Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

Catalin Marinas
2014-07-18T11:07:45Z

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();