[PATCH Resend] virtio-snd: improve PCM command lifecycle description

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

The PCM command lifecycle can be described as a state-machine in which
the switching between states is triggered by commands. This commit
explicitly presents the different states and the commands that are
allowed in a given state. This commit leaves to the implementation the
decision the state reached depending on the command.

Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>
---
 device-types/sound/description.tex | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/device-types/sound/description.tex b/device-types/sound/description.tex
index 54c9c8e..55e93af 100644
--- a/device-types/sound/description.tex
+++ b/device-types/sound/description.tex
@@ -397,9 +397,21 @@ \subsubsection{PCM Control Messages}\label{sec:Device Types / Sound Device / Dev
 \item[\field{stream_id}] specifies a PCM stream identifier from 0 to \field{streams} - 1.
 \end{description}
 
-\paragraph{PCM Command Lifecycle}
+\paragraph{PCM Command Lifecycle State Machine}
 
-A PCM stream has the following command lifecycle:
+A PCM stream can be represented by a state-machine made up of the following
+states:
+\begin{description}
+\item SET PARAMETERS
+\item PREPARE
+\item START
+\item STOP
+\item RELEASE
+\end{description}
+
+A command triggers the transition between these states. To what state to switch
+depends on the implementation. Depending on the state, only certain commands
+are valid. An error may occur if an invalid command is triggered.
 
 \begin{enumerate}
 \item SET PARAMETERS
@@ -407,13 +419,13 @@ \subsubsection{PCM Control Messages}\label{sec:Device Types / Sound Device / Dev
 The driver negotiates the stream parameters (format, transport, etc) with
 the device.
 
-Possible valid transitions: set parameters, prepare.
+Possible valid commands: set parameters, prepare.
 
 \item PREPARE
 
 The device prepares the stream (allocates resources, etc).
 
-Possible valid transitions: set parameters, prepare, start, release.
+Possible valid commands: set parameters, prepare, start, release.
 
 \item Output only: the driver transfers data for pre-buffing.
 
@@ -421,7 +433,7 @@ \subsubsection{PCM Control Messages}\label{sec:Device Types / Sound Device / Dev
 
 The device starts the stream (unmute, putting into running state, etc).
 
-Possible valid transitions: stop.
+Possible valid commands: stop.
 
 \item The driver transfers data to/from the stream.
 
@@ -429,13 +441,13 @@ \subsubsection{PCM Control Messages}\label{sec:Device Types / Sound Device / Dev
 
 The device stops the stream (mute, putting into non-running state, etc).
 
-Possible valid transitions: start, release.
+Possible valid commands: start, release.
 
 \item RELEASE
 
 The device releases the stream (frees resources, etc).
 
-Possible valid transitions: set parameters, prepare.
+Possible valid commands: set parameters, prepare.
 
 \end{enumerate}
 
-- 
2.41.0