Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

Mark Salter
2014-07-21T21:57:18Z

On Fri, 2014-07-18 at 12:07 +0100, Catalin Marinas wrote:
> 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.
>
> -------8<-----------------------
>
> From 133656f8378dbb838ad5f12ea29aa9303d7ca922 Mon Sep 17 00:00:00 2001
> From: Catalin Marinas <catalin.marinas@arm.com>
> Date: Fri, 18 Jul 2014 11:54:37 +0100
> Subject: [PATCH] arm64: Create non-empty ZONE_DMA when DRAM starts above 4GB
>
> ZONE_DMA is created to allow 32-bit only devices to access memory in the
> absence of an IOMMU. On systems where the memory starts above 4GB, it is
> expected that some devices have a DMA offset hardwired to be able to
> access the bottom of the memory. Linux currently supports DT bindings
> for the DMA offsets but they are not (easily) available early during
> boot.
>
> This patch tries to guess a DMA offset and assumes that ZONE_DMA
> corresponds to the 32-bit mask above the start of DRAM.
>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Mark Salter <msalter@redhat.com>
> ---

Tested-by: Mark Salter <msalter@redhat.com>

Thanks.