Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

[Catalin Marinas]

On Tue, Jun 24, 2014 at 03:38:34PM +0100, Mark Salter wrote:
> On Tue, 2014-06-24 at 15:14 +0100, Catalin Marinas wrote:
> > On Mon, Jun 23, 2014 at 02:17:03PM +0100, Mark Salter wrote:
> > > On Mon, 2014-06-23 at 12:09 +0100, Catalin Marinas wrote:
> > > > My proposal (in the absence of any kind of description) is to still
> > > > create a ZONE_DMA if we have DMA memory below 32-bit, otherwise just add
> > > > everything (>32-bit) to ZONE_DMA. Basically an extension from your CMA
> > > > patch, make dma_phys_limit static in that file and set it to
> > > > memblock_end_of_DRAM() if no 32-bit DMA. Re-use it in the
> > > > zone_sizes_init() function for ZONE_DMA (maybe with a pr_info for no
> > > > 32-bit only DMA zone).
> > > 
> > > There's a performance issue with all memory being in ZONE_DMA. It means
> > > all normal allocations will fail on ZONE_NORMAL and then have to fall
> > > back to ZONE_DMA. It would be better to put some percentage of memory
> > > in ZONE_DMA.
> > 
> > Is the performance penalty real or just theoretical? I haven't run any
> > benchmarks myself.
> 
> It is real insofar as you must eat cycles eliminating ZONE_NORMAL from
> consideration in the page allocation hot path. How much that really
> costs, I don't know. But it seems like it could be easily avoided by
> limiting ZONE_DMA size. Is there any reason it needs to be larger than
> 4GiB?

Basically ZONE_DMA should allow a 32-bit dma mask. When memory starts
above 4G, in the absence of an IOMMU, it is likely that 32-bit devices
get some offset for the top bits to be able to address the bottom of the
memory. The problem is that dma_to_phys() that early in the kernel has
no idea about DMA offsets until later (they can be specified in DT per
device).

The patch belows tries to guess a DMA offset and use the bottom 32-bit
of the DRAM as ZONE_DMA.

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