Re: [PATCH 5/5] nvmem: Add support for write-only instances

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Mar 23, 2020 at 03:00:07PM +0000, Srinivas Kandagatla wrote:
> From: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> 
> There is at least one real-world use-case for write-only nvmem
> instances. Refer to 03cd45d2e219 ("thunderbolt: Prevent crash if
> non-active NVMem file is read").
> 
> Add support for write-only nvmem instances by adding attrs for 0200.
> 
> Change nvmem_register() to abort if NULL group is returned from
> nvmem_sysfs_get_groups().
> 
> Return NULL from nvmem_sysfs_get_groups() in invalid cases.
> 
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
> ---
>  drivers/nvmem/core.c        | 10 +++++--
>  drivers/nvmem/nvmem-sysfs.c | 56 +++++++++++++++++++++++++++++++------
>  2 files changed, 56 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 77d890d3623d..ddc7be5149d5 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -381,6 +381,14 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  	nvmem->type = config->type;
>  	nvmem->reg_read = config->reg_read;
>  	nvmem->reg_write = config->reg_write;
> +	nvmem->dev.groups = nvmem_sysfs_get_groups(nvmem, config);
> +	if (!nvmem->dev.groups) {
> +		ida_simple_remove(&nvmem_ida, nvmem->id);
> +		gpiod_put(nvmem->wp_gpio);
> +		kfree(nvmem);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
>  	if (!config->no_of_node)
>  		nvmem->dev.of_node = config->dev->of_node;
>  
> @@ -395,8 +403,6 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  	nvmem->read_only = device_property_present(config->dev, "read-only") ||
>  			   config->read_only || !nvmem->reg_write;
>  
> -	nvmem->dev.groups = nvmem_sysfs_get_groups(nvmem, config);
> -
>  	device_initialize(&nvmem->dev);
>  
>  	dev_dbg(&nvmem->dev, "Registering nvmem device %s\n", config->name);
> diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
> index 8759c4470012..9728da948988 100644
> --- a/drivers/nvmem/nvmem-sysfs.c
> +++ b/drivers/nvmem/nvmem-sysfs.c
> @@ -202,16 +202,49 @@ static const struct attribute_group *nvmem_ro_root_dev_groups[] = {
>  	NULL,
>  };
>  
> +/* write only permission, root only */
> +static struct bin_attribute bin_attr_wo_root_nvmem = {
> +	.attr	= {
> +		.name	= "nvmem",
> +		.mode	= 0200,
> +	},
> +	.write	= bin_attr_nvmem_write,
> +};

You are adding a new sysfs file without a Documentation/ABI/ new entry
as well?


> +
> +static struct bin_attribute *nvmem_bin_wo_root_attributes[] = {
> +	&bin_attr_wo_root_nvmem,
> +	NULL,
> +};
> +
> +static const struct attribute_group nvmem_bin_wo_root_group = {
> +	.bin_attrs	= nvmem_bin_wo_root_attributes,
> +	.attrs		= nvmem_attrs,
> +};
> +
> +static const struct attribute_group *nvmem_wo_root_dev_groups[] = {
> +	&nvmem_bin_wo_root_group,
> +	NULL,
> +};
> +
>  const struct attribute_group **nvmem_sysfs_get_groups(
>  					struct nvmem_device *nvmem,
>  					const struct nvmem_config *config)
>  {
> -	if (config->root_only)
> -		return nvmem->read_only ?
> -			nvmem_ro_root_dev_groups :
> -			nvmem_rw_root_dev_groups;
> +	/* Read-only */
> +	if (nvmem->reg_read && (!nvmem->reg_write || nvmem->read_only))
> +		return config->root_only ?
> +			nvmem_ro_root_dev_groups : nvmem_ro_dev_groups;
> +
> +	/* Read-write */
> +	if (nvmem->reg_read && nvmem->reg_write && !nvmem->read_only)
> +		return config->root_only ?
> +			nvmem_rw_root_dev_groups : nvmem_rw_dev_groups;
> +
> +	/* Write-only, do not honour request for global writable entry */
> +	if (!nvmem->reg_read && nvmem->reg_write && !nvmem->read_only)
> +		return config->root_only ? nvmem_wo_root_dev_groups : NULL;


What is this supposed to be doing, I am totally lost.

greg k-h