Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1625953 > unrolled thread

Re: [PATCH v13 03/10] mux: minimal mux subsystem and gpio-based mux controller

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: [PATCH v13 03/10] mux: minimal mux subsystem and gpio-based mux  controller Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-19 11:10 +0200
    Re: [PATCH v13 03/10] mux: minimal mux subsystem and gpio-based mux  controller Philipp Zabel <p.zabel@pengutronix.de> - 2017-04-19 15:50 +0200

#1625953 — Re: [PATCH v13 03/10] mux: minimal mux subsystem and gpio-based mux controller

On Thu, 2017-04-13 at 18:43 +0200, Peter Rosin wrote:
> Add a new minimalistic subsystem that handles multiplexer controllers.
> When multiplexers are used in various places in the kernel, and the
> same multiplexer controller can be used for several independent things,
> there should be one place to implement support for said multiplexer
> controller.
> 
> A single multiplexer controller can also be used to control several
> parallel multiplexers, that are in turn used by different subsystems
> in the kernel, leading to a need to coordinate multiplexer accesses.
> The multiplexer subsystem handles this coordination.
> 
> This new mux controller subsystem initially comes with a single backend
> driver that controls gpio based multiplexers. Even though not needed by
> this initial driver, the mux controller subsystem is prepared to handle
> chips with multiple (independent) mux controllers.
> 
> Reviewed-by: Jonathan Cameron <jic23@kernel.org>
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  Documentation/driver-model/devres.txt |   8 +
>  MAINTAINERS                           |   2 +
>  drivers/Kconfig                       |   2 +
>  drivers/Makefile                      |   1 +
>  drivers/mux/Kconfig                   |  34 +++
>  drivers/mux/Makefile                  |   6 +
>  drivers/mux/mux-core.c                | 422 ++++++++++++++++++++++++++++++++++
>  drivers/mux/mux-gpio.c                | 114 +++++++++
>  include/linux/mux.h                   | 252 ++++++++++++++++++++
>  9 files changed, 841 insertions(+)
>  create mode 100644 drivers/mux/Kconfig
>  create mode 100644 drivers/mux/Makefile
>  create mode 100644 drivers/mux/mux-core.c
>  create mode 100644 drivers/mux/mux-gpio.c
>  create mode 100644 include/linux/mux.h
> 
> diff --git a/Documentation/driver-model/devres.txt b/Documentation/driver-model/devres.txt
> index efb8200819d6..e2343d9cbec7 100644
> --- a/Documentation/driver-model/devres.txt
> +++ b/Documentation/driver-model/devres.txt
> @@ -337,6 +337,14 @@ MEM
>  MFD
>    devm_mfd_add_devices()
>  
> +MUX
> +  devm_mux_chip_alloc()
> +  devm_mux_chip_free()
> +  devm_mux_chip_register()
> +  devm_mux_chip_unregister()
> +  devm_mux_control_get()
> +  devm_mux_control_put()
> +
>  PER-CPU MEM
>    devm_alloc_percpu()
>    devm_free_percpu()
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 7fc06739c8ad..591eba737678 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8563,6 +8563,8 @@ M:	Peter Rosin <peda@axentia.se>
>  S:	Maintained
>  F:	Documentation/devicetree/bindings/mux/
>  F:	include/linux/dt-bindings/mux/
> +F:	include/linux/mux.h
> +F:	drivers/mux/
>  
>  MULTISOUND SOUND DRIVER
>  M:	Andrew Veliath <andrewtv@usa.net>
> diff --git a/drivers/Kconfig b/drivers/Kconfig
> index 117ca14ccf85..a7ea13e1b869 100644
> --- a/drivers/Kconfig
> +++ b/drivers/Kconfig
> @@ -204,4 +204,6 @@ source "drivers/fpga/Kconfig"
>  
>  source "drivers/fsi/Kconfig"
>  
> +source "drivers/mux/Kconfig"
> +
>  endmenu
> diff --git a/drivers/Makefile b/drivers/Makefile
> index 2eced9afba53..c0436f6dd5a9 100644
> --- a/drivers/Makefile
> +++ b/drivers/Makefile
> @@ -177,3 +177,4 @@ obj-$(CONFIG_ANDROID)		+= android/
>  obj-$(CONFIG_NVMEM)		+= nvmem/
>  obj-$(CONFIG_FPGA)		+= fpga/
>  obj-$(CONFIG_FSI)		+= fsi/
> +obj-$(CONFIG_MULTIPLEXER)	+= mux/
> diff --git a/drivers/mux/Kconfig b/drivers/mux/Kconfig
> new file mode 100644
> index 000000000000..41dfe08ead84
> --- /dev/null
> +++ b/drivers/mux/Kconfig
> @@ -0,0 +1,34 @@
> +#
> +# Multiplexer devices
> +#
> +
> +menuconfig MULTIPLEXER
> +	tristate "Multiplexer subsystem"
> +	help
> +	  Multiplexer controller subsystem. Multiplexers are used in a
> +	  variety of settings, and this subsystem abstracts their use
> +	  so that the rest of the kernel sees a common interface. When
> +	  multiple parallel multiplexers are controlled by one single
> +	  multiplexer controller, this subsystem also coordinates the
> +	  multiplexer accesses.
> +
> +	  To compile the subsystem as a module, choose M here: the module will
> +	  be called mux-core.
> +
> +if MULTIPLEXER
> +
> +config MUX_GPIO
> +	tristate "GPIO-controlled Multiplexer"
> +	depends on OF && GPIOLIB
> +	help
> +	  GPIO-controlled Multiplexer controller.
> +
> +	  The driver builds a single multiplexer controller using a number
> +	  of gpio pins. For N pins, there will be 2^N possible multiplexer
> +	  states. The GPIO pins can be connected (by the hardware) to several
> +	  multiplexers, which in that case will be operated in parallel.
> +
> +	  To compile the driver as a module, choose M here: the module will
> +	  be called mux-gpio.
> +
> +endif
> diff --git a/drivers/mux/Makefile b/drivers/mux/Makefile
> new file mode 100644
> index 000000000000..bb16953f6290
> --- /dev/null
> +++ b/drivers/mux/Makefile
> @@ -0,0 +1,6 @@
> +#
> +# Makefile for multiplexer devices.
> +#
> +
> +obj-$(CONFIG_MULTIPLEXER)	+= mux-core.o
> +obj-$(CONFIG_MUX_GPIO)		+= mux-gpio.o
> diff --git a/drivers/mux/mux-core.c b/drivers/mux/mux-core.c
> new file mode 100644
> index 000000000000..66a8bccfc3d7
> --- /dev/null
> +++ b/drivers/mux/mux-core.c
> @@ -0,0 +1,422 @@
> +/*
> + * Multiplexer subsystem
> + *
> + * Copyright (C) 2017 Axentia Technologies AB
> + *
> + * Author: Peter Rosin <peda@axentia.se>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +
> +#define pr_fmt(fmt) "mux-core: " fmt
> +
> +#include <linux/device.h>
> +#include <linux/err.h>
> +#include <linux/export.h>
> +#include <linux/idr.h>
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <linux/mux.h>
> +#include <linux/of.h>
> +#include <linux/of_platform.h>
> +#include <linux/slab.h>
> +
> +/*
> + * The idle-as-is "state" is not an actual state that may be selected, it
> + * only implies that the state should not be changed. So, use that state
> + * as indication that the cached state of the multiplexer is unknown.
> + */
> +#define MUX_CACHE_UNKNOWN MUX_IDLE_AS_IS
> +
> +static struct class mux_class = {
> +	.name = "mux",
> +	.owner = THIS_MODULE,
> +};
> +
> +static int __init mux_init(void)
> +{
> +	return class_register(&mux_class);
> +}
> +
> +static DEFINE_IDA(mux_ida);
> +
> +static void mux_chip_release(struct device *dev)
> +{
> +	struct mux_chip *mux_chip = to_mux_chip(dev);
> +
> +	ida_simple_remove(&mux_ida, mux_chip->id);
> +	kfree(mux_chip);
> +}
> +
> +static struct device_type mux_type = {
> +	.name = "mux-chip",
> +	.release = mux_chip_release,
> +};
> +
> +struct mux_chip *mux_chip_alloc(struct device *dev,
> +				unsigned int controllers, size_t sizeof_priv)
> +{
> +	struct mux_chip *mux_chip;
> +	int i;
> +
> +	if (WARN_ON(!dev || !controllers))
> +		return NULL;
> +
> +	mux_chip = kzalloc(sizeof(*mux_chip) +
> +			   controllers * sizeof(*mux_chip->mux) +
> +			   sizeof_priv, GFP_KERNEL);
> +	if (!mux_chip)
> +		return NULL;
> +
> +	mux_chip->mux = (struct mux_control *)(mux_chip + 1);
> +	mux_chip->dev.class = &mux_class;
> +	mux_chip->dev.type = &mux_type;
> +	mux_chip->dev.parent = dev;
> +	mux_chip->dev.of_node = dev->of_node;
> +	dev_set_drvdata(&mux_chip->dev, mux_chip);
> +
> +	mux_chip->id = ida_simple_get(&mux_ida, 0, 0, GFP_KERNEL);
> +	if (mux_chip->id < 0) {
> +		pr_err("muxchipX failed to get a device id\n");
> +		kfree(mux_chip);
> +		return NULL;
> +	}
> +	dev_set_name(&mux_chip->dev, "muxchip%d", mux_chip->id);
> +
> +	mux_chip->controllers = controllers;
> +	for (i = 0; i < controllers; ++i) {
> +		struct mux_control *mux = &mux_chip->mux[i];
> +
> +		mux->chip = mux_chip;
> +		init_rwsem(&mux->lock);
> +		mux->cached_state = MUX_CACHE_UNKNOWN;
> +		mux->idle_state = MUX_IDLE_AS_IS;
> +	}
> +
> +	device_initialize(&mux_chip->dev);
> +
> +	return mux_chip;
> +}
> +EXPORT_SYMBOL_GPL(mux_chip_alloc);
> +
> +static int mux_control_set(struct mux_control *mux, int state)
> +{
> +	int ret = mux->chip->ops->set(mux, state);
> +
> +	mux->cached_state = ret < 0 ? MUX_CACHE_UNKNOWN : state;
> +
> +	return ret;
> +}
> +
> +int mux_chip_register(struct mux_chip *mux_chip)
> +{
> +	int i;
> +	int ret;
> +
> +	for (i = 0; i < mux_chip->controllers; ++i) {
> +		struct mux_control *mux = &mux_chip->mux[i];
> +
> +		if (mux->idle_state == mux->cached_state)
> +			continue;
> +
> +		ret = mux_control_set(mux, mux->idle_state);
> +		if (ret < 0) {
> +			dev_err(&mux_chip->dev, "unable to set idle state\n");
> +			return ret;
> +		}
> +	}
> +
> +	ret = device_add(&mux_chip->dev);
> +	if (ret < 0)
> +		dev_err(&mux_chip->dev,
> +			"device_add failed in mux_chip_register: %d\n", ret);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(mux_chip_register);
> +
> +void mux_chip_unregister(struct mux_chip *mux_chip)
> +{
> +	device_del(&mux_chip->dev);
> +}
> +EXPORT_SYMBOL_GPL(mux_chip_unregister);
> +
> +void mux_chip_free(struct mux_chip *mux_chip)
> +{
> +	if (!mux_chip)
> +		return;
> +
> +	put_device(&mux_chip->dev);
> +}
> +EXPORT_SYMBOL_GPL(mux_chip_free);
> +
> +static void devm_mux_chip_release(struct device *dev, void *res)
> +{
> +	struct mux_chip *mux_chip = *(struct mux_chip **)res;
> +
> +	mux_chip_free(mux_chip);
> +}
> +
> +struct mux_chip *devm_mux_chip_alloc(struct device *dev,
> +				     unsigned int controllers,
> +				     size_t sizeof_priv)
> +{
> +	struct mux_chip **ptr, *mux_chip;
> +
> +	ptr = devres_alloc(devm_mux_chip_release, sizeof(*ptr), GFP_KERNEL);
> +	if (!ptr)
> +		return NULL;
> +
> +	mux_chip = mux_chip_alloc(dev, controllers, sizeof_priv);
> +	if (!mux_chip) {
> +		devres_free(ptr);
> +		return NULL;
> +	}
> +
> +	*ptr = mux_chip;
> +	devres_add(dev, ptr);
> +
> +	return mux_chip;
> +}
> +EXPORT_SYMBOL_GPL(devm_mux_chip_alloc);
> +
> +static int devm_mux_chip_match(struct device *dev, void *res, void *data)
> +{
> +	struct mux_chip **r = res;
> +
> +	if (WARN_ON(!r || !*r))
> +		return 0;
> +
> +	return *r == data;
> +}
> +
> +void devm_mux_chip_free(struct device *dev, struct mux_chip *mux_chip)
> +{
> +	WARN_ON(devres_release(dev, devm_mux_chip_release,
> +			       devm_mux_chip_match, mux_chip));
> +}
> +EXPORT_SYMBOL_GPL(devm_mux_chip_free);
> +
> +static void devm_mux_chip_reg_release(struct device *dev, void *res)
> +{
> +	struct mux_chip *mux_chip = *(struct mux_chip **)res;
> +
> +	mux_chip_unregister(mux_chip);
> +}
> +
> +int devm_mux_chip_register(struct device *dev,
> +			   struct mux_chip *mux_chip)
> +{
> +	struct mux_chip **ptr;
> +	int res;
> +
> +	ptr = devres_alloc(devm_mux_chip_reg_release, sizeof(*ptr), GFP_KERNEL);
> +	if (!ptr)
> +		return -ENOMEM;
> +
> +	res = mux_chip_register(mux_chip);
> +	if (res) {
> +		devres_free(ptr);
> +		return res;
> +	}
> +
> +	*ptr = mux_chip;
> +	devres_add(dev, ptr);
> +
> +	return res;
> +}
> +EXPORT_SYMBOL_GPL(devm_mux_chip_register);
> +
> +void devm_mux_chip_unregister(struct device *dev, struct mux_chip *mux_chip)
> +{
> +	WARN_ON(devres_release(dev, devm_mux_chip_reg_release,
> +			       devm_mux_chip_match, mux_chip));
> +}
> +EXPORT_SYMBOL_GPL(devm_mux_chip_unregister);
> +
> +int mux_control_select(struct mux_control *mux, int state)

If we let two of these race, ...

> +{
> +	int ret;
> +
> +	if (down_read_trylock(&mux->lock)) {
> +		if (mux->cached_state == state)
> +			return 0;
> +		/* Sigh, the mux needs updating... */
> +		up_read(&mux->lock);

... and both decide the mux needs updating ...

> +	}
> +
> +	/* ...or it's just contended. */
> +	down_write(&mux->lock);

... then the last to get to down_write will just wait here forever (or
until the first consumer calls mux_control_deselect, which may never
happen)?

> +
> +	if (mux->cached_state == state) {
> +		/*
> +		 * Hmmm, someone else changed the mux to my liking.
> +		 * That makes me wonder how long I waited for nothing?
> +		 */
> +		downgrade_write(&mux->lock);
> +		return 0;
> +	}
> +
> +	ret = mux_control_set(mux, state);
> +	if (ret < 0) {
> +		if (mux->idle_state != MUX_IDLE_AS_IS)
> +			mux_control_set(mux, mux->idle_state);
> +
> +		up_write(&mux->lock);
> +		return ret;
> +	}
> +
> +	downgrade_write(&mux->lock);
> +
> +	return 1;
> +}
> +EXPORT_SYMBOL_GPL(mux_control_select);

I wonder if these should be called mux_control_lock/unlock instead,
which would allow for try_lock and lock_timeout variants.

regards
Philipp

[toc] | [next] | [standalone]

#1626122

On Wed, 2017-04-19 at 14:00 +0200, Peter Rosin wrote:
[...]
> >> +int mux_control_select(struct mux_control *mux, int state)
> > 
> > If we let two of these race, ...
>
> The window for this "race" is positively huge. If there are several
> mux consumers of a single mux controller, it is self-evident that
> if one of them grabs the mux for a long time, the others will suffer.
> 
> The design is that the rwsem is reader-locked for the full duration
> of a select/deselect operation by the mux consumer.

I was not clear. I meant: I think this can also happen if we let them
race with the same state target.

> >> +{
> >> +	int ret;
> >> +
> >> +	if (down_read_trylock(&mux->lock)) {
> >> +		if (mux->cached_state == state)
> >> +			return 0;

This check makes it clear that a second select call is not intended to
block if the intended state is already selected. But if the instance we
will lose the race against has not yet updated cached_state, ...

> >> +		/* Sigh, the mux needs updating... */
> >> +		up_read(&mux->lock);
> >> +	}
> >> +
> >> +	/* ...or it's just contended. */
> >> +	down_write(&mux->lock);

... we are blocking here until the other instance calls up_read. Even
though in this case (same state target) we would only have to block
until the other instance calls downgrade_write after the mux control is
set to the correct state.

Basically there is a small window before down_write with no lock at all,
where multiple instances can already have decided they must change the
mux (to the same state). If this happens, they go on to block each other
unnecessarily.

> > ... then the last to get to down_write will just wait here forever (or
> > until the first consumer calls mux_control_deselect, which may never
> > happen)?
> 
> It is vital that the mux consumer call _deselect when it is done with
> the mux. Not doing so will surely starve out any other mux consumers.
> The whole thing is designed around the fact that mux consumers should
> deselect the mux as soon as it's no longer needed.

I'd like to use this for video bus multiplexers. Those would not be
selected for the duration of an i2c transfer, but for the duration of a
running video capture stream, or for the duration of an enabled display
path. While I currently have no use case for multiple consumers
controlling the same mux, this scenario is what shapes my perspective.
For such long running selections the consumer should have the option to
return -EBUSY instead of blocking when the lock can't be taken.

regards
Philipp

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web