gain_groups: Add zero-value gain groups

Add convenience factory for making a gain group that has a single
zero-valued element. This factory requires a name, which should
probably be ALL_GAINS, or something similar (these constants are
device-specific).

Using this new make_zero factory in the X300 radio control when we
don't find any gain elements so that our gain groups aren't empty. This
simplifies our later setters/getters because we know that we'll always
have _something_ cached.
Note that we only register this zero value gain group for TX, as our
ADC is registered as a gain element, so our RX gain groups are never
empty.

1 files changed, 7 insertions, 0 deletions

diff --git a/host/include/uhd/utils/gain_group.hpp b/host/include/uhd/utils/gain_group.hpp
index f85e841dc..4a3a01a65 100644
--- a/host/include/uhd/utils/gain_group.hpp
+++ b/host/include/uhd/utils/gain_group.hpp
@@ -91,6 +91,13 @@ public:
      * \return a gain group object.
      */
     static sptr make(void);
+
+    /*!
+     * Make a new gain group with all zero values.
+     * \param name the name of the (only and zero-valued) gain element
+     * \return a gain group object populated with zeroes
+     */
+    static sptr make_zero();
 };
 
 } // namespace uhd