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Subject: Re: [PATCH 6/9] of: add support for 'dynamic' DT property
To:     Zev Weiss <zev@bewilderbeest.net>, openbmc@lists.ozlabs.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jeremy Kerr <jk@codeconstruct.com.au>,
        Joel Stanley <joel@jms.id.au>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20211007000954.30621-1-zev@bewilderbeest.net>
 <20211007000954.30621-7-zev@bewilderbeest.net>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <7bf5cfce-e84d-f0e8-e6e8-8e6fedffd154@gmail.com>
Date:   Fri, 8 Oct 2021 13:51:47 -0500
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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;
> +	}
> +
>  	rc = sysfs_create_bin_file(&np->kobj, &pp->attr);
>  	WARN(rc, "error adding attribute %s to node %pOF\n", pp->name, np);
>  	return rc;
>