linux-kernel - Re: [PATCH 3/4] arm64: Default to 32-bit ZONE

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Date:   Thu, 1 Oct 2020 18:19:22 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     robh+dt@...nel.org, hch@....de, robin.murphy@....com,
        devicetree@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] arm64: Default to 32-bit ZONE_DMA

On Thu, Oct 01, 2020 at 06:17:39PM +0200, Nicolas Saenz Julienne wrote:
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index e1a69a618832..3c3f462466eb 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -43,8 +43,6 @@
>  #include <asm/tlb.h>
>  #include <asm/alternative.h>
>  
> -#define ARM64_ZONE_DMA_BITS	30
> -
>  /*
>   * We need to be able to catch inadvertent references to memstart_addr
>   * that occur (potentially in generic code) before arm64_memblock_init()
> @@ -388,8 +386,14 @@ void __init arm64_memblock_init(void)
>  	early_init_fdt_scan_reserved_mem();
>  
>  	if (IS_ENABLED(CONFIG_ZONE_DMA)) {
> -		zone_dma_bits = ARM64_ZONE_DMA_BITS;
> -		arm64_dma_phys_limit = max_zone_phys(ARM64_ZONE_DMA_BITS);
> +		/*
> +		 * early_init_dt_scan() might alter zone_dma_bits based on the
> +		 * device's DT. Otherwise, have it cover the 32-bit address
> +		 * space.
> +		 */
> +		if (zone_dma_bits == ZONE_DMA_BITS_DEFAULT)
> +			zone_dma_bits = 32;
> +		arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);

So here we assume that if zone_dma_bits is 24, it wasn't initialised. I
think it may be simpler if we just set it in setup_machine_fdt() to 32
or 30 if RPi4. This way we don't have to depend on what the core kernel
sets.

-- 
Catalin