Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable

[Catalin Marinas]

On Mon, Jun 23, 2014 at 06:05:24AM +0100, Mark Salter wrote:
> Commit 19e7640d1f (arm64: Replace ZONE_DMA32 with ZONE_DMA)
> moves support for 32-bit DMA addressing into ZONE_DMA and renames
> CONFIG_ZONE_DMA32 to CONFIG_ZONE_DMA.
> 
> Commit 2d5a5612bc (arm64: Limit the CMA buffer to 32-bit if ZONE_DMA)
> forces the CMA buffer to be 32-bit addressable if CONFIG_ZONE_DMA is
> defined.
> 
> These two patches pose a problem for platforms which have no 32-bit
> addressable DRAM.

It's actually the bus/dma address that needs to be 32-bit rather than
the DRAM as seen by the CPU (which can be beyond 32-bit like the
Keystone platform).

> If CONFIG_ZONE_DMA is turned on for such platforms,
> CMA is unable to reserve a buffer for allocations. CONFIG_ZONE_DMA is
> not user settable however, so there is no way to turn it off without
> editing arch/arm64/Kconfig. Even if one edits Kconfig to turn off
> CONFIG_ZONE_DMA, the kernel fails to build with such a config:
> 
>   arch/arm64/mm/init.c: In function âzone_sizes_initâ:
>   arch/arm64/mm/init.c:76:13: error: âZONE_DMAâ undeclared (first use in this function)
>      zone_size[ZONE_DMA] = max_dma - min;
>                ^
> 
> This patch makes CONFIG_ZONE_DMA user settable and fixes the kernel
> build when it is turned off.

The reason I left CONFIG_ZONE_DMA not user settable is because on arm64
we aim for single Image by default. So ZONE_DMA would most likely be
needed on at least one platform. Even with ZONE_DMA it's just a guess
without proper system topology description. dma_to_phys() just takes a
NULL argument for device when trying to guess the physical address for a
32-bit DMA mask (and 32-bit devices may have some physical offset wired
already).

With the CMA fix, does the kernel work properly with a zero sized DMA
zone? Any GFP_DMA allocations will fail, including the swiotlb bounce
buffer. It may be fine if no devices require 32-bit DMA memory but I
wonder whether on such platforms it would be better to simply add
all the memory to ZONE_DMA.

Which gets us back to finding a way for describing such system topology
in hardware. We may be able to find a way with DT but unlikely for ACPI.

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).

-- 
Catalin
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