linux-kernel - Re: [PATCH] riscv: select DMA_DIRECT_REMAP by RISCV_ISA_SVPBMT and ERRATA_THEAD

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Message-ID: <199526de-4351-4fd7-8f6a-9e8dbf05c184@ghiti.fr>
Date: Mon, 17 Feb 2025 13:23:28 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Sergey Matyukevich <geomatsi@...il.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...belt.com>,
 Heiko Stuebner <heiko@...ech.de>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Conor Dooley <conor.dooley@...rochip.com>,
 Robin Murphy <robin.murphy@....com>,
 Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
 Geert Uytterhoeven <geert+renesas@...der.be>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] riscv: select DMA_DIRECT_REMAP by RISCV_ISA_SVPBMT and
 ERRATA_THEAD_MAE

Hi Sergey,

On 16/01/2025 18:29, Sergey Matyukevich wrote:
> Select DMA_DIRECT_REMAP for the RISC-V extensions that allow to set
> page-based memory types in PTEs according to the requested DMA
> attributes. This is the purpose of Svpbmt or XTheadMae extensions.
> Zicbom or XTheadCmo serve a different purpose, providing instructions
> to flush/invalidate cache blocks.
>
> Fixes: 381cae169853 ("riscv: only select DMA_DIRECT_REMAP from RISCV_ISA_ZICBOM and ERRATA_THEAD_PBMT")
>
> Signed-off-by: Sergey Matyukevich <geomatsi@...il.com>
> ---
>   arch/riscv/Kconfig        | 2 +-
>   arch/riscv/Kconfig.errata | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d4a7ca0388c0..a5dabb744009 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -603,6 +603,7 @@ config RISCV_ISA_SVPBMT
>   	depends on 64BIT && MMU
>   	depends on RISCV_ALTERNATIVE
>   	default y
> +	select DMA_DIRECT_REMAP


 From what I read, DMA_DIRECT_MAP relies on the ability to map pages 
uncached (pgprot_dmacoherent() here 
https://elixir.bootlin.com/linux/v6.13.2/source/kernel/dma/pool.c#L104). 
But CONFIG_RISCV_ISA_SVPBMT does not guarantee that the underlying 
platform supports svpbmt so to me it is wrong to select DMA_DIRECT_MAP, 
we would need some runtime check instead.


>   	help
>   	   Adds support to dynamically detect the presence of the Svpbmt
>   	   ISA-extension (Supervisor-mode: page-based memory types) and
> @@ -787,7 +788,6 @@ config RISCV_ISA_ZICBOM
>   	depends on RISCV_ALTERNATIVE
>   	default y
>   	select RISCV_DMA_NONCOHERENT


And in the same way, we should not enable RISCV_DMA_NONCOHERENT since 
CONFIG_RISCV_ISA_ZICBOM does guarantee the presence of zicbom. Because 
then in mm/dma-noncoherent.c, the cache flush operations are nops.

Or am I missing something?

Thanks,

Alex


> -	select DMA_DIRECT_REMAP
>   	help
>   	   Adds support to dynamically detect the presence of the ZICBOM
>   	   extension (Cache Block Management Operations) and enable its
> diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> index 2acc7d876e1f..3bcae5bd3231 100644
> --- a/arch/riscv/Kconfig.errata
> +++ b/arch/riscv/Kconfig.errata
> @@ -86,6 +86,7 @@ config ERRATA_THEAD_MAE
>   	bool "Apply T-Head's memory attribute extension (XTheadMae) errata"
>   	depends on ERRATA_THEAD && 64BIT && MMU
>   	select RISCV_ALTERNATIVE_EARLY
> +	select DMA_DIRECT_REMAP
>   	default y
>   	help
>   	  This will apply the memory attribute extension errata to handle the
> @@ -96,7 +97,6 @@ config ERRATA_THEAD_MAE
>   config ERRATA_THEAD_CMO
>   	bool "Apply T-Head cache management errata"
>   	depends on ERRATA_THEAD && MMU
> -	select DMA_DIRECT_REMAP
>   	select RISCV_DMA_NONCOHERENT
>   	select RISCV_NONSTANDARD_CACHE_OPS
>   	default y