stand: Introduce new dv_parsedev routine

Allow device classes to define a parsing routine. Most device classes
already have these routines, but there's much duplication in their
use. Define an interface for a common routine to parse an individual
device. By convetion, files have the form "[device:]/path/to/file"
where device is optional (filled in to be the value of currdev)
and it starts with the dv_name field of the device, with the rest
of the name up to the device (typically a unit number, but disks
add partition inforation, and other devices may do artibtrary
otehr things).

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D37337

(cherry picked from commit a0aad69f9556221283b8f6ddcd332329b50463e4)

2 files changed, 32 insertions, 1 deletions

diff --git a/stand/libsa/libsa.3 b/stand/libsa/libsa.3
index 7bd7a848cd3d..f732595bc647 100644
--- a/stand/libsa/libsa.3
+++ b/stand/libsa/libsa.3
@@ -710,7 +710,9 @@ struct devsw {
     int		(*dv_ioctl)(struct open_file *f, u_long cmd, void *data);
     int		(*dv_print)(int verbose);
     void	(*dv_cleanup)(void);
-    void	(*dv_fmtdev)(struct devdesc *);
+    char *	(*dv_fmtdev)(struct devdesc *);
+    int		(*dv_parsedev)(struct devdesc **dev, const char *devpart,
+    		const char **path);
 };
 .Ed
 .Bl -tag -width ".Fn dv_strategy"
@@ -751,6 +753,34 @@ Cleans up any memory used by the device before the next stage is run.
 .It Fn dv_fmtdev
 Converts the specified devdesc to the canonical string representation
 for that device.
+.It Fn dv_parsedev
+Parses the device portion of a file path.
+The
+.Dv devpart
+will point to the
+.Sq tail
+of device name, possibly followed by a colon and a path within the device.
+The
+.Sq tail
+is, by convention, the part of the device specification that follows the
+.Fa dv_name
+part of the string.
+So when parsing the string
+.Dq disk3p5:/xxx
+.Dv devpart
+will point to the
+.Sq 3
+in that string.
+The parsing routine is expected to allocate a new
+.Dv struct devdesc
+or subclass and return it in
+.Dv dev
+when successful.
+This routine should set
+.Dv path
+to point to the portion of the string after device specification, or
+.Dq /xxx
+in the earlier example.
 .El
 .Sh HISTORY
 The
diff --git a/stand/libsa/stand.h b/stand/libsa/stand.h
index 3b2627355ed6..716486107b27 100644
--- a/stand/libsa/stand.h
+++ b/stand/libsa/stand.h
@@ -160,6 +160,7 @@ struct devsw {
     int		(*dv_print)(int verbose);	/* print device information */
     void	(*dv_cleanup)(void);
     char *	(*dv_fmtdev)(struct devdesc *);
+    int		(*dv_parsedev)(struct devdesc **, const char *, const char **);
 };
 
 /*