Re: [PATCH 3/4] arm64: Default to 32-bit ZONE_DMA

[Catalin Marinas]

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 
>  #include 
>  
> -#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
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[PATCH 3/4] arm64: Default to 32-bit ZONE_DMA

[Nicolas Saenz Julienne]

The Raspberry Pi 4 needs two DMA zones as some of its devices can only
DMA into the 30-bit physical address space. We solved that by creating
an extra ZONE_DMA covering the 30-bit. It turns out that creating extra
zones unnecessarily broke Kdump on large systems. So default to a single
32-bit wide ZONE_DMA and only define both zones if running on RPi4.

Signed-off-by: Nicolas Saenz Julienne 
---
 arch/arm64/mm/init.c | 12 
 1 file changed, 8 insertions(+), 4 deletions(-)

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 
 #include 
 
-#define ARM64_ZONE_DMA_BITS30
-
 /*
  * 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);
}
 
if (IS_ENABLED(CONFIG_ZONE_DMA32))
-- 
2.28.0

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