From mboxrd@z Thu Jan 1 00:00:00 1970 From: klaus.goger@theobroma-systems.com (klaus.goger at theobroma-systems.com) Date: Thu, 17 Nov 2016 21:57:43 +0100 Subject: [PATCH] ARM: dts: sunxi: Explicitly enable pull-ups for MMC pins In-Reply-To: <20161117093438.17988-1-wens@csie.org> References: <20161117093438.17988-1-wens@csie.org> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 2016-11-17 10:34, Chen-Yu Tsai wrote: > Given that MMC starts in open-drain mode and the pull-ups are required, > it's best to enable it for all the pin settings. It's even more complicated than that with MMC. It starts in open-drain mode for CMD during initialization but changes to push-pull afterwards. The card has internal pull-ups to prevent bus floating during setup and will disable them after switching to 4bit mode (or 8bit for eMMC when available). But even after switching to push-pull drivers there are states the bus would float and pull ups have to ensure a high level on the bus. See JESD84-A441 chapter 7.15 ff as reference. The difference between the P-bit and Z-bit is that a P-bit is actively driven to HIGH by the card respectively host output driver, while Z-bit is driven to (respectively kept) HIGH by the pull-up resistors Rcmd respectively Rdat. Enabling the pull ups on the CPU would be the right choice considering that most boards will not have external pull-ups. Even if they would have one, adding the internal in parallel would work in almost all cases and the increase in power consumption would be negligible. Cheers, Klaus