Date:	Tue, 23 Aug 2011 15:03:11 -0700
From:	Greg KH <greg@...ah.com>
To:	Jonathan Cameron <jic23@....ac.uk>
Cc:	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] Sysfs group create for empty groups.

On Tue, Aug 23, 2011 at 08:25:29PM +0100, Jonathan Cameron wrote:
> On 08/23/11 12:01, Jonathan Cameron wrote:
> > On 08/23/11 01:33, Greg KH wrote:
> >> On Wed, Aug 17, 2011 at 05:27:41PM +0100, Jonathan Cameron wrote:
> >>> On 08/17/11 11:17, Jonathan Cameron wrote:
> >>>> The following is a quick stab at avoiding a hideous work around
> >>>> we are currently using in IIO.  In some cases we have entire
> >>>> attribute groups that are created from descriptions an array
> >>>> of struct iio_chan_spec.  To keep the reference counts sane
> >>>> and cause subdirectories to be created we are currently using
> >>>> dummy groups and dummy attribute arrays (provided once in the
> >>>> core).  This series is an initial probably stupid approach
> >>>> to avoiding this.
> >>>>
> >>>> Greg has expressed some doubts about whether subdirectories are
> >>>> ever a good idea, but the problem occurs for the top level
> >>>> directory as well (handled by patch 1).
> >>>>
> >>>> Note, all attributes are created at probe time.  Ultimately we
> >>>> are just respinning the create_group code to allow us to create
> >>>> the attributes from a device description rather than statically
> >>>> allocating them in each driver (results in a massive reduction
> >>>> in repeated code).
> >>>>
> >>>> All opinions welcomed.
> >>>>
> >>>> (this is definitely an rfc, the code isn't even tested yet)
> >>> Now tested and looks fine, so I've pushed it to our iio dev tree
> >>> (which is re based a few times a day currently so I can always
> >>> drop it again ;)
> >>
> >> Can you show me how you would use this so I could try to understand it a
> >> bit better?
> > Sorry, should have pointed you at an example.
> > 
> > See our iio-dev tree for the full code (some of it is in your staging tree
> > as well).
> > http://git.kernel.org/?p=linux/kernel/git/jic23/iio-blue.git;a=summary
> > 
> > Summary:
> > 
> > Sysfs attributes are created from struct iio_chan_spec arrays. These structures
> > describe the facilities and characteristics of a physical device channel in
> > a compact fashion. Sometimes there are no other attributes thus we currently
> > end up with dummy attribute_groups in the core that are registered.  The
> > purpose of this is to keep the reference counts consistent.
> > 
> > Details:
> > 
> > The core of the matter is that we have channel descriptions for every
> > channel on a device.  This encodes most of the information about the
> > channel in a consistent compact way (there are a few more unusual cases
> > we are still working out how to handle).  The sysfs control and data reading
> > attributes are generated from these struct iio_chan_spec structure arrays
> > rather than existing as a static set of attributes as they previously did.
> > 
> > This change (suggested by Arnd) has two big advantages:
> > 1) Massive reduction in boilerplate code.
> > 2) Enforced consistency of interface across drivers.
> > 
> > The base directory contains simple read attributes for the files and
> > stuff like calibration offsets.  In many cases there are other elements
> > in there not handled by iio_chan_spec. When that happens we register the
> > group of other attributes first and then use sysfs_add_file_to_group to
> > add the other attributes as they are created from the description.
> > A klist keeps track of the attributes added so they can be cleanly
> > removed later.
> > 
> > Key function is __iio_add_chan_devattr in industrialio-core.c
> > int __iio_add_chan_devattr(const char *postfix,
> > 			   const char *group,
> > 			   struct iio_chan_spec const *chan,
> > 			   ssize_t (*readfunc)(struct device *dev,
> > 					       struct device_attribute *attr,
> > 					       char *buf),
> > 			   ssize_t (*writefunc)(struct device *dev,
> > 						struct device_attribute *attr,
> > 						const char *buf,
> > 						size_t len),
> > 			   u64 mask,
> > 			   bool generic,
> > 			   struct device *dev,
> > 			   struct list_head *attr_list)
> > {
> > 	int ret;
> > 	struct iio_dev_attr *iio_attr, *t;
> > 
> > 	iio_attr = kzalloc(sizeof *iio_attr, GFP_KERNEL);
> > 	if (iio_attr == NULL) {
> > 		ret = -ENOMEM;
> > 		goto error_ret;
> > 	}
> > 	ret = __iio_device_attr_init(&iio_attr->dev_attr,
> > 				     postfix, chan,
> > 				     readfunc, writefunc, generic);
> > 	if (ret)
> > 		goto error_iio_dev_attr_free;
> > 	iio_attr->c = chan;
> > 	iio_attr->address = mask;
> > 	list_for_each_entry(t, attr_list, l)
> > 		if (strcmp(t->dev_attr.attr.name,
> > 			   iio_attr->dev_attr.attr.name) == 0) {
> > 			if (!generic)
> > 				dev_err(dev, "tried to double register : %s\n",
> > 					t->dev_attr.attr.name);
> > 			ret = -EBUSY;
> > 			goto error_device_attr_deinit;
> > 		}
> > 
> > 	ret = sysfs_add_file_to_group(&dev->kobj,
> > 				      &iio_attr->dev_attr.attr, group);
> > 	if (ret < 0)
> > 		goto error_device_attr_deinit;
> > 
> > 	list_add(&iio_attr->l, attr_list);
> > 
> > 	return 0;
> > 
> > error_device_attr_deinit:
> > 	__iio_device_attr_deinit(&iio_attr->dev_attr);
> > error_iio_dev_attr_free:
> > 	kfree(iio_attr);
> > error_ret:
> > 	return ret;
> > }
> > 
> > 
> > Call to __iio_device_attr_init builds the attribute name up and then
> > it performs checks for double registration (valid to try for 'generic'
> > attributes - but not others. When it is marked generic it means
> > it applies to a number of channels in_accel_scale for example applies to
> > all registered in_accel channels and it simply will not be added if
> > already there).
> > 
> > The file is added with sysfs_add_file_to_group.
> > 
> > So ultimately we have a dynamic equivalent of what goes on in
> > sysfs_create_group with a const group.  We could in theory
> > do two passes - first to work out what space is needed and second
> > to create a suitable attribute_group, but it's a lot uglier than
> > doing it in a single pass through the chan_spec structure array (made
> > so by the need to handle those 'generic' attributes.)  An alternative
> > is to do all but the sysfs_add_file_to_group and then build an
> > attribute group from our existing klist of attributes.
> > 
> > Basically I'm happy to do anything that makes this work. If we
> > agree there is a reason to have sysfs_add_file_to_group available
> > then to my mind we should also allow for empty groups. Right now
> > it looks like there are only a few users of that (I count about 6 with
> > 2 of them in IIO).
> > 
> > The group add of an empty group is simply to get the reference count
> > consistent. 
> > 
> > The slightly more involved case is for the scanelements subdirectory.
> > This describes the contents of the any buffers that the driver supports
> > and is often (but not quite always) generated entirely from the iio_chan_spec
> > structure arrays provided by the drivers.  It's in a subdirectory as purely
> > a means of grouping elements related to a particular task (buffer description)
> > rather than any inherent requirement. This case motivates the allowing a group
> > with a null pointer in .attrs to avoid having a
> > struct attribute *dummy_attrs[] = {NULL};
> > which is ugly.
> > 
> > So what do you recommend?
> Having just come across Bart's documentation patches to the driver-model
> docs I now understand the userspace issue (misunderstood what you said
> the other day).  Basically if it isn't done through the groups element
> of struct device userspace won't know the attributes have been created.
> 
> Will see if I can rework the registration code to build up a suitable cache
> of what will be in the attribute groups for each device. This dynamic
> cache can then be used to build up everything needed before the
> device_register occurs.  It's going to be somewhat clunky but contained
> within the IIO core so not too bad.
> 
> So upshot is ignore this patch set.  The hack with the dummy groups
> will have to stand for 3.1 in IIO.

Ok, consider it ignored :)

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