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Subject: [PATCH] dvb: Allow MAC addresses to be mapped to stable device
 names with udev
From:   David Howells <dhowells@redhat.com>
To:     mchehab@kernel.org
Cc:     dhowells@redhat.com, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Mon, 24 Sep 2018 11:10:31 +0100
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Some devices, such as the DVBSky S952 and T982 cards, are dual port cards
that provide two cx23885 devices on the same PCI device, which means the
attributes available for writing udev rules are exactly the same, apart
from the adapter number.  Unfortunately, the adapter numbers are dependent
on the order in which things are initialised, so this can change over
different releases of the kernel.

Devices have a MAC address available, which is printed during boot:

	[   10.951517] DVBSky T982 port 1 MAC address: 00:11:22:33:44:55
	...
	[   10.984875] DVBSky T982 port 2 MAC address: 00:11:22:33:44:56

To make it possible to distinguish these in udev, provide sysfs attributes
to make the MAC address, adapter number and type available.  There are
other fields that could perhaps be exported also.  In particular, it would
be nice to provide the port number, but somehow that doesn't manage to
propagate through the labyrinthine initialisation process.

The new sysfs attributes can be seen from userspace as:

	[root@deneb ~]# ls /sys/class/dvb/dvb0.frontend0/
	dev  device  dvb_adapter  dvb_mac  dvb_type
	power  subsystem  uevent
	[root@deneb ~]# cat /sys/class/dvb/dvb0.frontend0/dvb_*
	0
	00:11:22:33:44:55
	frontend

They can be used in udev rules:

	SUBSYSTEM=="dvb", ATTRS{vendor}=="0x14f1", ATTRS{device}=="0x8852", ATTRS{subsystem_device}=="0x0982", ATTR{dvb_mac}=="00:11:22:33:44:55", PROGRAM="/bin/sh -c 'K=%k; K=$${K#dvb}; printf dvb/adapter9820/%%s $${K#*.}'", SYMLINK+="%c"
	SUBSYSTEM=="dvb", ATTRS{vendor}=="0x14f1", ATTRS{device}=="0x8852", ATTRS{subsystem_device}=="0x0982", ATTR{dvb_mac}=="00:11.22.33.44.56", PROGRAM="/bin/sh -c 'K=%k; K=$${K#dvb}; printf dvb/adapter9821/%%s $${K#*.}'", SYMLINK+="%c"

where the match is made with ATTR{dvb_mac} or similar.  The rules above
make symlinks from /dev/dvb/adapter982/* to /dev/dvb/adapterXX/*.

Note that binding the dvb-net device to a network interface and changing it
there does not reflect back into the the dvb_adapter struct and doesn't
change the MAC address here.  This means that a system with two identical
cards in it may need to distinguish them by some other means than MAC
address.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 Documentation/ABI/testing/sysfs-class-dvb     |   29 +++++++++++
 Documentation/media/dvb-drivers/udev.rst      |   29 +++++++++++
 Documentation/media/uapi/dvb/intro.rst        |    7 +++
 Documentation/media/uapi/dvb/stable_names.rst |   66 +++++++++++++++++++++++++
 drivers/media/dvb-core/dvbdev.c               |   36 ++++++++++++++
 5 files changed, 167 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-class-dvb
 create mode 100644 Documentation/media/uapi/dvb/stable_names.rst

diff --git a/Documentation/ABI/testing/sysfs-class-dvb b/Documentation/ABI/testing/sysfs-class-dvb
new file mode 100644
index 000000000000..09e3be329c92
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-dvb
@@ -0,0 +1,29 @@
+What:		/sys/class/dvb/.../dvb_adapter
+Date:		September 2018
+KernelVersion:	4.20
+Contact:	David Howells <dhowells@redhat.com>
+Description:
+		This displays the assigned adapter number of a DVB device.
+
+What:		/sys/class/dvb/.../dvb_mac
+Date:		September 2018
+KernelVersion:	4.20
+Contact:	David Howells <dhowells@redhat.com>
+Description:
+		This displays the mac address of a DVB device.  This can be
+		used by udev to name stable device files for DVB devices and
+		avoid problems with changes in the order of device
+		initialisation changing the assigned device numbers.  See:
+
+			Documentation/media/dvb-drivers/udev.rst
+			Documentation/media/uapi/dvb/stable_names.rst
+
+		for information on how to actually do this.
+
+What:		/sys/class/dvb/.../dvb_type
+Date:		September 2018
+KernelVersion:	4.20
+Contact:	David Howells <dhowells@redhat.com>
+Description:
+		This displays the object type of a DVB device interface, such
+		as "frontend" or "demux".
diff --git a/Documentation/media/dvb-drivers/udev.rst b/Documentation/media/dvb-drivers/udev.rst
index 7d7d5d82108a..df754312f1f4 100644
--- a/Documentation/media/dvb-drivers/udev.rst
+++ b/Documentation/media/dvb-drivers/udev.rst
@@ -59,3 +59,32 @@ have a look at "man udev".
 For every device that registers to the sysfs subsystem with a "dvb" prefix,
 the helper script /etc/udev/scripts/dvb.sh is invoked, which will then
 create the proper device node in your /dev/ directory.
+
+2. A DVB device's adapter number, type and MAC addresses are exposed through
+the sysfs interface as files dvb_adapter, dvb_type and dvb_mac in the various
+dvb object directories, e.g. /sys/class/dvb/dvb0.demux0/dvb_mac.
+
+These can be used to influence the binding of devices to names in /dev to avoid
+problems when the order in which names are assigned changes.  This is of
+particular interest when you have, say, a PCI card with multiple identical
+devices on board under the same PCI function slot.  The only way to distinguish
+them is either by the DVB port number or the DVB MAC address.
+
+To make use of this with udev, a rule needs to be emplaced in a file under
+/etc/udev/rules.d/ that has an appropriate ATTR{} clause in it.  Something like
+the following, for example::
+
+	SUBSYSTEM=="dvb", ATTRS{vendor}=="0x14f1", ATTRS{device}=="0x8852", ATTRS{subsystem_device}=="0x0982", ATTR{dvb_mac}=="00:11:22:33:44:55", PROGRAM="/bin/sh -c 'K=%k; K=$${K#dvb}; printf dvb/adapter9820/%%s $${K#*.}'", SYMLINK+="%c"
+
+Note the 'ATTR{dvb_mac}' clause that indicates the MAC address to look for.
+This should be different for every device, even if the devices are otherwise
+identical.  The other ATTR{} clauses in this example refer to PCI parameters.
+
+This example generates a directory called /dev/dvb/adapter9820/ and places
+symlinks in it to the device files under the appropriate /dev/dvb/adapterX/
+directory - whatever X happens to be today.
+
+The generated name is then stable and can be relied on by programs that need to
+pick it up without user interaction.
+
+Note that this facility does not exist in v4.19 kernels and earlier.
diff --git a/Documentation/media/uapi/dvb/intro.rst b/Documentation/media/uapi/dvb/intro.rst
index 79b4d0e4e920..074fb3b3ee21 100644
--- a/Documentation/media/uapi/dvb/intro.rst
+++ b/Documentation/media/uapi/dvb/intro.rst
@@ -153,6 +153,13 @@ where ``N`` enumerates the Digital TV cards in a system starting from 0, and
 from 0, too. We will omit the “``/dev/dvb/adapterN/``\ ” in the further
 discussion of these devices.
 
+Note that the automatic numbering of adapters isn't stable and may vary
+depending on changes to the order in which devices are initialised, both in
+the order in which individual devices get initialised and also the order in
+which subdevices get initialised (e.g. a PCI card with multiple identical DVB
+devices attached to the same PCI function).  :ref:`stable_names` shows use
+udev rules to create stable names.
+
 More details about the data structures and function calls of all the
 devices are described in the following chapters.
 
diff --git a/Documentation/media/uapi/dvb/stable_names.rst b/Documentation/media/uapi/dvb/stable_names.rst
new file mode 100644
index 000000000000..1b5dc5171ee3
--- /dev/null
+++ b/Documentation/media/uapi/dvb/stable_names.rst
@@ -0,0 +1,66 @@
+.. -*- coding: utf-8; mode: rst -*-
+
+.. _stable_names:
+
+*********************************
+Creating stable device file names
+*********************************
+
+From time to time the order in which the Linux kernel initialises devices and
+initialises subdevices within those devices has changed.  This can cause the
+assignment of user-visible device numbers to devices to fluctuate - leading to
+the failure of services to operate correctly in non-obvious ways when multiple,
+otherwise identical devices are available in a system.
+
+To counteract this, udev rules can be defined that map devices onto stable
+names.  This must, however, be done in relation to attributes of a device that
+don't vary, such as the MAC address.
+
+Take, for example, a PCI DVB card that has two identical DVB devices attached
+to the same PCI function.  The devices cannot be distinguished on PCI
+parameters and the DVB port number - which could otherwise distinguish these
+subdevices - is not easily accessible by userspace.
+
+The MAC address, however, *is* made available, and this is supposed to be
+unique to each individual DVB device, and won't vary even if the device is
+moved to another slot.  This is exported to userspace through sysfs.  It can
+be found by looking in the dvb_mac file that can be found in a device
+interface's directory, for example:
+
+	/sys/class/dvb/dvb0.demux0/dvb_mac
+
+Two other files can be found there that export the adapter number and the
+interface type:
+
+	/sys/class/dvb/dvb0.demux0/dvb_adapter
+	/sys/class/dvb/dvb0.demux0/dvb_type
+
+Note that the two numbers in the path are assigned based on the order in which
+the devices are registered with the core code, and not necessarily on the
+physical arrangement of the device - and thus should not be considered stable.
+
+
+The creation of stable names can be done by writing rules for udev to match on
+the MAC addresses of the devices.  Rules needs to be placed in a file in the
+/etc/udev/rules.d/ directory for udev to pick up.  They need appropriate
+ATTR{} clauses to specify the attribute matches to make.  Any of the above
+mentioned files can be used.  For example::
+
+	SUBSYSTEM=="dvb", ATTRS{vendor}=="0x14f1", ATTRS{device}=="0x8852", ATTRS{subsystem_device}=="0x0982", ATTR{dvb_mac}=="00:11:22:33:44:55", PROGRAM="/bin/sh -c 'K=%k; K=$${K#dvb}; printf dvb/adapter9820/%%s $${K#*.}'", SYMLINK+="%c"
+	SUBSYSTEM=="dvb", ATTRS{vendor}=="0x14f1", ATTRS{device}=="0x8852", ATTRS{subsystem_device}=="0x0982", ATTR{dvb_mac}=="00:11.22.33.44.56", PROGRAM="/bin/sh -c 'K=%k; K=$${K#dvb}; printf dvb/adapter9821/%%s $${K#*.}'", SYMLINK+="%c"
+
+In each of these example rules, the first three ATTR{} clauses specify the PCI
+card to match - in this case the same DVBsky T982 dual T2 receiver card.  The
+ATTR{dvb_mac} attribute in each specifies the card MAC address of that
+receiver unit (the name of the attribute refers to the name of sysfs file to
+read).
+
+This example generates a pair of directories called /dev/dvb/adapter9820/ and
+/dev/dvb/adapter9821/ and places in each symlinks to the device files under
+the appropriate /dev/dvb/adapterX/ and /dev/dvb/adapterY/ directories -
+whatever X and Y happens to be today.
+
+The generated names are then stable and can be relied on by programs that need
+to pick it up without user interaction.
+
+Note that this facility does not exist in v4.19 kernels and earlier.
diff --git a/drivers/media/dvb-core/dvbdev.c b/drivers/media/dvb-core/dvbdev.c
index 64d6793674b9..41be3ba66341 100644
--- a/drivers/media/dvb-core/dvbdev.c
+++ b/drivers/media/dvb-core/dvbdev.c
@@ -995,6 +995,41 @@ void dvb_module_release(struct i2c_client *client)
 EXPORT_SYMBOL_GPL(dvb_module_release);
 #endif
 
+static ssize_t dvb_adapter_show(struct device *dev,
+				struct device_attribute *attr, char *buf)
+{
+	struct dvb_device *dvbdev = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%d\n", dvbdev->adapter->num);
+}
+static DEVICE_ATTR_RO(dvb_adapter);
+
+static ssize_t dvb_mac_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	struct dvb_device *dvbdev = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%pM\n", dvbdev->adapter->proposed_mac);
+}
+static DEVICE_ATTR_RO(dvb_mac);
+
+static ssize_t dvb_type_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	struct dvb_device *dvbdev = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%s\n", dnames[dvbdev->type]);
+}
+static DEVICE_ATTR_RO(dvb_type);
+
+static struct attribute *dvb_class_attrs[] = {
+	&dev_attr_dvb_adapter.attr,
+	&dev_attr_dvb_mac.attr,
+	&dev_attr_dvb_type.attr,
+	NULL
+};
+ATTRIBUTE_GROUPS(dvb_class);
+
 static int dvb_uevent(struct device *dev, struct kobj_uevent_env *env)
 {
 	struct dvb_device *dvbdev = dev_get_drvdata(dev);
@@ -1035,6 +1070,7 @@ static int __init init_dvbdev(void)
 		retval = PTR_ERR(dvb_class);
 		goto error;
 	}
+	dvb_class->dev_groups = dvb_class_groups,
 	dvb_class->dev_uevent = dvb_uevent;
 	dvb_class->devnode = dvb_devnode;
 	return 0;