Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

Anup Patel
2014-07-18T11:58:34Z

Hi Catalin,

On 18 July 2014 16:37, Catalin Marinas <catalin.marinas@arm.com> 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>
> ---
> arch/arm64/mm/init.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 7f68804814a1..160bbaa4fc78 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -60,6 +60,17 @@ static int __init early_initrd(char *p)
> early_param("initrd", early_initrd);
> #endif
>
> +/*
> + * Return the maximum physical address for ZONE_DMA (DMA_BIT_MASK(32)). It
> + * currently assumes that for memory starting above 4G, 32-bit devices will
> + * use a DMA offset.
> + */
> +static phys_addr_t max_zone_dma_phys(void)
> +{
> + phys_addr_t offset = memblock_start_of_DRAM() & GENMASK_ULL(63, 32);
> + return min(offset + (1ULL << 32), memblock_end_of_DRAM());
> +}
> +
> static void __init zone_sizes_init(unsigned long min, unsigned long max)
> {
> struct memblock_region *reg;
> @@ -70,9 +81,7 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
>
> /* 4GB maximum for 32-bit only capable devices */
> if (IS_ENABLED(CONFIG_ZONE_DMA)) {
> - unsigned long max_dma_phys =
> - (unsigned long)(dma_to_phys(NULL, DMA_BIT_MASK(32)) + 1);
> - max_dma = max(min, min(max, max_dma_phys >> PAGE_SHIFT));
> + max_dma = PFN_DOWN(max_zone_dma_phys());
> zone_size[ZONE_DMA] = max_dma - min;
> }
> zone_size[ZONE_NORMAL] = max - max_dma;
> @@ -142,7 +151,7 @@ void __init arm64_memblock_init(void)
>
> /* 4GB maximum for 32-bit only capable devices */
> if (IS_ENABLED(CONFIG_ZONE_DMA))
> - dma_phys_limit = dma_to_phys(NULL, DMA_BIT_MASK(32)) + 1;
> + dma_phys_limit = max_zone_dma_phys();
> dma_contiguous_reserve(dma_phys_limit);
>
> memblock_allow_resize();
>
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Linux-3.16-rcX is broken on X-Gene Mustang because
on X-Gene Mustang the DRAM starts at 0x4000000000.

I have tested your patch and the original patch from
this thread. Both patches fixes the issue for X-Gene
Mustang and Linux-3.16-rc5 happily boots on X-Gene.

Can you to send your patch as Linux-3.16-rcX fix?

For your patch, you can have:
Tested-by: Anup Patel <anup.patel@linaro.org>

Thanks,
Anup