[linux-sunxi] Re: [PATCH] mmc: pwrseq-simple: Add an optional post-power-on-delay

Rob Herring robh at kernel.org
Thu Jun 30 17:47:32 PDT 2016

On Thu, Jun 23, 2016 at 11:33:27AM +0200, Hans de Goede wrote:
> Hi,
> On 23-06-16 00:25, Rob Herring wrote:
> >On Wed, Jun 22, 2016 at 12:59 PM, Hans de Goede <hdegoede at redhat.com> wrote:
> >>Some devices need a while to boot their firmware after providing clks /
> >>de-asserting resets before they are ready to receive sdio commands.
> >>
> >>This commits adds a post-power-on-delay-ms devicetree property to
> >>mmc-pwrseq-simple for use with such devices.
> >>
> >>Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> >>---
> >> Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt | 2 ++
> >> drivers/mmc/core/pwrseq_simple.c                            | 9 +++++++++
> >> 2 files changed, 11 insertions(+)
> >>
> >>diff --git a/Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt b/Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt
> >>index ce0e767..e254368 100644
> >>--- a/Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt
> >>+++ b/Documentation/devicetree/bindings/mmc/mmc-pwrseq-simple.txt
> >>@@ -16,6 +16,8 @@ Optional properties:
> >>   See ../clocks/clock-bindings.txt for details.
> >> - clock-names : Must include the following entry:
> >>   "ext_clock" (External clock provided to the card).
> >>+- post-power-on-delay-ms : Delay in ms after powering the card and
> >>+       de-asserting the reset-gpios (if any)
> >
> >Presumably you need this delay post any reset, not just after power on
> mmc-pwrseq is only about doing power-on / off, not about providing
> reset functionality.

Yes, but the property (e.g. the delay) is relevant for both and reset is 
part of the power seq.

> >if you are waiting for firmware to boot. So the name is not all that
> >clear. How about a "reset-timing-ms" property that takes 3 values for
> >pre-assert time (normally 0), assertion time, post assert time. Of
> >course, I can still think of ways that breaks like when in this
> >sequence do clocks need to be turned on.
> If you look at bindings/mmc/mmc-pwrseq-simple.txt out side of
> the diff context it contains:
> - reset-gpios : contains a list of GPIO specifiers. The reset GPIOs are asserted
>         at initialization and prior we start the power up procedure of the card.
>         They will be de-asserted right after the power has been provided to the
>         card.
> - clocks : Must contain an entry for the entry in clock-names.
>   See ../clocks/clock-bindings.txt for details.
> - clock-names : Must include the following entry:
>   "ext_clock" (External clock provided to the card).
> Notice how the existing docs talk about a power-up procedure (which matches
> the pwrseq name and purpose of these bindings).
> I actually started with calling the property "post-reset-delay-ms", but then
> I realized that what we really want is the ability to specify a time to
> wait (for e.g. firmware to boot) after completing the power-up procedure
> (and before starting the probe). The power-up procedure currently can
> also includes enabling external clocks to the sdio device (if any) and
> enabling regulators, so having a "reset-timing-ms" property does not
> seem right, as that would suggest it is ok to do the wait after deasserting
> reset, but before e.g. enabling external clocks. Where what we really
> want is to enable all necessary resources (or iow complete the powerup
> procedure) and then wait.
> You're right that in some cases more complicated timings may be necessary,
> but that can get really complicated like e.g.: enable regulator1, wait 10 ms,
> enable regulator2, wait 15ms, enable external clock1, ...
> And such complex timings fall outside of the scope of the mmc-pwrseq-simple
> binding, the idea being that for complex cases we do a device specific
> pwrseq binding, and then the smarts are in implementation of that specific
> pwrseq driver. As you've said yourself before we do not want to turn
> devicetree into a scripting language.

Exactly. The challenge is any single property is hard to push back on 
that we've crossed that line. I don't want to see this expanded one 
property at a time without any foresight on additional needs. If we can 
add a property that is more flexible, but doesn't add to the complexity 
then that would be better. So this one alone is fine, but the next one 
I'll be less receptive.


P.S. I'm on vacation ATM, so I'll be slow responding.

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