[PATCH v4 2/2] firmware: google: Expose coreboot tables over sysfs

[patrick.rudolph@9elements.com]

From: Patrick Rudolph <patrick.rudolph@9elements.com>

Make all coreboot table entries available to userland. This is useful for
tools that are currently using /dev/mem.

Besides the tag and size also expose the raw table data itself.

Update the ABI documentation to explain the new sysfs interface.

Tools can easily scan for the right coreboot table by reading
/sys/bus/coreboot/devices/coreboot*/attributes/id
The binary table data can then be read from
/sys/bus/coreboot/devices/coreboot*/attributes/data

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
---
 -v2:
        - Add ABI documentation
        - Add 0x prefix on hex values
        - Remove wrong ioremap hint as found by CI
 -v3:
        - Use BIN_ATTR_RO
 -v4:
        - Updated ABI documentation
---
 Documentation/ABI/stable/sysfs-bus-coreboot | 30 +++++++++++
 drivers/firmware/google/coreboot_table.c    | 58 +++++++++++++++++++++
 2 files changed, 88 insertions(+)

diff --git a/Documentation/ABI/stable/sysfs-bus-coreboot b/Documentation/ABI/stable/sysfs-bus-coreboot
index 6055906f41f2..328153a1b3f4 100644
--- a/Documentation/ABI/stable/sysfs-bus-coreboot
+++ b/Documentation/ABI/stable/sysfs-bus-coreboot
@@ -42,3 +42,33 @@ Description:
 		buffer.
 		The file holds a read-only binary representation of the CBMEM
 		buffer.
+
+What:		/sys/bus/coreboot/devices/.../attributes/id
+Date:		Apr 2020
+KernelVersion:	5.6
+Contact:	Patrick Rudolph <patrick.rudolph@9elements.com>
+Description:
+		coreboot device directory can contain a file named attributes/id.
+		The file holds an ASCII representation of the coreboot table ID
+		in hex (e.g. 0x000000ef). On coreboot enabled platforms the ID is
+		usually called TAG.
+
+What:		/sys/bus/coreboot/devices/.../attributes/size
+Date:		Apr 2020
+KernelVersion:	5.6
+Contact:	Patrick Rudolph <patrick.rudolph@9elements.com>
+Description:
+		coreboot device directory can contain a file named
+		attributes/size.
+		The file holds an ASCII representation as decimal number of the
+		coreboot table size (e.g. 64).
+
+What:		/sys/bus/coreboot/devices/.../attributes/data
+Date:		Apr 2020
+KernelVersion:	5.6
+Contact:	Patrick Rudolph <patrick.rudolph@9elements.com>
+Description:
+		coreboot device directory can contain a file named
+		attributes/data.
+		The file holds a read-only binary representation of the coreboot
+		table.
diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
index 0205987a4fd4..d0fc3eb93f4f 100644
--- a/drivers/firmware/google/coreboot_table.c
+++ b/drivers/firmware/google/coreboot_table.c
@@ -3,9 +3,11 @@
  * coreboot_table.c
  *
  * Module providing coreboot table access.
+ * Exports coreboot tables as attributes in sysfs.
  *
  * Copyright 2017 Google Inc.
  * Copyright 2017 Samuel Holland <samuel@sholland.org>
+ * Copyright 2019 9elements Agency GmbH
  */
 
 #include <linux/acpi.h>
@@ -84,6 +86,60 @@ void coreboot_driver_unregister(struct coreboot_driver *driver)
 }
 EXPORT_SYMBOL(coreboot_driver_unregister);
 
+static ssize_t id_show(struct device *dev,
+		       struct device_attribute *attr, char *buffer)
+{
+	struct coreboot_device *device = CB_DEV(dev);
+
+	return sprintf(buffer, "0x%08x\n", device->entry.tag);
+}
+
+static ssize_t size_show(struct device *dev,
+			 struct device_attribute *attr, char *buffer)
+{
+	struct coreboot_device *device = CB_DEV(dev);
+
+	return sprintf(buffer, "%u\n", device->entry.size);
+}
+
+static DEVICE_ATTR_RO(id);
+static DEVICE_ATTR_RO(size);
+
+static struct attribute *cb_dev_attrs[] = {
+	&dev_attr_id.attr,
+	&dev_attr_size.attr,
+	NULL
+};
+
+static ssize_t data_read(struct file *filp, struct kobject *kobj,
+			 struct bin_attribute *bin_attr,
+			 char *buffer, loff_t offset, size_t count)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct coreboot_device *device = CB_DEV(dev);
+
+	return memory_read_from_buffer(buffer, count, &offset,
+				       &device->entry, device->entry.size);
+}
+
+static BIN_ATTR_RO(data, 0);
+
+static struct bin_attribute *cb_dev_bin_attrs[] = {
+	&bin_attr_data,
+	NULL
+};
+
+static const struct attribute_group cb_dev_attr_group = {
+	.name = "attributes",
+	.attrs = cb_dev_attrs,
+	.bin_attrs = cb_dev_bin_attrs,
+};
+
+static const struct attribute_group *cb_dev_attr_groups[] = {
+	&cb_dev_attr_group,
+	NULL
+};
+
 static int coreboot_table_populate(struct device *dev, void *ptr)
 {
 	int i, ret;
@@ -104,6 +160,8 @@ static int coreboot_table_populate(struct device *dev, void *ptr)
 		device->dev.parent = dev;
 		device->dev.bus = &coreboot_bus_type;
 		device->dev.release = coreboot_device_release;
+		device->dev.groups = cb_dev_attr_groups;
+
 		memcpy(&device->entry, ptr_entry, entry->size);
 
 		ret = device_register(&device->dev);
-- 
2.24.1