Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

[Catalin Marinas <catalin.marinas@arm.com>]

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.

-- 
Catalin