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[91.79.175.49]) by smtp.googlemail.com with ESMTPSA id d19-v6sm2348828ljc.37.2019.02.03.08.48.10 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 03 Feb 2019 08:48:10 -0800 (PST) Subject: Re: [PATCH V9 3/5] i2c: tegra: Add DMA support To: Sowjanya Komatineni , "thierry.reding@gmail.com" , Jonathan Hunter , Mantravadi Karthik , Shardar Mohammed , Timo Alho Cc: "linux-tegra@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-i2c@vger.kernel.org" References: <1549040867-18149-1-git-send-email-skomatineni@nvidia.com> <1549040867-18149-3-git-send-email-skomatineni@nvidia.com> From: Dmitry Osipenko Message-ID: Date: Sun, 3 Feb 2019 19:48:09 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 02.02.2019 21:32, Sowjanya Komatineni пишет: >>> This patch adds DMA support for Tegra I2C. >>> >>> Tegra I2C TX and RX FIFO depth is 8 words. PIO mode is used for >>> transfer size of the max FIFO depth and DMA mode is used for transfer >>> size higher than max FIFO depth to save CPU overhead. >>> >>> PIO mode needs full intervention of CPU to fill or empty FIFO's and >>> also need to service multiple data requests interrupt for the same >>> transaction. This adds delay between data bytes of the same transfer >>> when CPU is fully loaded and some slave devices has internal timeout >>> for no bus activity and stops transaction to avoid bus hang. DMA mode >>> is helpful in such cases. >>> >>> DMA mode is also helpful for Large transfers during downloading or >>> uploading FW over I2C to some external devices. >>> >>> Signed-off-by: Sowjanya Komatineni >>> --- >> >> >>> +static int tegra_i2c_init_dma(struct tegra_i2c_dev *i2c_dev) { >>> + struct dma_chan *dma_chan; >>> + u32 *dma_buf; >>> + dma_addr_t dma_phys; >>> + int err = 0; >>> + >>> + if (!IS_ENABLED(CONFIG_TEGRA20_APB_DMA)) >>> + return -ENODEV; >> >> Another detail is that we need to keep older kernels working on T186+ after its device-tree will get the "dmas" property, device-tree changes shall be backwards-compatible with older kernels. Hence we need to check that platform actually wants the APB DMA driver, otherwise T186+ will be failing with -EPROBE_DEFER. > > Yes, that will be a separate patch later for adding DMA support for Tegra186 and later chips once we check on GPCDMA upstream > > Sure, and there is a requirement in upstream to keep older kernel versions working with the later device-tree updates, as I pointed above. Please include the ".has_apb_dma checking" that I'm suggesting into the next version.