Re: [PATCH v3 4/4] nvmem: core: Expose cells through sysfs

[Miquel Raynal]

Hi Srinivas,

srinivas.kandagatla@xxxxxxxxxx wrote on Tue, 6 Jun 2023 11:31:07 +0100:

> Thanks for doing this Miquel

Great, I'm glad you like the idea :)

> 
> Few minor comments below.
> 
> On 05/06/2023 14:34, Miquel Raynal wrote:
> > 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. Of course the relevant NVMEM sysfs Kconfig option must be
> > enabled for this support to be available.
> > 
> > Not all nvmem devices expose cells. Indeed, the .bin_attrs attribute
> > group member will be filled at runtime only when relevant and will
> > remain empty otherwise. In this case, as the cells attribute group will
> > be empty, it will not lead to any additional folder/file creation.
> > 
> > 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.
> > 
> > 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 | 128 +++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 124 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> > index 342cd380b420..625e3de273b7 100644
> > --- a/drivers/nvmem/core.c
> > +++ b/drivers/nvmem/core.c
> > @@ -325,6 +325,61 @@ 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;  
> I see no point in this, as we do not set any private data for this attribute.
> 
> How about storing nvmem_cell_entry in private and using it here.
> This should also avoid looping for every cell in the follow on code.

Mmmh, excellent idea, that will simplify a lot the helper, I'll do
that, thanks a lot for the suggestion!

> 
> > +	else
> > +		dev = kobj_to_dev(kobj);  
> 
> new line here.
> 
> > +	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;
> > +}
> > +
> >   /* default read/write permissions */
> >   static struct bin_attribute bin_attr_rw_nvmem = {
> >   	.attr	= {
> > @@ -346,8 +401,14 @@ 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",
> > +};
> > +
> >   static const struct attribute_group *nvmem_dev_groups[] = {
> >   	&nvmem_bin_group,
> > +	&nvmem_cells_group,
> >   	NULL,
> >   };  
> >   > @@ -406,6 +467,59 @@ 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);
> > +
> > +	if (list_empty(&nvmem->cells))
> > +		goto unlock_mutex;
> > +
> > +	list_for_each_entry(entry, &nvmem->cells, node)
> > +		ncells++;  
> 
> list_count_nodes?

I didn't even know about this macro. Of course I'll use it!

> > +
> > +	/* 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;
> > +	}
> > +
> > +	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++;
> > +	}
> > +
> > +	nvmem_cells_group.bin_attrs = cells_attrs;
> > +
> > +unlock_mutex:
> > +	mutex_unlock(&nvmem_mutex);
> > +
> > +	return ret;
> > +}
> > +
> >   #else /* CONFIG_NVMEM_SYSFS */  
> >   >   static int nvmem_sysfs_setup_compat(struct nvmem_device *nvmem,  
> > @@ -976,16 +1090,22 @@ 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;  


Thanks,
Miquèl