[PATCH v2] ref-manual/variables.rst: document the QB_DEFAULT_BIOS variable

[Antonin Godard <antonin.godard@bootlin.com>]

The QB_DEFAULT_BIOS allows setting the -bios parameter of QEMU, which
can be used to emulate U-Boot, for example.

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
---
Changes in v2:
- Address comments from Quentin (thanks!)
- Link to v1: https://patch.msgid.link/20260804-qb-default-bios-v1-1-bc851931973d@bootlin.com
---
 documentation/ref-manual/variables.rst | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index e75f42afe..f4ed9a883 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -8084,6 +8084,25 @@ system and gives an overview of their function and contents.
       replaces the ``@CLIENT@`` and ``@GATEWAY@`` place holders by the IP and
       the gateway address of the QEMU guest.
 
+   :term:`QB_DEFAULT_BIOS`
+      The :term:`QB_DEFAULT_BIOS` variable can be used to provide a default
+      value for the path the of a file located in :term:`DEPLOY_DIR_IMAGE` and
+      used by ``runqemu`` to specify the `-bios <https://www.qemu.org/docs/master/system/invocation.html#hxtool-8>`__
+      option of QEMU. For example, this parameter can be set as follows to
+      emulate U-Boot for the :oecore_path:`qemuarm64 <meta/conf/machine/qemuarm64.conf>`
+      machine::
+
+         QB_DEFAULT_BIOS = "u-boot.bin"
+
+      The above example makes the assumption the U-Boot recipe was built
+      and that the ``u-boot.bin`` is deployed in the :term:`DEPLOY_DIR_IMAGE`
+      directory.
+
+      .. note::
+
+         When using ``runqemu``, the ``BIOS`` environment variable takes
+         precedence over this variable.
+
    :term:`QB_DEFAULT_FSTYPE`
 
       The :term:`QB_DEFAULT_FSTYPE` variable controls the default filesystem

---
base-commit: 41dae3c3da3ada1745fc60228ff6269c64ee2361
change-id: 20260804-qb-default-bios-7c12e88f0db2