Re: [PATCH] arm64: Remove arm64_dma32_phys_limit and its uses

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jan 07, 2021 at 06:40:32PM +0000, Catalin Marinas wrote:
> With the introduction of a dynamic ZONE_DMA range based on DT or IORT
> information, there's no need for CMA allocations from the wider
> ZONE_DMA32 since on most platforms ZONE_DMA will cover the 32-bit
> addressable range. Remove the arm64_dma32_phys_limit and set
> arm64_dma_phys_limit to cover the smallest DMA range required on the
> platform. CMA allocation and crashkernel reservation now goes in
> the dynamically sized ZONE_DMA.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> Cc: Chen Zhou <chenzhou10@huawei.com>
> ---
> 
> This patch goes on top of the ARCH_LOW_ADDRESS_LIMIT fix from Nicolas,
> already in the arm64 for-next/fixes which fixes a 5.5 issue with
> !CONFIG_ZONE_DMA. The changes here depend on the patches in 5.11-rc1.
> While they look mostly like clean-ups, they still fix a potential issue
> with !CONFIG_ZONE_DMA32 configurations where arm64_dma32_phys_limit
> would be equal to PHYS_MASK but we still have a limited ZONE_DMA. In
> addition, it now allows proper CMA and crashkernel reservations for
> RPi4.
[...]
> @@ -394,16 +399,9 @@ void __init arm64_memblock_init(void)
>  
>  	early_init_fdt_scan_reserved_mem();
>  
> -	if (IS_ENABLED(CONFIG_ZONE_DMA32))
> -		arm64_dma32_phys_limit = max_zone_phys(32);
> -	else
> -		arm64_dma32_phys_limit = PHYS_MASK + 1;
> -
>  	reserve_elfcorehdr();
>  
>  	high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
> -
> -	dma_contiguous_reserve(arm64_dma32_phys_limit);
>  }
>  
>  void __init bootmem_init(void)
> @@ -438,6 +436,11 @@ void __init bootmem_init(void)
>  	sparse_init();
>  	zone_sizes_init(min, max);
>  
> +	/*
> +	 * Reserve the CMA area after arm64_dma_phys_limit was initialised.
> +	 */
> +	dma_contiguous_reserve(arm64_dma_phys_limit);

Prior to this patch, disabling CONFIG_ZONE_DMA32 leads to CMA allocation
from the whole RAM as arm64_dma32_phys_limit becomes PHYS_MASK+1. With
this patch (which I plan to merge in 5.11-rc4), we limit the CMA
allocation to the 32-bit addressable RAM (if any) even if we don't
describe anything in DT or IORT since ZONE_DMA is capped at 32-bit. We
could relax this so that ZONE_DMA can be expanded beyond 32-bit with
ZONE_DMA32 disabled and no DT/IORT information. Is there a real use-case
for such configuration? We might as well make ZONE_DMA depend on
ZONE_DMA32 (though given the EXPERT dependency, people should know what
they are doing...).

An alternative would be to change max_zone_phys() to avoid the U32_MAX
cap if ZONE_DMA32 is disabled but I don't think it's worth as I don't
see much point in a kernel config with ZONE_DMA enabled and ZONE_DMA32
disabled.

-- 
Catalin

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