Messages in this thread | | | Date | Thu, 1 Jun 2023 10:51:14 +0200 | From | Miquel Raynal <> | Subject | Re: [PATCH v2 2/2] nvmem: core: Expose cells through sysfs |
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Hi Greg,
gregkh@linuxfoundation.org wrote on Wed, 31 May 2023 20:16:37 +0100:
> On Tue, May 30, 2023 at 12:09:29PM +0200, 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. > > > > 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@bootlin.com> > > --- > > 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); > > As the list can change right after you have unlocked this, why care > about the locking at all?
Just to mimic the existing code which protects this list. I don't think a race can happen here anyway, the locks are not needed indeed.
> > But in looking deeper here, is this really even needed? As you manually > create the attributes in here anyway, the group starts out empty and > then you manually add them, so this should never fail, right?
Absolutely. I put these lines in the commit log:
"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."
It was a try to prepare the future :) But I agree it is not needed, statically defining the rights is more than enough, so I'll just get rid of it.
> > > + > > + 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; > > Shouldn't this check be higher up _before_ you allocate any memory? > If the attribute group list is empty, nothing should be created, > right? Or will the driver core crash?
As you rightfully guessed it, the core will crash if no list is provided at all. I need to provide an empty list with just an empty member and everything goes smoothly.
Thanks, Miquèl
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