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Document: virtio-v1.0-csprd01
Number: 3
Date: Tue, 21 Jan 2014 15:09:54 +0530
Link to Mail: https://lists.oasis-open.org/archives/virtio-comment/201401/msg00037.html
Commenter name: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Decision: Agreed at meeting 2014-01-28.
This patch updates virtio spec lyx for adding emergency write to
virtio console.
Original patch series related to this is -:
http://lkml.org/lkml/2013/5/6/167
Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Proposal:
diff --git a/content.tex b/content.tex
index 17d1bd7..96645df 100644
--- a/content.tex
+++ b/content.tex
@@ -3225,6 +3225,9 @@ data and outgoing characters are placed in the transmit queue.
\item[VIRTIO_CONSOLE_F_MULTIPORT (1)] Device has support for multiple
ports; configuration fields nr_ports and max_nr_ports are
valid and control virtqueues will be used.
+
+\item[VIRTIO_CONSOLE_F_EMERG_WRITE (2)] Device has support for emergency write.
+ Configuration field emerg_wr is valid.
\end{description}
\subsection{Device configuration layout}\label{sec:Device Types / Console Device / Device configuration layout}
@@ -3235,11 +3238,16 @@ data and outgoing characters are placed in the transmit queue.
is set, the maximum number of ports supported by the device can
be fetched.
+ If VIRTIO_CONSOLE_F_EMERG_WRITE is set then the driver can use emergency write
+ to output a single character without initializing virtio queues, or even
+ acknowledging the feature.
+
\begin{lstlisting}
struct virtio_console_config {
le16 cols;
le16 rows;
le32 max_nr_ports;
+ le32 emerg_wr;
};
\end{lstlisting}
@@ -3250,6 +3258,11 @@ native endian of the guest rather than (necessarily) little-endian.
\subsection{Device Initialization}\label{sec:Device Types / Console Device / Device Initialization}
\begin{enumerate}
+\item If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the
+ emerg_wr field of the configuration can be written at any time.
+ Thus it should work for very early boot debugging output as well as
+ catastophic OS failures (eg. virtio ring corruption).
+
\item If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
can read the console dimensions from the configuration fields.
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