Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA_NONCOHERENT is not set

From: Conor Dooley <conor@kernel.org>
To: Maxim Kochetkov <fido_max@inbox.ru>, hch@lst.de
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	robh@kernel.org, jiaxun.yang@flygoat.com, mpe@ellerman.id.au,
	aou@eecs.berkeley.edu, palmer@dabbelt.com,
	paul.walmsley@sifive.com
Subject: Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA_NONCOHERENT is not set
Date: Thu, 21 Dec 2023 20:29:32 +0000	[thread overview]
Message-ID: <20231221-discount-decade-e306e5878c46@spud> (raw)
In-Reply-To: <20231221185152.327231-1-fido_max@inbox.ru>

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+ Christoph

I don't think this patch is correct. Regardless of whether we support
cache management operations, DMA is assumed to be coherent unless
peripherals etc are specified to otherwise in DT (or however ACPI deals
with that kind of thing).

What problem are you trying to solve here?

On Thu, Dec 21, 2023 at 09:51:52PM +0300, Maxim Kochetkov wrote:
> Not all the RISCV are DMA coherent by default. 

What is a "RISCV"? I believe this sentence should be "not all RISC-V
systems are DMA coherent." but that is provided for by the
"dma-noncoherent" property, set for peripherals (or buses) that are not
DMA coherent.

> Moreover we have
> RISCV_DMA_NONCOHERENT option.
> So set ARCH_DMA_DEFAULT_COHERENT only when RISCV_DMA_NONCOHERENT is not set
> 
> Fixes: c00a60d6f4a1 ("of: address: always use dma_default_coherent for default coherency")
> Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
> ---
>  arch/riscv/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d6824bec2c00..111c5d92d503 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -14,7 +14,7 @@ config RISCV
>  	def_bool y
>  	select ACPI_GENERIC_GSI if ACPI
>  	select ACPI_REDUCED_HARDWARE_ONLY if ACPI
> -	select ARCH_DMA_DEFAULT_COHERENT
> +	select ARCH_DMA_DEFAULT_COHERENT if !RISCV_DMA_NONCOHERENT

I think this is actually buggy, for things like distro kernels
RISCV_DMA_COHERENT will always be set, but those kernels are expected
to be used on systems that are cache coherent also.

Thanks,
Conor.

>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
>  	select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
>  	select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
> -- 
> 2.40.1
> 
> 
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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