 major SoC vendors (I would be surprised if
> > > Nvidia didn't also need to do this) - so I think some configurable
> > > method to control allocations is necessarily. The chips can't do
> > > decode without it (and by can't do I mean 1080P and higher decode is
> > > not functionally useful). Far from special, this would appear to be
> > > the default.

We at Qualcomm have some platforms that have memory of different
performance characteristics, some drivers will need a way of
specifying that they need fast memory for an allocation (and would prefer
an error if it is not available rather than a fallback to slower
memory). It would also be bad if allocators who don't need fast
memory got it "accidentally", depriving those who really need it.

> > 
> > Thanks for the insight, that's a much better argument than 'something
> > may need it'. Are those all chips without an IOMMU or do we also
> > need to solve the IOMMU case with split bank allocation?
> > 
> > I think I'd still prefer to see the support for multiple regions split
> > out into one of the later patches, especially since that would defer
> > the question of how to do the initialization for this case and make
> > sure we first get a generic way.
> > 
> > You've convinced me that we need to solve the problem of allocating
> > memory from a specific bank eventually, but separating it from the
> > one at hand (contiguous allocation) should help getting the important
> > groundwork in at first.
> >
> > The possible conflict that I still see with per-bank CMA regions are:
> > 
> > * It completely destroys memory power management in cases where that
> >   is based on powering down entire memory banks.
> 
> I don't think that per-bank CMA regions destroys memory power management
> more than the global CMA pool. Please note that the contiguous buffers
> (or in general dma-buffers) right now are unmovable so they don't fit
> well into memory power management.

We also have platforms where a well-defined part of the memory
can be powered off, and other parts can't (or won't). We need a way
to steer the place allocations come from to the memory that won't be
turned off (so that CMA allocations are not an obstacle to memory
hotremove).

> 
> Best regards
> -- 
> Marek Szyprowski
> Samsung Poland R&D Center
> 
> 
> 
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Larry Bassel

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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