Re: [PATCH] New config added to handle 64-bit systems with 32-bit DMA support

[Robin Murphy <robin.murphy@arm.com>]

On 2024-06-19 10:17 am, Alberto Secondi wrote:
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> From: Alberto Secondi <alberto.secondi@abinsula.com>
> 
> The kernel assumes that 64-bit systems have 64-bit DMA support through
> CONFIG_ARCH_DMA_ADDR_T_64BIT. This is not always true; for example, several
> iMX8 systems (verified on iMX8MM and iMX8MP) have DMA with only 32-bit support.
> This results in several drivers requesting DMA_BIT_MASK(64), which causes
> malfunctions, particularly when systems have more than 3GB of DRAM (verified
> with the lan743x driver and iMX8 systems with 4GB of DRAM). Therefore, a new
> config ARCH_64BIT_HAS_DMA32_ONLY was added to manage 64-bit systems with 32-bit
> DMA, which adjusts DMA_BIT_MASK(n) accordingly.

No. If a system has devices naturally capable of >32-bit DMA, and memory 
at >32-bit system physical addresses, but only a 32-bit interconnect in 
between, that needs to be described properly in Devicetree/ACPI, not 
hacked around with completely non-portable kernel bodges.

Thanks,
Robin.

> Signed-off-by: Alberto Secondi <alberto.secondi@abinsula.com>
> Co-developed-by: Davide Salaris <davide.salaris@abinsula.com>
> ---
>   include/linux/dma-mapping.h | 4 ++++
>   kernel/dma/Kconfig          | 8 ++++++++
>   2 files changed, 12 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index f693aafe221f..629220a777e3 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -74,7 +74,11 @@
>    */
>   #define DMA_MAPPING_ERROR		(~(dma_addr_t)0)
>   
> +#ifdef CONFIG_ARCH_64BIT_HAS_DMA32_ONLY
> +#define DMA_BIT_MASK(n)	(((n) > 32) ? ((1ULL<<(32))-1) : ((1ULL<<(n))-1))
> +#else
>   #define DMA_BIT_MASK(n)	(((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> +#endif
>   
>   #ifdef CONFIG_DMA_API_DEBUG
>   void debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
> diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> index c06e56be0ca1..0a27eafed808 100644
> --- a/kernel/dma/Kconfig
> +++ b/kernel/dma/Kconfig
> @@ -36,6 +36,14 @@ config NEED_DMA_MAP_STATE
>   config ARCH_DMA_ADDR_T_64BIT
>   	def_bool 64BIT || PHYS_ADDR_T_64BIT
>   
> +config ARCH_64BIT_HAS_DMA32_ONLY
> +        bool "64bit System has DMA32 only"
> +        depends on ARCH_DMA_ADDR_T_64BIT
> +        default n
> +	help
> +	  This enables forcing the maximum DMA_BIT_MASK to 32 bits for
> +	  64-bit systems that have DMA support limited to 32 bits.
> +
>   config ARCH_HAS_DMA_SET_MASK
>   	bool
>