linux-kernel - Re: [PATCH v4 3/4] arm64: use both ZONE_DMA and ZONE

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Date:   Sun, 8 Sep 2019 22:27:11 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     hch@....de, wahrenst@....net, marc.zyngier@....com,
        robh+dt@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        Will Deacon <will@...nel.org>, f.fainelli@...il.com,
        robin.murphy@....com, linux-kernel@...r.kernel.org,
        mbrugger@...e.com, linux-rpi-kernel@...ts.infradead.org,
        phill@...pberrypi.org, m.szyprowski@...sung.com
Subject: Re: [PATCH v4 3/4] arm64: use both ZONE_DMA and ZONE_DMA32

On Fri, Sep 06, 2019 at 02:06:14PM +0200, Nicolas Saenz Julienne wrote:
> @@ -430,7 +454,7 @@ void __init arm64_memblock_init(void)
>  
>  	high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
>  
> -	dma_contiguous_reserve(arm64_dma32_phys_limit);
> +	dma_contiguous_reserve(arm64_dma_phys_limit ? : arm64_dma32_phys_limit);
>  }
>  
>  void __init bootmem_init(void)
> @@ -534,6 +558,7 @@ static void __init free_unused_memmap(void)
>  void __init mem_init(void)
>  {
>  	if (swiotlb_force == SWIOTLB_FORCE ||
> +	    max_pfn > (arm64_dma_phys_limit >> PAGE_SHIFT) ||
>  	    max_pfn > (arm64_dma32_phys_limit >> PAGE_SHIFT))
>  		swiotlb_init(1);

So here we want to initialise the swiotlb only if we need bounce
buffers. Prior to this patch, we assumed that swiotlb is needed if
max_pfn is beyond the reach of 32-bit devices. With ZONE_DMA, we need to
lower this limit to arm64_dma_phys_limit.

If ZONE_DMA is enabled, just comparing max_pfn with arm64_dma_phys_limit
is sufficient since the dma32 one limit always higher. However, if
ZONE_DMA is disabled, arm64_dma_phys_limit is 0, so we may initialise
swiotlb unnecessarily. I guess you need a similar check to the
dma_contiguous_reserve() above.

With that:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Unless there are other objections, I can queue this series for 5.5 in a
few weeks time (too late for 5.4).

-- 
Catalin