linuxkpi: Define `MINORBITS`

We can't really define a proper value for this constant because minor
and major are encoded in a complex way on FreeBSD which cannot be
represented with a simple shift.

The DRM generic code started to use it in Linux 6.12.

In this context, `MINORBITS` is used to define an upper limit passed to
`xa_alloc()`. Therefore it is not used to encode or decode minors. It is
used as an arbitrary value. Therefore, we define the constant to 20,
like on Linux.

Reviewed by:	bz
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56447

1 files changed, 15 insertions, 0 deletions

diff --git a/sys/compat/linuxkpi/common/include/linux/kdev_t.h b/sys/compat/linuxkpi/common/include/linux/kdev_t.h
index 988dd771254a..964b7078e7f5 100644
--- a/sys/compat/linuxkpi/common/include/linux/kdev_t.h
+++ b/sys/compat/linuxkpi/common/include/linux/kdev_t.h
@@ -31,6 +31,21 @@
 
 #include <sys/types.h>
 
+/*
+ * The encoding of major/minor on FreeBSD is complex to take into account the
+ * compatibility with 16-bit dev_t (see <sys/types.h>). It is not possible to
+ * use a simple shift and thus, we can't have a proper definition of
+ * `MINORBITS`.
+ *
+ * At the time this is added to linuxkpi, this is used by the DRM generic code
+ * as the upper limit to allocate an ID using an xarray. It is not used to
+ * decode a minor, therefore any arbitrary value here is fine in this context.
+ *
+ * On Linux, the value is set to 20. Because of the above, we reuse this value,
+ * even so it does not correspond to the actual minor encoding on FreeBSD.
+ */
+#define MINORBITS	20
+
 #define MAJOR(dev)      major(dev)
 #define MINOR(dev)      minor(dev)
 #define MKDEV(ma, mi)   makedev(ma, mi)