[RFC,v5,2/2] alsa: jack: add document for sound jack injection.

diff --git a/Documentation/sound/designs/index.rst b/Documentation/sound/designs/index.rst
index f0749943ccb2..1eb08e7bae52 100644
--- a/Documentation/sound/designs/index.rst
+++ b/Documentation/sound/designs/index.rst
@@ -14,3 +14,4 @@  Designs and Implementations
    powersave
    oss-emulation
    seq-oss
+   jack-injection
diff --git a/Documentation/sound/designs/jack-injection.rst b/Documentation/sound/designs/jack-injection.rst
new file mode 100644
index 000000000000..0915048a0dbd
--- /dev/null
+++ b/Documentation/sound/designs/jack-injection.rst
@@ -0,0 +1,124 @@ 
+============================
+ALSA Jack Software Injection
+============================
+
+Simple Introduction On Jack Injection
+=====================================
+
+Here jack injection means users could inject plugin or plugout events
+to the audio jacks through debugfs interface, it is helpful to
+validate ALSA userspace changes. For example, we change the audio
+profile switching code in the pulseaudio, and we want to verify if the
+change works as expected and if the change introduce the regression,
+in this case, we could inject plugin or plugout events to an audio
+jack or to some audio jacks, we don't need to physically access the
+machine and plug/unplug physical devices to the audio jack.
+
+In this design, an audio jack doesn't equal to a physical audio jack.
+Sometimes a physical audio jack contains multi functions, and the
+ALSA driver creates multi ``jack_kctl`` for a ``snd_jack``, here the
+``snd_jack`` represents a physical audio jack and the ``jack_kctl``
+represents a function, for example a physical jack has two functions:
+headphone and mic_in, the ALSA ASoC driver will build 2 ``jack_kctl``
+for this jack. The jack injection is implemented based on the
+``jack_kctl`` instead of ``snd_jack``.
+
+To inject events to audio jacks, we need to enable the jack injection
+via ``sw_inject_enable`` first, once it is enabled, this jack will not
+change the state by hardware events anymore, we could inject plugin or
+plugout events via ``jackin_inject`` and check the jack state via
+``status``, after we finish our test, we need to disable the jack
+injection via ``sw_inject_enable`` too, once it is disabled, the jack
+state will be restored according to the last reported hardware events
+and will change by future hardware events.
+
+The Layout of Jack Injection Interface
+======================================
+
+If users enable the SND_JACK_INJECTION_DEBUG in the kernel, the audio
+jack injection interface will be created as below:
+::
+
+   $debugfs_mount_dir/sound
+   |-- card0
+   |-- |-- HDMI_DP_pcm_10_Jack
+   |-- |-- |-- jackin_inject
+   |-- |-- |-- kctl_id
+   |-- |-- |-- mask_bits
+   |-- |-- |-- status
+   |-- |-- |-- sw_inject_enable
+   |-- |-- |-- type
+   ...
+   |-- |-- HDMI_DP_pcm_9_Jack
+   |--     |-- jackin_inject
+   |--     |-- kctl_id
+   |--     |-- mask_bits
+   |--     |-- status
+   |--     |-- sw_inject_enable
+   |--     |-- type
+   |-- card1
+       |-- HDMI_DP_pcm_5_Jack
+       |-- |-- jackin_inject
+       |-- |-- kctl_id
+       |-- |-- mask_bits
+       |-- |-- status
+       |-- |-- sw_inject_enable
+       |-- |-- type
+       ...
+       |-- Headphone_Jack
+       |-- |-- jackin_inject
+       |-- |-- kctl_id
+       |-- |-- mask_bits
+       |-- |-- status
+       |-- |-- sw_inject_enable
+       |-- |-- type
+       |-- Headset_Mic_Jack
+           |-- jackin_inject
+           |-- kctl_id
+           |-- mask_bits
+           |-- status
+           |-- sw_inject_enable
+           |-- type
+
+
+Explanation for the debugfs nodes:
+::
+
+ - kctl_id, read-only, get jack_kctl->kctl's id
+   sound/card1/Headphone_Jack# cat kctl_id
+   Headphone Jack
+
+ - mask_bits, read-only, get jack_kctl's supported events mask_bits
+   sound/card1/Headphone_Jack# cat mask_bits
+   0x0001 HEADPHONE(0x0001)
+
+ - status, read-only, get jack_kctl's current status
+   headphone unplugged:
+   sound/card1/Headphone_Jack# cat status
+   Unplugged
+   headphone plugged:
+   sound/card1/Headphone_Jack# cat status
+   Plugged
+
+ - type, read-only, get snd_jack's supported events type (all
+   supported events on the physical audio jack)
+   sound/card1/Headphone_Jack# cat type
+   0x7803 HEADPHONE(0x0001) MICROPHONE(0x0002) BTN_3(0x0800) BTN_2(0x1000) BTN_1(0x2000) BTN_0(0x4000)
+
+ - sw_inject_enable, read-write, enable or disable injection
+   injection disabled:
+   sound/card1/Headphone_Jack# cat sw_inject_enable
+   Jack: Headphone Jack		Inject Enabled: 0
+   injection enabled:
+   sound/card1/Headphone_Jack# cat sw_inject_enable
+   Jack: Headphone Jack		Inject Enabled: 1
+   to enable jack injection:
+   sound/card1/Headphone_Jack# echo 1 > sw_inject_enable
+   to disable jack injection:
+   sound/card1/Headphone_Jack# echo 0 > sw_inject_enable
+
+ - jackin_inject, write-only, inject plugin or plugout
+   to inject plugin:
+   sound/card1/Headphone_Jack# echo 1 > jackin_inject
+   to inject plugout:
+   sound/card1/Headphone_Jack# echo 0 > jackin_inject