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Subject: Re: [PATCH 6/9] of: add support for 'dynamic' DT property
From: Frank Rowand <frowand.list@gmail.com>
To: Zev Weiss <zev@bewilderbeest.net>, openbmc@lists.ozlabs.org
References: <20211007000954.30621-1-zev@bewilderbeest.net>
 <20211007000954.30621-7-zev@bewilderbeest.net>
 <7bf5cfce-e84d-f0e8-e6e8-8e6fedffd154@gmail.com>
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Date: Fri, 8 Oct 2021 14:19:10 -0500
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Cc: devicetree@vger.kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 linux-kernel@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
 Jeremy Kerr <jk@codeconstruct.com.au>
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 <openbmc-bounces+openbmc=archiver.kernel.org@lists.ozlabs.org>

On 10/8/21 1:51 PM, Frank Rowand wrote:
> On 10/6/21 7:09 PM, Zev Weiss wrote:
>> Nodes marked with this (boolean) property will have a writable status
>> sysfs file, which can be used to toggle them between "okay" and
>> "reserved", effectively hot-plugging them.  Note that this will only
>> be effective for devices on busses that register for OF reconfig
>> notifications (currently spi, i2c, and platform), and only if
>> CONFIG_OF_DYNAMIC is enabled.
>>
>> Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
>> ---
>>  drivers/of/kobj.c | 69 +++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 69 insertions(+)
>>
>> diff --git a/drivers/of/kobj.c b/drivers/of/kobj.c
>> index 378cb421aae1..141ae23f3130 100644
>> --- a/drivers/of/kobj.c
>> +++ b/drivers/of/kobj.c
>> @@ -36,6 +36,69 @@ static ssize_t of_node_property_read(struct file *filp, struct kobject *kobj,
>>  	return memory_read_from_buffer(buf, count, &offset, pp->value, pp->length);
>>  }
>>  
>> +static ssize_t of_node_status_write(struct file *filp, struct kobject *kobj,
>> +                                    struct bin_attribute *bin_attr, char *buf,
>> +                                    loff_t offset, size_t count)
>> +{
>> +	int rc;
>> +	char *newstatus;
>> +	struct property **deadprev;
>> +	struct property *newprop = NULL;
>> +	struct property *oldprop = container_of(bin_attr, struct property, attr);
>> +	struct device_node *np = container_of(kobj, struct device_node, kobj);
>> +
>> +	if (WARN_ON_ONCE(strcmp(oldprop->name, "status")))
>> +		return -EIO;
>> +
>> +	if (offset)
>> +		return -EINVAL;
>> +
>> +	if (sysfs_buf_streq(buf, count, "okay") || sysfs_buf_streq(buf, count, "ok"))
>> +		newstatus = "okay";
>> +	else if (sysfs_buf_streq(buf, count, "reserved"))
>> +		newstatus = "reserved";
>> +	else if (sysfs_buf_streq(buf, count, "disabled"))
>> +		newstatus = "disabled";
>> +	else
>> +		return -EINVAL;
>> +
>> +	if (!strcmp(newstatus, oldprop->value))
>> +		return count;
>> +
> 
> If the general approach of this patch set is the correct way to provide the desired
> functionality (I'm still pondering that), then a version of the following code
> probably belongs in drivers/of/dynamic.c so that it is easier to maintain and keep
> consistent with other dynamic devicetree updates.  If you look at the code there
> that touches deadprops (eg __of_changeset_entry_apply()) you will notice that the
> locking issues are more extensive than what is implemented here.
> 
> I'm still thinking about how this interacts with other forms of dynamic devicetree
> changes (eg drivers/of/dynamic.c and also overlays).
> 
>> +	/*
>> +	 * of_update_property_self() doesn't free replaced properties, so
>> +	 * rifle through deadprops first to see if there's an equivalent old
>> +	 * status property we can reuse instead of allocating a new one.
>> +	 */
>> +	mutex_lock(&of_mutex);
>> +	for (deadprev = &np->deadprops; *deadprev; deadprev = &(*deadprev)->next) {
>> +		struct property *deadprop = *deadprev;
>> +		if (!strcmp(deadprop->name, "status") &&
>> +		    deadprop->length == strlen(newstatus) + 1 &&
>> +		    !strcmp(deadprop->value, newstatus)) {
>> +			*deadprev = deadprop->next;
>> +			deadprop->next = NULL;
>> +			newprop = deadprop;
>> +			break;
>> +		}
>> +	}
>> +	mutex_unlock(&of_mutex);
>> +
>> +	if (!newprop) {
>> +		newprop = kzalloc(sizeof(*newprop), GFP_KERNEL);
>> +		if (!newprop)
>> +			return -ENOMEM;
>> +
>> +		newprop->name = oldprop->name;
>> +		newprop->value = newstatus;
>> +		newprop->length = strlen(newstatus) + 1;
>> +	}
>> +
>> +	rc = of_update_property_self(np, newprop, true);
> 
> -Frank
> 
>> +
>> +	return rc ? rc : count;
>> +}
>> +
>>  /* always return newly allocated name, caller must free after use */
>>  static const char *safe_name(struct kobject *kobj, const char *orig_name)
>>  {
>> @@ -79,6 +142,12 @@ int __of_add_property_sysfs(struct device_node *np, struct property *pp)
>>  	pp->attr.size = secure ? 0 : pp->length;
>>  	pp->attr.read = of_node_property_read;
>>  
>> +	if (!strcmp(pp->name, "status") && of_property_read_bool(np, "dynamic")) {
>> +		pp->attr.attr.mode |= 0200;
>> +		pp->attr.write = of_node_status_write;
>> +		pp->attr.growable = true;
>> +	}

This isn't (yet) a request for a change to the patch, but more exposing a
potential issue of interaction with overlays.

The current in tree instances of updating a property value are fairly
limited.  Adding a userspace interface to update a property value
(although limited to only a node's status value) has me thinking about
the interaction with dynamic devicetree updates (of/overlay/dynamic.c)
and also overlays.

If a node exists in a base devicetree, containing only the property
"status", then an overlay subsequently populates the node with a
"dynamic" property then the sysfs status file will not be updated
to be writable.  If we do not allow an overlay to add a property
to an existing node, then not an issue.  But we have not created
such overlay rules yet.

Again, not a request for a change to this patch yet, just leaving
a breadcrumb for myself.

-Frank

>> +
>>  	rc = sysfs_create_bin_file(&np->kobj, &pp->attr);
>>  	WARN(rc, "error adding attribute %s to node %pOF\n", pp->name, np);
>>  	return rc;
>>
>