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

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Message-ID: <Z4oXF2ewizIIk1cL@curiosity>
Date: Fri, 17 Jan 2025 11:38:47 +0300
From: Sergey Matyukevich <geomatsi@...il.com>
To: Christoph Hellwig <hch@....de>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Palmer Dabbelt <palmer@...belt.com>,
	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>
Subject: Re: [PATCH] riscv: select DMA_DIRECT_REMAP by RISCV_ISA_SVPBMT and
 ERRATA_THEAD_MAE

On Fri, Jan 17, 2025 at 08:52:38AM +0100, Christoph Hellwig wrote:
> On Thu, Jan 16, 2025 at 08:29:35PM +0300, 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.
> 
> Please explain what this is supposed to solve, because the above
> explanation dosn't make any sense.  DMA_DIRECT_REMAP is one
> of the implementations supporting dma coherent allocatiosn for
> non-coherent devices.  So selecting it from something that
> just keyes off support for an extension, but not the dma
> implementation is wrong.

Now DMA_DIRECT_REMAP is selected either by Zicbom (standard) or XTheadCmo
(vendor) RISC-V extensions. However neither of them can help to implement
DMA_DIRECT_REMAP on RISC-V. So selection of DMA_DIRECT_REMAP has been
moved in Kconfigs to Svpbmt and XTheadMae extensions.