ACPI: Add new sysfs interface to export device description

Add support to export the device description obtained from the ACPI _STR
method, if one exists for a device, to user-space via a sysfs interface.
This new interface provides a standard and platform neutral way for users
to obtain the description text stored in the ACPI _STR method.  If no
_STR method exists for the device, no sysfs 'description' file will be
created.  The 'description' file will be located in the /sys/devices/
directory using the device's path.

/sys/device/<bus>/<bridge path>/<device path>.../firmware_node/description

Example:

/sys/devices/pci0000:00/0000:00.07.0/0000:0e:00.0/firmware_node/description

It can also be located using the ACPI device path, for example:

/sys/devices/LNXSYSTM:00/device:00/ACPI0004:00/PNP0A08:00/device:13/device:15/description
/sys/devices/LNXSYSTM:00/device:00/ACPI0004:00/ACPI0004:01/ACPI0007:02/description

Execute the 'cat' command on the 'description' file to obtain the
description string for that device.

This patch also includes documentation describing how the new sysfs
interface works

Changes from v1-v2 based on comments by Len Brown and Fengguang Wu
* Removed output "No Description" and leaving a NULL attribute if the
_STR method failed to evaluate.

* In acpi_device_remove_files() removed the redundent check of
dev->pnp.str_obj before calling free.  This check triggered a message
from smatch.

Signed-off-by: Lance Ortiz <lance.ortiz@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>

1 files changed, 52 insertions, 2 deletions

diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
index d1ecca2b641a..04302835723d 100644
--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c
@@ -10,6 +10,7 @@
 #include <linux/signal.h>
 #include <linux/kthread.h>
 #include <linux/dmi.h>
+#include <linux/nls.h>
 
 #include <acpi/acpi_drivers.h>
 
@@ -232,8 +233,35 @@ end:
 }
 static DEVICE_ATTR(path, 0444, acpi_device_path_show, NULL);
 
+/* sysfs file that shows description text from the ACPI _STR method */
+static ssize_t description_show(struct device *dev,
+				struct device_attribute *attr,
+				char *buf) {
+	struct acpi_device *acpi_dev = to_acpi_device(dev);
+	int result;
+
+	if (acpi_dev->pnp.str_obj == NULL)
+		return 0;
+
+	/*
+	 * The _STR object contains a Unicode identifier for a device.
+	 * We need to convert to utf-8 so it can be displayed.
+	 */
+	result = utf16s_to_utf8s(
+		(wchar_t *)acpi_dev->pnp.str_obj->buffer.pointer,
+		acpi_dev->pnp.str_obj->buffer.length,
+		UTF16_LITTLE_ENDIAN, buf,
+		PAGE_SIZE);
+
+	buf[result++] = '\n';
+
+	return result;
+}
+static DEVICE_ATTR(description, 0444, description_show, NULL);
+
 static int acpi_device_setup_files(struct acpi_device *dev)
 {
+	struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
 	acpi_status status;
 	acpi_handle temp;
 	int result = 0;
@@ -257,6 +285,21 @@ static int acpi_device_setup_files(struct acpi_device *dev)
 			goto end;
 	}
 
+	/*
+	 * If device has _STR, 'description' file is created
+	 */
+	status = acpi_get_handle(dev->handle, "_STR", &temp);
+	if (ACPI_SUCCESS(status)) {
+		status = acpi_evaluate_object(dev->handle, "_STR",
+					NULL, &buffer);
+		if (ACPI_FAILURE(status))
+			buffer.pointer = NULL;
+		dev->pnp.str_obj = buffer.pointer;
+		result = device_create_file(&dev->dev, &dev_attr_description);
+		if (result)
+			goto end;
+	}
+
         /*
          * If device has _EJ0, 'eject' file is created that is used to trigger
          * hot-removal function from userland.
@@ -274,8 +317,15 @@ static void acpi_device_remove_files(struct acpi_device *dev)
 	acpi_handle temp;
 
 	/*
-	 * If device has _EJ0, 'eject' file is created that is used to trigger
-	 * hot-removal function from userland.
+	 * If device has _STR, remove 'description' file
+	 */
+	status = acpi_get_handle(dev->handle, "_STR", &temp);
+	if (ACPI_SUCCESS(status)) {
+		kfree(dev->pnp.str_obj);
+		device_remove_file(&dev->dev, &dev_attr_description);
+	}
+	/*
+	 * If device has _EJ0, remove 'eject' file.
 	 */
 	status = acpi_get_handle(dev->handle, "_EJ0", &temp);
 	if (ACPI_SUCCESS(status))