Date:	Thu, 09 Apr 2015 19:12:34 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Daniel Baluta <daniel.baluta@...el.com>
CC:	jlbec@...lplan.org, lars@...afoo.de, knaack.h@....de,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	octavian.purdila@...el.com, pebolle@...cali.nl,
	patrick.porlan@...el.com, adriana.reus@...el.com
Subject: Re: [PATCH v3 1/3] iio: configfs: Add configfs support to IIO

On 09/04/15 18:40, Jonathan Cameron wrote:
> On 09/04/15 18:15, Jonathan Cameron wrote:
>> On 06/04/15 15:18, Daniel Baluta wrote:
>>> This module is the core of IIO configfs. It creates the "iio" subsystem under
>>> configfs mount point root, with one default group for "triggers".
>>>
>>> It offers basic interface for registering software trigger types. Next patches
>>> will add "hrtimer" and "sysfs" trigger types. To add a new trigger type we must
>>> create a new kernel module which implements iio_configfs_trigger.h interface.
>>>
>>> See Documentation/iio/iio_configfs.txt for more details on how configfs
>>> support for IIO works.
>>>
>>> Signed-off-by: Daniel Baluta <daniel.baluta@...el.com>
>> Looks good and is actually a fair bit simpler than I expected which is nice!
>> As an ideal aim, I'd like there to be no need for the core to have any
>> idea what triggers exist and can be registered (beyond preventing naming
>> clashes etc). Actually, not much would be needed to implement that I think, just
>> using a list and looking up in it (we aren't on a particularly fast path so
>> don't need anything clever) instead of a simple array.  A touch of locking
>> probably to avoid two many simultaneous users of that list...
>>
>> Hmm. having read through the patches, are we fundamentally limited to having to
>> specify the entire directory structure before bringing up configfs?
> As I read more on this, I think the definition of a 'subsystem' in configfs is
> very restricted. It means a tight group of devices sharing a prior
> known interface with no ability to add extras later.
> It's sort of like the difference between a class and a bus in sysfs (but more limited
> actually that's a rubbish analogy).  Thus either we have to do as here
> and have the core know all about everything it might want to setup in advance
> of registering the iio configfs subsystem or we need to make a whole load of
> different configfs subsystems. Basically boils down to one per module.  We lose
> the grouping convenience of the current structure but gain in separation.
> 
> So option 1 we have effectively to predefine anything that might turn up
> in a module...
> 
> /config/iio/triggers/hrtimer/instance1/..
> /config/iio/triggers/sysfs/instance2/..
> /config/iio/virtualdevs/hwmon/iiohwmoninstance1/..
> /config/iio/virtualdevs/input/iioinputinstance1/..
> /config/iio/maps/channelmapisntance1/.. 
> etc (actually fine for the maps as they are coming from the core anyway
> and will be known in advance)
> 
> Option 2 we have complete flexibility as its down to the relevant drivers
> 
> /config/iio-trigger-hrtimer/instance/..
> /config/iio-trigger-sysfs/instance/..
> /config/iio-virtual-hwmon/instance/..
> /config/iio-virtual-input/instance/..
> 
> but we lose the grouping by directory which is a conceptual shame.
> 
> Am I missing something important here?  (pretty new to configfs!)
> 
> Just for others not familiar with IIO who might read this. We have a number
> of userspace created elements but each type is provided by a different module,
> hence it's a real pain having to have them all defined at configfs subsystem
> initialization as it adds tight coupling we don't otherwise have!

For reference of others the usb gadget code is a good example of how to handle
these sort of cross dependencies..

the approach used for function drivers there would give us the cleaner option of

/config/iio/triggers/hrtimer-instance1
/config/iio/triggers/hrtimer-instance2
/config/iio/triggers/sysfs-instance1
/config/iio/virtualdevs/hwmon-instance1
etc


> 
> J
>>
>>> ---
>>>
>>> Changes since v2:
>>> 	* Fix build errors by compiling industrialio-configfs.ko 
>>> 	as a separate module, as reported by Paul.
>>>
>>> Changes since v1:
>>> 	* addressed feedback from v1:
>>> 		* https://lkml.org/lkml/2015/3/25/647
>>> 	* create a directory per trigger type
>>> 	* removed activate and type attributes
>>> 	
>>>  drivers/iio/Kconfig                      |   8 ++
>>>  drivers/iio/Makefile                     |   1 +
>>>  drivers/iio/industrialio-configfs.c      | 127 +++++++++++++++++++++++++++++++
>>>  include/linux/iio/iio_configfs_trigger.h |  40 ++++++++++
>>>  4 files changed, 176 insertions(+)
>>>  create mode 100644 drivers/iio/industrialio-configfs.c
>>>  create mode 100644 include/linux/iio/iio_configfs_trigger.h
>>>
>>> diff --git a/drivers/iio/Kconfig b/drivers/iio/Kconfig
>>> index 4011eff..41d5863 100644
>>> --- a/drivers/iio/Kconfig
>>> +++ b/drivers/iio/Kconfig
>>> @@ -18,6 +18,14 @@ config IIO_BUFFER
>>>  	  Provide core support for various buffer based data
>>>  	  acquisition methods.
>>>  
>>> +config IIO_CONFIGFS
>>> +	tristate "Enable IIO configuration via configfs"
>>> +	select CONFIGFS_FS
>>> +	help
>>> +	  This allows configuring various IIO bits through configfs
>>> +	  (e.g. software triggers). For more info see
>>> +	  Documentation/iio/iio_configfs.txt.
>>> +
>>>  if IIO_BUFFER
>>>  
>>>  config IIO_BUFFER_CB
>>> diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
>>> index 698afc2..72e85c42 100644
>>> --- a/drivers/iio/Makefile
>>> +++ b/drivers/iio/Makefile
>>> @@ -8,6 +8,7 @@ industrialio-$(CONFIG_IIO_BUFFER) += industrialio-buffer.o
>>>  industrialio-$(CONFIG_IIO_TRIGGER) += industrialio-trigger.o
>>>  industrialio-$(CONFIG_IIO_BUFFER_CB) += buffer_cb.o
>>>  
>>> +obj-$(CONFIG_IIO_CONFIGFS) += industrialio-configfs.o
>>>  obj-$(CONFIG_IIO_TRIGGERED_BUFFER) += industrialio-triggered-buffer.o
>>>  obj-$(CONFIG_IIO_KFIFO_BUF) += kfifo_buf.o
>>>  
>>> diff --git a/drivers/iio/industrialio-configfs.c b/drivers/iio/industrialio-configfs.c
>>> new file mode 100644
>>> index 0000000..d8ffd76
>>> --- /dev/null
>>> +++ b/drivers/iio/industrialio-configfs.c
>>> @@ -0,0 +1,127 @@
>>> +/*
>>> + * Industrial I/O configfs bits
>>> + *
>>> + * Copyright (c) 2015 Intel Corporation
>>> + *
>>> + * 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.
>>> + */
>>> +
>>> +#include <linux/configfs.h>
>>> +#include <linux/module.h>
>>> +#include <linux/init.h>
>>> +#include <linux/kmod.h>
>>> +#include <linux/slab.h>
>>> +
>>> +#include <linux/iio/iio.h>
>>> +#include <linux/iio/iio_configfs_trigger.h>
>>> +
>>> +static struct iio_configfs_trigger_type *trigger_types[IIO_TRIGGER_TYPE_MAX];
>>> +static DEFINE_RWLOCK(trigger_types_lock);
>>> +
>>> +int register_configfs_trigger(struct iio_configfs_trigger_type *t)
>>> +{
>>> +	int index = t->type;
>>> +	int ret = 0;
>>> +
>>> +	if (index < 0 || index >= IIO_TRIGGER_TYPE_MAX)
>>> +		return -EINVAL;
>>> +
>>> +	write_lock(&trigger_types_lock);
>>> +	if (trigger_types[index])
>>> +		ret = -EBUSY;
>>> +	else
>>> +		trigger_types[index] = t;
>>> +	write_unlock(&trigger_types_lock);
>>> +
>>> +	return ret;
>>> +}
>>> +EXPORT_SYMBOL(register_configfs_trigger);
>>> +
>>> +int unregister_configfs_trigger(struct iio_configfs_trigger_type *t)
>>> +{
>>> +	int index = t->type;
>>> +
>>> +	if (index < 0 || index >= IIO_TRIGGER_TYPE_MAX)
>>> +		return -EINVAL;
>>> +
>>> +	write_lock(&trigger_types_lock);
>>> +	trigger_types[index] = NULL;
>>> +	write_unlock(&trigger_types_lock);
>>> +
>>> +	return 0;
>>> +}
>>> +EXPORT_SYMBOL(unregister_configfs_trigger);
>>> +
>>> +static
>>> +struct iio_configfs_trigger_type *iio_get_configfs_trigger_type(int type)
>>> +{
>>> +	struct iio_configfs_trigger_type *t;
>>> +
>>> +	if (type < 0 || type >= IIO_TRIGGER_TYPE_MAX)
>>> +		return ERR_PTR(-EINVAL);
>>> +
>>> +	read_lock(&trigger_types_lock);
>>> +	t = trigger_types[type];
>>> +	if (t && !try_module_get(t->owner))
>>> +		t = NULL;
>>> +	read_unlock(&trigger_types_lock);
>>> +
>>> +	return t;
>>> +}
>>> +
>>> +static struct config_group *iio_triggers_default_groups[] = {
>>> +	NULL
>>> +};
>>> +
>>> +static struct config_item_type iio_triggers_group_type = {
>>> +	.ct_owner       = THIS_MODULE,
>>> +};
>>> +
>>> +static struct config_group iio_triggers_group = {
>>> +	.cg_item = {
>>> +		.ci_namebuf = "triggers",
>>> +		.ci_type = &iio_triggers_group_type,
>>> +	},
>>> +	.default_groups = iio_triggers_default_groups,
>>> +};
>>> +
>>> +static struct config_group *iio_root_default_groups[] = {
>>> +	&iio_triggers_group,
>>> +	NULL
>>> +};
>>> +
>>> +static struct config_item_type iio_root_group_type = {
>>> +	.ct_owner       = THIS_MODULE,
>>> +};
>>> +
>>> +static struct configfs_subsystem iio_configfs_subsys = {
>>> +	.su_group = {
>>> +		.cg_item = {
>>> +			.ci_namebuf = "iio",
>>> +			.ci_type = &iio_root_group_type,
>>> +		},
>>> +		.default_groups = iio_root_default_groups,
>>> +	},
>>> +	.su_mutex = __MUTEX_INITIALIZER(iio_configfs_subsys.su_mutex),
>>> +};
>>> +
>>> +static int __init iio_configfs_init(void)
>>> +{
>>> +	config_group_init(&iio_triggers_group);
>>> +	config_group_init(&iio_configfs_subsys.su_group);
>>> +
>>> +	return configfs_register_subsystem(&iio_configfs_subsys);
>>> +}
>>> +module_init(iio_configfs_init);
>>> +
>>> +static void __exit iio_configfs_exit(void)
>>> +{
>>> +	configfs_unregister_subsystem(&iio_configfs_subsys);
>>> +}
>>> +module_exit(iio_configfs_exit);
>>> +
>>> +MODULE_AUTHOR("Daniel Baluta <daniel.baluta@...el.com>");
>>> +MODULE_DESCRIPTION("Industrial I/O configfs support");
>>> +MODULE_LICENSE("GPL v2");
>>> diff --git a/include/linux/iio/iio_configfs_trigger.h b/include/linux/iio/iio_configfs_trigger.h
>>> new file mode 100644
>>> index 0000000..75e76f2
>>> --- /dev/null
>>> +++ b/include/linux/iio/iio_configfs_trigger.h
>>> @@ -0,0 +1,40 @@
>>> +#ifndef __IIO_CONFIGFS_TRIGGER
>>> +#define __IIO_CONFIGFS_TRIGGER
>>> +
>>> +#include <linux/device.h>
>>> +#include <linux/iio/iio.h>
>>> +#include <linux/module.h>
>>> +
>>> +#define module_iio_configfs_trigger_driver(__iio_configfs_trigger) \
>>> +	module_driver(__iio_configfs_trigger, register_configfs_trigger, \
>>> +		      unregister_configfs_trigger)
>>> +
>>> +enum {
>> Is this enum actually needed as such? If we can avoid explicit need
>> to list triggers in one place it would be nice.
>>
>> As I understand it, the point of this is to allow allocating of various
>> arrays for easy lookup etc.  How about we switch over to a list and
>> lookup based on magic provided by the trigger? (string or address most likely).
>>> +	IIO_TRIGGER_TYPE_MAX,
>>> +};
>>> +
>>> +struct iio_configfs_trigger_ops;
>>> +
>>> +struct iio_configfs_trigger_type {
>>> +	int type;
>>> +	struct module *owner;
>>> +	struct iio_configfs_trigger_ops *trigger_ops;
>>> +};
>>> +
>>> +struct iio_configfs_trigger_info {
>>> +	char *name;
>>> +	struct iio_trigger *trigger;
>>> +	struct iio_configfs_trigger_type *trigger_type;
>>> +};
>>> +
>>> +struct iio_configfs_trigger_ops {
>> I wonder if we can make these more flexible from the start because
>> avoiding churn in callbacks is always nice as us ensuring we have
>> a manageable number in the long run!  Also we might want to play
>> the 'fixed point' games we play elsewhere in IIO to allow us to have
>> a very wide range of precisions.
>> enum iio_configfs_trigger_paramtype = {
>>      delay,
>> };
>>
>> As a final thought, we have standardised on sampling_frequency elsewhere in
>> IIO (largely based on that was what the first few device used) so perhaps
>> we can use that here as well instead of delay?
>>
>> int (*get_parameter)(struct iio_configfs_trigger_info *, enum iio_configfs_triger_paramtype param, unsigned long *);
>>
>>> +	int (*get_delay)(struct iio_configfs_trigger_info *, unsigned long *);
>>> +	int (*set_delay)(struct iio_configfs_trigger_info *, unsigned long);
>>> +	int (*probe)(struct iio_configfs_trigger_info *);
>>> +	int (*remove)(struct iio_configfs_trigger_info *);
>>> +};
>>> +
>>> +int register_configfs_trigger(struct iio_configfs_trigger_type *);
>>> +int unregister_configfs_trigger(struct iio_configfs_trigger_type *);
>>> +
>>> +#endif /* __IIO_CONFIGFS_TRIGGER */
>>>
>>
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