commands: implement devlookup to find device behind device file

For OF-enabled platforms with aliases, device file naming is pretty much
solved: If there is mmc2 = &something, then we have a mmc2 device and
a /dev/mmc2 device file. For other platforms like x86, EFI-provided
devices are harder to get ahold of. Add a command to make this
straight-forward to do in scripts. The main use of this is probably to
access parameters like nt_signature or guid:

  devlookup /dev/disk0 guid

This would print to console, but we have no output capture yet, so add
an optional -v VARIABLE parameter as well to allow easy use from
scripts.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20220630124035.4019644-2-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

3 files changed, 91 insertions, 0 deletions

diff --git a/commands/Kconfig b/commands/Kconfig
index 86e4714849..69d76a9c80 100644
--- a/commands/Kconfig
+++ b/commands/Kconfig
@@ -75,6 +75,19 @@ config CMD_DEVINFO
 	  If called with a device path being the argument, devinfo shows more
 	  default information about this device and its parameters.
 
+config CMD_DEVLOOKUP
+	tristate
+	prompt "devlookup"
+	help
+	  Look up device behind device file and its parameters
+
+	  devlookup [-v VAR] /dev/DEVICE [parameter]
+
+          Detects the device behind a device file and outputs it,
+          unless a second argument is given. In that case the device
+          parameter with that name is looked up. Specifying -v VARIABLE
+          will write output to VARIABLE instead of printing it.
+
 config CMD_DEVUNBIND
 	tristate
 	prompt "devunbind"
diff --git a/commands/Makefile b/commands/Makefile
index b3b7bafe6b..0aae8893d6 100644
--- a/commands/Makefile
+++ b/commands/Makefile
@@ -110,6 +110,7 @@ obj-$(CONFIG_CMD_DETECT)	+= detect.o
 obj-$(CONFIG_CMD_BOOT)		+= boot.o
 obj-$(CONFIG_CMD_DEVINFO)	+= devinfo.o
 obj-$(CONFIG_CMD_DEVUNBIND)	+= devunbind.o
+obj-$(CONFIG_CMD_DEVLOOKUP)	+= devlookup.o
 obj-$(CONFIG_CMD_DRVINFO)	+= drvinfo.o
 obj-$(CONFIG_CMD_READF)		+= readf.o
 obj-$(CONFIG_CMD_MENUTREE)	+= menutree.o
diff --git a/commands/devlookup.c b/commands/devlookup.c
new file mode 100644
index 0000000000..ffd6afbaba
--- /dev/null
+++ b/commands/devlookup.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <common.h>
+#include <command.h>
+#include <fs.h>
+#include <getopt.h>
+#include <malloc.h>
+#include <linux/stat.h>
+#include <linux/ctype.h>
+#include <environment.h>
+
+static int report(const char *variable, const char *val)
+{
+	if (!val)
+		return -(errno ?: EINVAL);
+
+	if (variable)
+		return setenv(variable, val);
+
+	printf("%s\n", val);
+	return 0;
+}
+
+static int do_devlookup(int argc, char *argv[])
+{
+	const char *variable = NULL, *devicefile, *paramname;
+	struct cdev *cdev;
+	int opt;
+
+	while ((opt = getopt(argc, argv, "v:")) > 0) {
+		switch(opt) {
+		case 'v':
+			variable = optarg;
+			break;
+		}
+	}
+
+	if (argc - optind == 0 || argc - optind > 2)
+		return COMMAND_ERROR_USAGE;
+
+	devicefile = argv[optind];
+	paramname  = argv[optind + 1];
+
+	devicefile = devpath_to_name(devicefile);
+
+	cdev = cdev_by_name(devicefile);
+	if (!cdev) {
+		printf("devlookup: cdev %s not found\n", devicefile);
+		return -ENOENT;
+	}
+
+	if (!cdev->dev) {
+		printf("devlookup: cdev %s not associated with a device\n", devicefile);
+		return -ENODEV;
+	}
+
+	if (paramname)
+		return report(variable, dev_get_param(cdev->dev, paramname));
+
+	return report(variable, dev_name(cdev->dev));
+}
+
+BAREBOX_CMD_HELP_START(devlookup)
+BAREBOX_CMD_HELP_TEXT("Detects the device behind a device file and outputs it,")
+BAREBOX_CMD_HELP_TEXT("unless a second argument is given. In that case the device")
+BAREBOX_CMD_HELP_TEXT("parameter with that name is looked up. Specifying -v VARIABLE")
+BAREBOX_CMD_HELP_TEXT("will write output to VARIABLE instead of printing it")
+BAREBOX_CMD_HELP_END
+
+BAREBOX_CMD_START(devlookup)
+	.cmd		= do_devlookup,
+	BAREBOX_CMD_DESC("look up device behind device file and its parameters")
+	BAREBOX_CMD_OPTS("[-v VAR] /dev/DEVICE [parameter]")
+	BAREBOX_CMD_GROUP(CMD_GRP_SCRIPT)
+	BAREBOX_CMD_HELP(cmd_devlookup_help)
+BAREBOX_CMD_END
+