[media] DocBook: partial rewrite of "Opening and Closing Devices"

This section was horribly out of date. A lot of references to old and
obsolete behavior have been dropped.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/common.xml

@@ -38,70 +38,41 @@ the basic concepts applicable to all devices.</para>
 
       <para>V4L2 drivers are implemented as kernel modules, loaded
 manually by the system administrator or automatically when a device is
-first opened. The driver modules plug into the "videodev" kernel
+first discovered. The driver modules plug into the "videodev" kernel
 module. It provides helper functions and a common application
 interface specified in this document.</para>
 
       <para>Each driver thus loaded registers one or more device nodes
-with major number 81 and a minor number between 0 and 255. Assigning
+with major number 81 and a minor number between 0 and 255. Minor numbers
-minor numbers to V4L2 devices is entirely up to the system administrator,
+are allocated dynamically unless the kernel is compiled with the kernel
-this is primarily intended to solve conflicts between devices.<footnote>
+option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers are
-	  <para>Access permissions are associated with character
+allocated in ranges depending on the device node type (video, radio, etc.).</para>
-device special files, hence we must ensure device numbers cannot
+
-change with the module load order. To this end minor numbers are no
+      <para>Many drivers support "video_nr", "radio_nr" or "vbi_nr"
-longer automatically assigned by the "videodev" module as in V4L but
+module options to select specific video/radio/vbi node numbers. This allows
-requested by the driver. The defaults will suffice for most people
+the user to request that the device node is named e.g. /dev/video5 instead
-unless two drivers compete for the same minor numbers.</para>
+of leaving it to chance. When the driver supports multiple devices of the same
-	</footnote> The module options to select minor numbers are named
+type more than one device node number can be assigned, separated by commas:
-after the device special file with a "_nr" suffix. For example "video_nr"
+	<informalexample>
-for <filename>/dev/video</filename> video capture devices. The number is
-an offset to the base minor number associated with the device type.
-<footnote>
-	  <para>In earlier versions of the V4L2 API the module options
-where named after the device special file with a "unit_" prefix, expressing
-the minor number itself, not an offset. Rationale for this change is unknown.
-Lastly the naming and semantics are just a convention among driver writers,
-the point to note is that minor numbers are not supposed to be hardcoded
-into drivers.</para>
-	</footnote> When the driver supports multiple devices of the same
-type more than one minor number can be assigned, separated by commas:
-<informalexample>
 	  <screen>
-&gt; insmod mydriver.o video_nr=0,1 radio_nr=0,1</screen>
+&gt; modprobe mydriver video_nr=0,1 radio_nr=0,1</screen>
 	</informalexample></para>
 
       <para>In <filename>/etc/modules.conf</filename> this may be
 written as: <informalexample>
 	  <screen>
-alias char-major-81-0 mydriver
+options mydriver video_nr=0,1 radio_nr=0,1
-alias char-major-81-1 mydriver
-alias char-major-81-64 mydriver              <co id="alias" />
-options mydriver video_nr=0,1 radio_nr=0,1   <co id="options" />
 	  </screen>
-	  <calloutlist>
+	</informalexample> When no device node number is given as module
-	    <callout arearefs="alias">
+option the driver supplies a default.</para>
-	      <para>When an application attempts to open a device
-special file with major number 81 and minor number 0, 1, or 64, load
-"mydriver" (and the "videodev" module it depends upon).</para>
-	    </callout>
-	    <callout arearefs="options">
-	      <para>Register the first two video capture devices with
-minor number 0 and 1 (base number is 0), the first two radio device
-with minor number 64 and 65 (base 64).</para>
-	    </callout>
-	  </calloutlist>
-	</informalexample> When no minor number is given as module
-option the driver supplies a default. <xref linkend="devices" />
-recommends the base minor numbers to be used for the various device
-types. Obviously minor numbers must be unique. When the number is
-already in use the <emphasis>offending device</emphasis> will not be
-registered. <!-- Blessed by Linus Torvalds on
-linux-kernel@vger.kernel.org, 2002-11-20. --></para>
 
-      <para>By convention system administrators create various
+      <para>Normally udev will create the device nodes in /dev automatically
-character device special files with these major and minor numbers in
+for you. If udev is not installed, then you need to enable the
-the <filename>/dev</filename> directory. The names recommended for the
+CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to correctly
-different V4L2 device types are listed in <xref linkend="devices" />.
+relate a minor number to a device node number. I.e., you need to be certain
+that minor number 5 maps to device node name video5. With this kernel option
+different device types have different minor number ranges. These ranges are
+listed in <xref linkend="devices" />.
 </para>
 
       <para>The creation of character special files (with
@@ -110,85 +81,66 @@ devices cannot be opened by major and minor number. That means
 applications cannot <emphasis>reliable</emphasis> scan for loaded or
 installed drivers. The user must enter a device name, or the
 application can try the conventional device names.</para>
-
-      <para>Under the device filesystem (devfs) the minor number
-options are ignored. V4L2 drivers (or by proxy the "videodev" module)
-automatically create the required device files in the
-<filename>/dev/v4l</filename> directory using the conventional device
-names above.</para>
     </section>
 
     <section id="related">
       <title>Related Devices</title>
 
-      <para>Devices can support several related functions. For example
+      <para>Devices can support several functions. For example
-video capturing, video overlay and VBI capturing are related because
+video capturing, VBI capturing and radio support.</para>
-these functions share, amongst other, the same video input and tuner
-frequency. V4L and earlier versions of V4L2 used the same device name
-and minor number for video capturing and overlay, but different ones
-for VBI. Experience showed this approach has several problems<footnote>
-	  <para>Given a device file name one cannot reliable find
-related devices. For once names are arbitrary and in a system with
-multiple devices, where only some support VBI capturing, a
-<filename>/dev/video2</filename> is not necessarily related to
-<filename>/dev/vbi2</filename>. The V4L
-<constant>VIDIOCGUNIT</constant> ioctl would require a search for a
-device file with a particular major and minor number.</para>
-	</footnote>, and to make things worse the V4L videodev module
-used to prohibit multiple opens of a device.</para>
 
-      <para>As a remedy the present version of the V4L2 API relaxed the
+      <para>The V4L2 API creates different nodes for each of these functions.</para>
-concept of device types with specific names and minor numbers. For
-compatibility with old applications drivers must still register different
-minor numbers to assign a default function to the device. But if related
-functions are supported by the driver they must be available under all
-registered minor numbers. The desired function can be selected after
-opening the device as described in <xref linkend="devices" />.</para>
 
-      <para>Imagine a driver supporting video capturing, video
+      <para>The V4L2 API was designed with the idea that one device node could support
-overlay, raw VBI capturing, and FM radio reception. It registers three
+all functions. However, in practice this never worked: this 'feature'
-devices with minor number 0, 64 and 224 (this numbering scheme is
+was never used by applications and many drivers did not support it and if
-inherited from the V4L API). Regardless if
+they did it was certainly never tested. In addition, switching a device
-<filename>/dev/video</filename> (81, 0) or
+node between different functions only works when using the streaming I/O
-<filename>/dev/vbi</filename> (81, 224) is opened the application can
+API, not with the read()/write() API.</para>
-select any one of the video capturing, overlay or VBI capturing
+
-functions. Without programming (e.&nbsp;g. reading from the device
+      <para>Today each device node supports just one function.</para>
-with <application>dd</application> or <application>cat</application>)
-<filename>/dev/video</filename> captures video images, while
-<filename>/dev/vbi</filename> captures raw VBI data.
-<filename>/dev/radio</filename> (81, 64) is invariable a radio device,
-unrelated to the video functions. Being unrelated does not imply the
-devices can be used at the same time, however. The &func-open;
-function may very well return an &EBUSY;.</para>
 
       <para>Besides video input or output the hardware may also
 support audio sampling or playback. If so, these functions are
-implemented as OSS or ALSA PCM devices and eventually OSS or ALSA
+implemented as ALSA PCM devices with optional ALSA audio mixer
-audio mixer. The V4L2 API makes no provisions yet to find these
+devices.</para>
-related devices. If you have an idea please write to the linux-media
+
-mailing list: &v4l-ml;.</para>
+      <para>One problem with all these devices is that the V4L2 API
+makes no provisions to find these related devices. Some really
+complex devices use the Media Controller (see <xref linkend="media_controller" />)
+which can be used for this purpose. But most drivers do not use it,
+and while some code exists that uses sysfs to discover related devices
+(see libmedia_dev in the <ulink url="http://git.linuxtv.org/v4l-utils/">v4l-utils</ulink>
+git repository), there is no library yet that can provide a single API towards
+both Media Controller-based devices and devices that do not use the Media Controller.
+If you want to work on this please write to the linux-media mailing list: &v4l-ml;.</para>
     </section>
 
     <section>
       <title>Multiple Opens</title>
 
-      <para>In general, V4L2 devices can be opened more than once.
+      <para>V4L2 devices can be opened more than once.<footnote><para>
+There are still some old and obscure drivers that have not been updated to
+allow for multiple opens. This implies that for such drivers &func-open; can
+return an &EBUSY; when the device is already in use.</para></footnote>
 When this is supported by the driver, users can for example start a
 "panel" application to change controls like brightness or audio
 volume, while another application captures video and audio. In other words, panel
-applications are comparable to an OSS or ALSA audio mixer application.
+applications are comparable to an ALSA audio mixer application.
-When a device supports multiple functions like capturing and overlay
+Just opening a V4L2 device should not change the state of the device.<footnote>
-<emphasis>simultaneously</emphasis>, multiple opens allow concurrent
+<para>Unfortunately, opening a radio device often switches the state of the
-use of the device by forked processes or specialized applications.</para>
+device to radio mode in many drivers. This behavior should be fixed eventually
+as it violates the V4L2 specification.</para></footnote></para>
 
-      <para>Multiple opens are optional, although drivers should
+      <para>Once an application has allocated the memory buffers needed for
-permit at least concurrent accesses without data exchange, &ie; panel
+streaming data (by calling the &VIDIOC-REQBUFS; or &VIDIOC-CREATE-BUFS; ioctls,
-applications. This implies &func-open; can return an &EBUSY; when the
+or implicitly by calling the &func-read; or &func-write; functions) that
-device is already in use, as well as &func-ioctl; functions initiating
+application (filehandle) becomes the owner of the device. It is no longer
-data exchange (namely the &VIDIOC-S-FMT; ioctl), and the &func-read;
+allowed to make changes that would affect the buffer sizes (e.g. by calling
-and &func-write; functions.</para>
+the &VIDIOC-S-FMT; ioctl) and other applications are no longer allowed to allocate
+buffers or start or stop streaming. The &EBUSY; will be returned instead.</para>
 
-      <para>Mere opening a V4L2 device does not grant exclusive
+      <para>Merely opening a V4L2 device does not grant exclusive
 access.<footnote>
 	  <para>Drivers could recognize the
 <constant>O_EXCL</constant> open flag. Presently this is not required,
@@ -206,12 +158,7 @@ additional access privileges using the priority mechanism described in
       <para>V4L2 drivers should not support multiple applications
 reading or writing the same data stream on a device by copying
 buffers, time multiplexing or similar means. This is better handled by
-a proxy application in user space. When the driver supports stream
+a proxy application in user space.</para>
-sharing anyway it must be implemented transparently. The V4L2 API does
-not specify how conflicts are solved. <!-- For example O_EXCL when the
-application does not want to be preempted, PROT_READ mmapped buffers
-which can be mapped twice, what happens when image formats do not
-match etc.--></para>
     </section>
 
     <section>

Documentation/DocBook/media/v4l/v4l2.xml

@@ -145,7 +145,7 @@ applications. -->
 	<revnumber>3.15</revnumber>
 	<date>2014-02-03</date>
 	<authorinitials>hv</authorinitials>
-	<revremark>Update several sections of "Common API Elements":
+	<revremark>Update several sections of "Common API Elements": "Opening and Closing Devices"
 "Querying Capabilities", "Application Priority", "Video Inputs and Outputs", "Audio Inputs and Outputs"
 "Tuners and Modulators", "Video Standards" and "Digital Video (DV) Timings".
 	</revremark>