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[121.45.221.81]) by smtp.gmail.com with ESMTPSA id e11sm12823809pfj.95.2020.03.23.01.58.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Mar 2020 01:58:06 -0700 (PDT) Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device To: Christoph Hellwig References: <20200320141640.366360-1-hch@lst.de> <20200320141640.366360-2-hch@lst.de> <2f31d0dd-aa7e-8b76-c8a1-5759fda5afc9@ozlabs.ru> <20200323083705.GA31245@lst.de> From: Alexey Kardashevskiy Autocrypt: addr=aik@ozlabs.ru; keydata= mQINBE+rT0sBEADFEI2UtPRsLLvnRf+tI9nA8T91+jDK3NLkqV+2DKHkTGPP5qzDZpRSH6mD EePO1JqpVuIow/wGud9xaPA5uvuVgRS1q7RU8otD+7VLDFzPRiRE4Jfr2CW89Ox6BF+q5ZPV /pS4v4G9eOrw1v09lEKHB9WtiBVhhxKK1LnUjPEH3ifkOkgW7jFfoYgTdtB3XaXVgYnNPDFo PTBYsJy+wr89XfyHr2Ev7BB3Xaf7qICXdBF8MEVY8t/UFsesg4wFWOuzCfqxFmKEaPDZlTuR tfLAeVpslNfWCi5ybPlowLx6KJqOsI9R2a9o4qRXWGP7IwiMRAC3iiPyk9cknt8ee6EUIxI6 t847eFaVKI/6WcxhszI0R6Cj+N4y+1rHfkGWYWupCiHwj9DjILW9iEAncVgQmkNPpUsZECLT 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XrB5p+6ML7Bd84xOEGsj/vdaXmz1esuH7BOZAGEZfLRCHJ0GVCSssg== Message-ID: <37ce1b7e-264d-292d-32b1-093b24b3525c@ozlabs.ru> Date: Mon, 23 Mar 2020 19:58:01 +1100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200323083705.GA31245@lst.de> Content-Language: en-US Cc: Greg Kroah-Hartman , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, "Aneesh Kumar K.V" , Robin Murphy X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On 23/03/2020 19:37, Christoph Hellwig wrote: > On Mon, Mar 23, 2020 at 12:28:34PM +1100, Alexey Kardashevskiy wrote: > > [full quote deleted, please follow proper quoting rules] > >>> +static bool dma_alloc_direct(struct device *dev, const struct dma_map_ops *ops) >>> +{ >>> + if (!ops) >>> + return true; >>> + >>> + /* >>> + * Allows IOMMU drivers to bypass dynamic translations if the DMA mask >>> + * is large enough. >>> + */ >>> + if (dev->dma_ops_bypass) { >>> + if (min_not_zero(dev->coherent_dma_mask, dev->bus_dma_limit) >= >>> + dma_direct_get_required_mask(dev)) >>> + return true; >>> + } >> >> >> Why not do this in dma_map_direct() as well? > > Mostly beacuse it is a relatively expensive operation, including a > fls64. Ah, ok. >> Or simply have just one dma_map_direct()? > > What do you mean with that? I mean use dma_alloc_direct() instead of dma_map_direct() everywhere, you explained just above. > >> And one more general question - we need a way to use non-direct IOMMU >> for RAM above certain limit. >> >> Let's say we have a system with: >> 0 .. 0x1.0000.0000 >> 0x100.0000.0000 .. 0x101.0000.0000 >> >> 2x4G, each is 1TB aligned. And we can map directly only the first 4GB >> (because of the maximum IOMMU table size) but not the other. And 1:1 on >> that "pseries" is done with offset=0x0800.0000.0000.0000. >> >> So we want to check every bus address against dev->bus_dma_limit, not >> dev->coherent_dma_mask. In the example above I'd set bus_dma_limit to >> 0x0800.0001.0000.0000 and 1:1 mapping for the second 4GB would not be >> tried. Does this sound reasonable? Thanks, > > bus_dma_limit is just another limiting factor applied on top of > coherent_dma_mask or dma_mask respectively. This is not enough for the task: in my example, I'd set bus limit to 0x0800.0001.0000.0000 but this would disable bypass for all RAM addresses - the first and the second 4GB blocks. -- Alexey _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu