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, 11 insertions, 0 deletions

diff --git a/host/lib/utils/gain_group.cpp b/host/lib/utils/gain_group.cpp
index 50edf2515..0e4df6469 100644
--- a/host/lib/utils/gain_group.cpp
+++ b/host/lib/utils/gain_group.cpp
@@ -182,3 +182,14 @@ private:
 gain_group::sptr gain_group::make(void){
     return sptr(new gain_group_impl());
 }
+
+gain_group::sptr gain_group::make_zero()
+{
+    gain_fcns_t gain_fcns;
+    gain_fcns.get_range = []() { return meta_range_t(0.0, 0.0); };
+    gain_fcns.get_value = []() { return 0.0; };
+    gain_fcns.set_value = [](const double) {};
+    auto gg             = make();
+    gg->register_fcns("null", gain_fcns);
+    return gg;
+}