linux-kernel - Re: [PATCH] ARM: dts: sunxi: Explicitly enable pull-ups for MMC pins

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Date:   Thu, 17 Nov 2016 21:57:43 +0100
From:   klaus.goger@...obroma-systems.com
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH] ARM: dts: sunxi: Explicitly enable pull-ups for MMC pins

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