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

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Message-ID: <20250117095832.GB2585@lst.de>
Date: Fri, 17 Jan 2025 10:58:32 +0100
From: Christoph Hellwig <hch@....de>
To: Sergey Matyukevich <geomatsi@...il.com>
Cc: Christoph Hellwig <hch@....de>, 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 11:38:47AM +0300, Sergey Matyukevich wrote:
> > 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.

Because they need DMA_DIRECT_REMAP to implement DMA coherent.

> 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.

But Svpbmt does not imply that you even need DMA_DIRECT_REMAP.

Are you tying to solve a problem here?  If so can you explain it?