linux-kernel - Re: [PATCH] arm64: fix CONFIG_ZONE_DMA on systems with no 32-bit addressable DRAM

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Date:	Tue, 24 Jun 2014 17:26:02 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mark Salter <msalter@...hat.com>
cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: fix CONFIG_ZONE_DMA on systems with no 32-bit
 addressable DRAM

On Mon, 23 Jun 2014, Mark Salter wrote:

> 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. This breaks CMA on platforms with no 32-bit addressable DRAM.
> This patch checks to make sure there is 32-bit addressable DRAM before
> setting the 32-bit limit. If there is none, no limit is placed on the
> CMA buffer. This allows a single kernel (with CONFIG_ZONE_DMA defined)
> to support platforms requiring the 32-bit limit and platforms with no
> 32-bit limit.
> 
> Signed-off-by: Mark Salter <msalter@...hat.com>
> ---
>  arch/arm64/mm/init.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index c5415e2..2925576 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -149,8 +149,17 @@ void __init arm64_memblock_init(void)
>  	early_init_fdt_scan_reserved_mem();
>  
>  	/* 4GB maximum for 32-bit only capable devices */
> -	if (IS_ENABLED(CONFIG_ZONE_DMA))
> +	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
>  		dma_phys_limit = dma_to_phys(NULL, DMA_BIT_MASK(32)) + 1;
> +		/*
> +		 * If platform doesn't have DRAM within the dma_phys_limit,
> +		 * remove the limit altogether. This allows one kernel (with
> +		 * CONFIG_ZONE_DMA defined) to support platforms with 32-bit
> +		 * only devices and platforms with no 32-bit DRAM.
> +		 */
> +		if (dma_phys_limit <= memblock_start_of_DRAM())
> +			dma_phys_limit = 0;
> +	}
>  	dma_contiguous_reserve(dma_phys_limit);
>  
>  	memblock_allow_resize();

It is a shame that we have very similar logic, all dependent on 
memblock_start_of_DRAM(), in both zone_sizes_init() and 
arm64_memblock_init() and the latter having to be called before the 
former.  I assume there's no way to make this cleaner in the bootstrap 
code so the dma_to_phys(NULL, DMA_BIT_MASK(32)) + 1 limit can be handled 
properly in a single location?
--
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