linux-kernel - [PATCH v2 0/3] add "delay" clock support to gpio

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Message-Id: <20210304221247.488173-1-linux@rasmusvillemoes.dk>
Date:   Thu,  4 Mar 2021 23:12:44 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Arnd Bergmann <arnd@...db.de>, Guenter Roeck <linux@...ck-us.net>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-watchdog@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH v2 0/3] add "delay" clock support to gpio_wdt

As Arnd and Guenther suggested, this adds support to the gpio_wdt
driver for being a consumer of the clock driving the ripple
counter. However, I don't think it should be merged as-is, see below.

The first patch makes sense on its own, quick grepping suggests plenty
of places that could benefit from this, and I thought it would be odd
to have to re-introduce a .remove callback in the gpio_wdt driver.

Unfortunately, this turns out to be a bit of an "operation succeeded,
patient (almost) died": We use CONFIG_GPIO_WATCHDOG_ARCH_INITCALL
because the watchdog has a rather short timeout (1.0-2.25s, 1.6s
typical according to data sheet). At first, I put the new code right
after the devm_gpiod_get(), but the problem is that this early, we get
-EPROBE_DEFER since the clock provider (the RTC which sits off i2c)
isn't probed yet. But then the board would reset because it takes way
too long for the rest of the machine to initialize. [The bootloader
makes sure to turn on the RTC's clock output so the watchdog is
actually functional, the task here is to figure out the proper way to
prevent clk_disable_unused() from disabling it.]

Moving the logic to after the first "is it always-running and if so
give it an initial ping" made the board survive, but unfortunately the
second, and succesful, probe happens a little more than a second
later, which happens to work on this particular board, but is
obviously not suitable for production given that it's already above
what the spec says, and other random changes in the future might make
the gap even wider.

So I don't know. The hardware is obviously misdesigned, and I don't
know how far the mainline kernel should stretch to support this; OTOH
the kernel does contain lots of workarounds for quirks and hardware
bugs. 

Rasmus Villemoes (3):
  clk: add devm_clk_prepare_enable() helper
  dt-bindings: watchdog: add optional "delay" clock to gpio-wdt binding
  watchdog: gpio_wdt: implement support for optional "delay" clock

 .../devicetree/bindings/watchdog/gpio-wdt.txt |  6 ++++
 .../driver-api/driver-model/devres.rst        |  1 +
 drivers/clk/clk-devres.c                      | 29 +++++++++++++++++++
 drivers/watchdog/gpio_wdt.c                   |  9 ++++++
 include/linux/clk.h                           | 13 +++++++++
 5 files changed, 58 insertions(+)

-- 
2.29.2