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Subject: [PATCH v2] introduction: document #define syntax

We use the C #define syntax to refer to numeric values.
Let's document that.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

changes from v1:
address Cornelia's comments

 introduction.tex | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/introduction.tex b/introduction.tex
index cc38e29..47bebf5 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -210,6 +210,29 @@ \section{Structure Specifications}
 CPU_TO_BE16(B << 15 | A)
+\section{Constant Specifications}
+In many cases, numeric values used in the interface between the device
+and the driver are documented using the C #define and /* */
+comment syntax. Multiple related values are grouped together with
+a common name as a prefix, using _ as a separator.
+Using _XXX as a suffix refers to all values in a group.
+For example:
+/* Field Fld value A description */
+#define VIRTIO_FLD_A        (1 << 0)
+/* Field Fld value B description */
+#define VIRTIO_FLD_B        (1 << 1)
+documents two numeric values for a field \field{Fld}, with
+\field{Fld} having value 1 referring to \field{A} and \field{Fld}
+having value 2 referring to \field{B}.
+Note that $<<$ refers to the shift-left operation.
+Further, in this case VIRTIO_FLD_A and VIRTIO_FLD_B
+refer to values 1 and 2 of Fld respectively. Further, VIRTIO_FLD_XXX refers to

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