From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from verein.lst.de (verein.lst.de [213.95.11.211]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2C1B82FB6 for ; Wed, 19 May 2021 06:53:57 +0000 (UTC) Received: by verein.lst.de (Postfix, from userid 2407) id C1D4B67373; Wed, 19 May 2021 08:53:52 +0200 (CEST) Date: Wed, 19 May 2021 08:53:52 +0200 From: Christoph Hellwig To: Drew Fustini Cc: Guo Ren , Christoph Hellwig , Anup Patel , Palmer Dabbelt , wefu@redhat.com, lazyparser@gmail.com, linux-riscv , Linux Kernel Mailing List , linux-arch , linux-sunxi@lists.linux.dev, Guo Ren , paul.walmsley@sifive.com, Nick Kossifidis , Benjamin Koch , Matteo Croce , Wei Fu Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support Message-ID: <20210519065352.GA31590@lst.de> References: <1621400656-25678-1-git-send-email-guoren@kernel.org> <20210519052048.GA24853@lst.de> <20210519064435.GA3076809@x1> X-Mailing-List: linux-sunxi@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210519064435.GA3076809@x1> User-Agent: Mutt/1.5.17 (2007-11-01) On Tue, May 18, 2021 at 11:44:35PM -0700, Drew Fustini wrote: > This patch series looks like it might be useful for the StarFive JH7100 > [1] [2] too as it has peripherals on a non-coherent interconnect. GMAC, > USB and SDIO require that the L2 cache must be manually flushed after > DMA operations if the data is intended to be shared with U74 cores [2]. Not too much, given that the SiFive lineage CPUs have an uncached window, that is a totally different way to allocate uncached memory. > There is the RISC-V Cache Management Operation, or CMO, task group [3] > but I am not sure if that can help the SoC's that have already been > fabbed like the the D1 and the JH7100. It does, because unimplemented instructions trap into M-mode, where they can be emulated. Or to summarize things. Non-coherent DMA (and not coherent as title in this series) requires two things: 1) allocating chunks of memory that is marked as not cachable 2) instructions to invalidate and/or writeback cache lines none of which currently exists in RISV-V. Hacking vendor specific cruft into the kernel doesn't scale, as shown perfectly by this series which requires to hard code vendor specific non-standardized extensions in a kernel that makes it specific to that implementation. What we need to do is to standardize a way to do this properly, and then after that figure out a way to quirk in non-compliant implementations in a way that does not harm the general kernel.