[PATCH v2 2/2] nvmem: core: Expose cells through sysfs

[Miquel Raynal]

The binary content of nvmem devices is available to the user so in the
easiest cases, finding the content of a cell is rather easy as it is
just a matter of looking at a known and fixed offset. However, nvmem
layouts have been recently introduced to cope with more advanced
situations, where the offset and size of the cells is not known in
advance or is dynamic. When using layouts, more advanced parsers are
used by the kernel in order to give direct access to the content of each
cell, regardless of its position/size in the underlying
device. Unfortunately, these information are not accessible by users,
unless by fully re-implementing the parser logic in userland.

Let's expose the cells and their content through sysfs to avoid these
situations.

Exposed cells are read-only. There is, in practice, everything in the
core to support a write path, but as I don't see any need for that, I
prefer to keep the interface simple (and probably safer). The interface
is documented as being in the "testing" state which means we can later
add a write attribute if though relevant.

Of course the relevant NVMEM sysfs Kconfig option must be enabled for
this support to be compiled-in.

The current implementation leads to the 'cells' folder to be always
present even when no cell is actually exposed. This is due to a sysfs
limitation which might in the future be overcome. In order to be as
close as how sysfs was designed, despite the cells .bin_attrs attribute
group member being assigned at runtime (and thus, not movable to a RO
section), we provide a .is_bin_visible hook which might become really
useful if the series avoiding the creation of empty directories borns.

There is one limitation though: if a layout is built as a module but is
not properly installed in the system and loaded manually with insmod
while the nvmem device driver was built-in, the cells won't appear in
sysfs. But if done like that, the cells won't be usable by the built-in
kernel drivers anyway.

Signed-off-by: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
---
 drivers/nvmem/core.c | 145 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 141 insertions(+), 4 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 342cd380b420..02b70f065ebc 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -325,6 +325,76 @@ static umode_t nvmem_bin_attr_is_visible(struct kobject *kobj,
 	return nvmem_bin_attr_get_umode(nvmem);
 }
 
+static struct nvmem_cell *nvmem_create_cell(struct nvmem_cell_entry *entry,
+					    const char *id, int index);
+
+static ssize_t nvmem_cell_attr_read(struct file *filp, struct kobject *kobj,
+				    struct bin_attribute *attr, char *buf,
+				    loff_t pos, size_t count)
+{
+	struct nvmem_cell_entry *entry;
+	struct nvmem_cell *cell = NULL;
+	struct nvmem_device *nvmem;
+	size_t cell_sz, read_len;
+	struct device *dev;
+	void *content;
+
+	if (attr->private)
+		dev = attr->private;
+	else
+		dev = kobj_to_dev(kobj);
+	nvmem = to_nvmem_device(dev);
+
+	mutex_lock(&nvmem_mutex);
+	list_for_each_entry(entry, &nvmem->cells, node) {
+		if (strncmp(entry->name, attr->attr.name, XATTR_NAME_MAX))
+			continue;
+
+		cell = nvmem_create_cell(entry, entry->name, 0);
+		if (IS_ERR(cell)) {
+			mutex_unlock(&nvmem_mutex);
+			return PTR_ERR(cell);
+		}
+
+		break;
+	}
+	mutex_unlock(&nvmem_mutex);
+
+	if (!cell)
+		return -EINVAL;
+
+	content = nvmem_cell_read(cell, &cell_sz);
+	if (IS_ERR(content)) {
+		read_len = PTR_ERR(content);
+		goto destroy_cell;
+	}
+
+	read_len = min_t(unsigned int, cell_sz - pos, count);
+	memcpy(buf, content + pos, read_len);
+	kfree(content);
+
+destroy_cell:
+	kfree_const(cell->id);
+	kfree(cell);
+
+	return read_len;
+}
+
+static umode_t nvmem_cells_is_visible(struct kobject *kobj,
+				      struct bin_attribute *attr, int i)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct nvmem_device *nvmem = to_nvmem_device(dev);
+	umode_t mode = 0444;
+
+	mutex_lock(&nvmem_mutex);
+	if (list_empty(&nvmem->cells))
+		mode = 0;
+	mutex_unlock(&nvmem_mutex);
+
+	return mode;
+}
+
 /* default read/write permissions */
 static struct bin_attribute bin_attr_rw_nvmem = {
 	.attr	= {
@@ -346,8 +416,15 @@ static const struct attribute_group nvmem_bin_group = {
 	.is_bin_visible = nvmem_bin_attr_is_visible,
 };
 
+/* Cell attributes will be dynamically allocated */
+static struct attribute_group nvmem_cells_group = {
+	.name		= "cells",
+	.is_bin_visible = nvmem_cells_is_visible,
+};
+
 static const struct attribute_group *nvmem_dev_groups[] = {
 	&nvmem_bin_group,
+	&nvmem_cells_group,
 	NULL,
 };
 
@@ -406,6 +483,60 @@ static void nvmem_sysfs_remove_compat(struct nvmem_device *nvmem,
 		device_remove_bin_file(nvmem->base_dev, &nvmem->eeprom);
 }
 
+static int nvmem_populate_sysfs_cells(struct nvmem_device *nvmem)
+{
+	struct bin_attribute **cells_attrs, *attrs;
+	struct nvmem_cell_entry *entry;
+	unsigned int ncells = 0, i = 0;
+	int ret = 0;
+
+	mutex_lock(&nvmem_mutex);
+
+	list_for_each_entry(entry, &nvmem->cells, node)
+		ncells++;
+
+	/* Allocate an array of attributes with a sentinel */
+	cells_attrs = devm_kcalloc(&nvmem->dev, ncells + 1,
+				   sizeof(struct bin_attribute *), GFP_KERNEL);
+	if (!cells_attrs) {
+		ret = -ENOMEM;
+		goto unlock_mutex;
+	}
+
+	nvmem_cells_group.bin_attrs = cells_attrs;
+
+	/* Without exposed cells, successfully exit after assigning an empty attributes array */
+	if (!ncells)
+		goto unlock_mutex;
+
+	attrs = devm_kcalloc(&nvmem->dev, ncells, sizeof(struct bin_attribute), GFP_KERNEL);
+	if (!attrs) {
+		ret = -ENOMEM;
+		goto unlock_mutex;
+	}
+
+	/* Initialize each attribute to take the name and size of the cell */
+	list_for_each_entry(entry, &nvmem->cells, node) {
+		sysfs_bin_attr_init(&attrs[i]);
+		attrs[i].attr.name = devm_kstrdup(&nvmem->dev, entry->name, GFP_KERNEL);
+		attrs[i].attr.mode = 0444;
+		attrs[i].size = entry->bytes;
+		attrs[i].read = &nvmem_cell_attr_read;
+		if (!attrs[i].attr.name) {
+			ret = -ENOMEM;
+			goto unlock_mutex;
+		}
+
+		cells_attrs[i] = &attrs[i];
+		i++;
+	}
+
+unlock_mutex:
+	mutex_unlock(&nvmem_mutex);
+
+	return ret;
+}
+
 #else /* CONFIG_NVMEM_SYSFS */
 
 static int nvmem_sysfs_setup_compat(struct nvmem_device *nvmem,
@@ -976,16 +1107,22 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
 	if (rval)
 		goto err_remove_cells;
 
+	rval = nvmem_add_cells_from_layout(nvmem);
+	if (rval)
+		goto err_remove_cells;
+
+#ifdef CONFIG_NVMEM_SYSFS
+	rval = nvmem_populate_sysfs_cells(nvmem);
+	if (rval)
+		goto err_remove_cells;
+#endif
+
 	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
 
 	rval = device_add(&nvmem->dev);
 	if (rval)
 		goto err_remove_cells;
 
-	rval = nvmem_add_cells_from_layout(nvmem);
-	if (rval)
-		goto err_remove_cells;
-
 	blocking_notifier_call_chain(&nvmem_notifier, NVMEM_ADD, nvmem);
 
 	return nvmem;
-- 
2.34.1