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Message-ID: <20221006122634.GW986@pengutronix.de>
Date: Thu, 6 Oct 2022 14:26:34 +0200
From: Sascha Hauer <s.hauer@...gutronix.de>
To: Serge Semin <fancer.lancer@...il.com>
Cc: linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
linux-kernel@...r.kernel.org,
Linus Walleij <linus.walleij@...aro.org>,
Bartosz Golaszewski <brgl@...ev.pl>, kernel@...gutronix.de
Subject: Re: [PATCH v4 1/2] gpio: Add gpio latch driver
On Thu, Oct 06, 2022 at 12:15:06PM +0300, Serge Semin wrote:
> On Thu, Oct 06, 2022 at 10:30:30AM +0200, Sascha Hauer wrote:
> > This driver implements a GPIO multiplexer based on latches connected to
> > other GPIOs. A set of data GPIOs is connected to the data input of
> > multiple latches. The clock input of each latch is driven by another
> > set of GPIOs. With two 8-bit latches 10 GPIOs can be multiplexed into
> > 16 GPIOs. GPOs might be a better term as in fact the multiplexed pins
> > are output only.
> >
> > Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> > ---
> >
> > Notes:
> > Changes since v3:
> > - Introduce delays between GPIO toggles as suggested by Serge Semin
> >
> > Changes since v2:
> > - Fix inconsistent licensing
> > - document locks
> > - use regular bit operations
> > - include linux/mod_devicetable.h rather than linux/of_device.h
> > - Put spinlock and mutex into a union to make clear that only one of them is used
> > - rename __gpio_latch_set to gpio_latch_set_unlocked
> >
> > Changes since v1:
> > - Use gpiod_set_value_cansleep when the underlying GPIOs might sleep
> > - Move MODULE_DEVICE_TABLE near to the end
> >
> > drivers/gpio/Kconfig | 6 ++
> > drivers/gpio/Makefile | 1 +
> > drivers/gpio/gpio-latch.c | 220 ++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 227 insertions(+)
> > create mode 100644 drivers/gpio/gpio-latch.c
> >
> > diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
> > index 0642f579196f2..e4603810ec910 100644
> > --- a/drivers/gpio/Kconfig
> > +++ b/drivers/gpio/Kconfig
> > @@ -1690,6 +1690,12 @@ config GPIO_AGGREGATOR
> > industrial control context, to be operated from userspace using
> > the GPIO chardev interface.
> >
> > +config GPIO_LATCH
> > + tristate "GPIO latch driver"
> > + help
> > + Say yes here to enable a driver for GPIO multiplexers based on latches
> > + connected to other GPIOs.
> > +
> > config GPIO_MOCKUP
> > tristate "GPIO Testing Driver"
> > select IRQ_SIM
> > diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
> > index a0985d30f51bb..310fa08decc69 100644
> > --- a/drivers/gpio/Makefile
> > +++ b/drivers/gpio/Makefile
> > @@ -75,6 +75,7 @@ obj-$(CONFIG_GPIO_IT87) += gpio-it87.o
> > obj-$(CONFIG_GPIO_IXP4XX) += gpio-ixp4xx.o
> > obj-$(CONFIG_GPIO_JANZ_TTL) += gpio-janz-ttl.o
> > obj-$(CONFIG_GPIO_KEMPLD) += gpio-kempld.o
> > +obj-$(CONFIG_GPIO_LATCH) += gpio-latch.o
> > obj-$(CONFIG_GPIO_LOGICVC) += gpio-logicvc.o
> > obj-$(CONFIG_GPIO_LOONGSON1) += gpio-loongson1.o
> > obj-$(CONFIG_GPIO_LOONGSON) += gpio-loongson.o
> > diff --git a/drivers/gpio/gpio-latch.c b/drivers/gpio/gpio-latch.c
> > new file mode 100644
> > index 0000000000000..a4ab8f1240450
> > --- /dev/null
> > +++ b/drivers/gpio/gpio-latch.c
> > @@ -0,0 +1,220 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * GPIO latch driver
> > + *
> > + * Copyright (C) 2022 Sascha Hauer <s.hauer@...gutronix.de>
> > + *
> > + * This driver implements a GPIO (or better GPO as there is no input)
> > + * multiplexer based on latches like this:
> > + *
> > + * CLK0 ----------------------. ,--------.
> > + * CLK1 -------------------. `--------|> #0 |
> > + * | | |
> > + * OUT0 ----------------+--|-----------|D0 Q0|-----|<
> > + * OUT1 --------------+-|--|-----------|D1 Q1|-----|<
> > + * OUT2 ------------+-|-|--|-----------|D2 Q2|-----|<
> > + * OUT3 ----------+-|-|-|--|-----------|D3 Q3|-----|<
> > + * OUT4 --------+-|-|-|-|--|-----------|D4 Q4|-----|<
> > + * OUT5 ------+-|-|-|-|-|--|-----------|D5 Q5|-----|<
> > + * OUT6 ----+-|-|-|-|-|-|--|-----------|D6 Q6|-----|<
> > + * OUT7 --+-|-|-|-|-|-|-|--|-----------|D7 Q7|-----|<
> > + * | | | | | | | | | `--------'
> > + * | | | | | | | | |
> > + * | | | | | | | | | ,--------.
> > + * | | | | | | | | `-----------|> #1 |
> > + * | | | | | | | | | |
> > + * | | | | | | | `--------------|D0 Q0|-----|<
> > + * | | | | | | `----------------|D1 Q1|-----|<
> > + * | | | | | `------------------|D2 Q2|-----|<
> > + * | | | | `--------------------|D3 Q3|-----|<
> > + * | | | `----------------------|D4 Q4|-----|<
> > + * | | `------------------------|D5 Q5|-----|<
> > + * | `--------------------------|D6 Q6|-----|<
> > + * `----------------------------|D7 Q7|-----|<
> > + * `--------'
> > + *
> > + * The above is just an example. The actual number of number of latches and
> > + * the number of inputs per latch is derived from the number of GPIOs given
> > + * in the corresponding device tree properties.
> > + */
> > +
> > +#include <linux/err.h>
> > +#include <linux/gpio/consumer.h>
> > +#include <linux/gpio/driver.h>
> > +#include <linux/module.h>
> > +#include <linux/mod_devicetable.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/delay.h>
> > +
> > +#include "gpiolib.h"
> > +
> > +struct gpio_latch_priv {
> > + struct gpio_chip gc;
> > + struct gpio_descs *clk_gpios;
> > + struct gpio_descs *latched_gpios;
> > + int n_latched_gpios;
> > + unsigned int setup_duration_ns;
> > + unsigned int clock_duration_ns;
> > + unsigned long *shadow;
> > + /*
> > + * Depending on whether any of the underlying GPIOs may sleep we either
> > + * use a mutex or a spinlock to protect our shadow map.
> > + */
> > + union {
> > + struct mutex mutex; /* protects @shadow */
> > + spinlock_t spinlock; /* protects @shadow */
> > + };
> > +};
> > +
> > +static int gpio_latch_get_direction(struct gpio_chip *gc, unsigned int offset)
> > +{
> > + return GPIO_LINE_DIRECTION_OUT;
> > +}
> > +
> > +static void gpio_latch_set_unlocked(struct gpio_latch_priv *priv,
> > + void (*set)(struct gpio_desc *desc, int value),
> > + unsigned int offset, bool val)
> > +{
> > + int latch = offset / priv->n_latched_gpios;
> > + int i;
> > +
> > + assign_bit(offset, priv->shadow, val);
> > +
> > + for (i = 0; i < priv->n_latched_gpios; i++)
> > + set(priv->latched_gpios->desc[i],
> > + test_bit(latch * priv->n_latched_gpios + i, priv->shadow));
> > +
> > + ndelay(priv->setup_duration_ns);
> > + set(priv->clk_gpios->desc[latch], 1);
>
> > + udelay(priv->clock_duration_ns);
>
> These are supposed to be !n!delay()'s. Aren't they?
Yes, indeed.
>
> > + set(priv->clk_gpios->desc[latch], 0);
>
> > + udelay(priv->clock_duration_ns);
>
> Why do you need the second clock-duration? AFAICS at least from the
> TI SNx4LV74A specification the outputs get updated on the positive
> edge of the pulse. So the clock-duration value shall contain both
> the "CLK pulse duration" and "CLK hold time", which should be enough
> for the latches states being perceived by the device.
The rationale was that there should be some delay between two subsequent
calls to gpio_latch_set_unlocked(). You are right though, on my latch
the update happens on the positive edge as well, so this shouldn't be
needed.
Sascha
> > + /*
> > + * Some value which is still acceptable to delay in atomic context.
> > + * If we need to go higher we might have to switch to usleep_range(),
> > + * but that cannot ne used in atomic context and the driver would have
> > + * to be adjusted to support that.
> > + */
> > + unsigned int duration_ns_max = 5000;
>
> I don't see you changing this parameter. So it's better to be a macro.
Why should it?
Sascha
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