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From:   Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Mika Westerberg <mika.westerberg@linux.intel.com>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Subject: Re: [PATCH v3 2/2] nvmem: Add support for write-only instances
Thread-Topic: [PATCH v3 2/2] nvmem: Add support for write-only instances
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Date:   Tue, 10 Mar 2020 01:10:24 +0000
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On Mon, Mar 09, 2020 at 05:50:17PM +0000, Nicholas Johnson wrote:
> 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.
Oops, late night. Please drop the above line if you accept this. This 
above line applies to patch 1/2 now.

> 
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> ---
>  drivers/nvmem/nvmem-sysfs.c | 56 +++++++++++++++++++++++++++++++------
>  1 file changed, 48 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/nvmem/nvmem-sysfs.c b/drivers/nvmem/nvmem-sysfs.c
> index 9e0c429cd..846112786 100644
> --- a/drivers/nvmem/nvmem-sysfs.c
> +++ b/drivers/nvmem/nvmem-sysfs.c
> @@ -196,16 +196,49 @@ 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,
> +};
> +
> +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;
>  
> -	return nvmem->read_only ? nvmem_ro_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;
> +
> +	return NULL;
>  }
>  
>  /*
> @@ -224,17 +257,24 @@ int nvmem_sysfs_setup_compat(struct nvmem_device *nvmem,
>  	if (!config->base_dev)
>  		return -EINVAL;
>  
> -	if (nvmem->read_only) {
> +	if (nvmem->reg_read && (!nvmem->reg_write || nvmem->read_only)) {
>  		if (config->root_only)
>  			nvmem->eeprom = bin_attr_ro_root_nvmem;
>  		else
>  			nvmem->eeprom = bin_attr_ro_nvmem;
> -	} else {
> +	} else if (!nvmem->reg_read && nvmem->reg_write && !nvmem->read_only) {
> +		if (config->root_only)
> +			nvmem->eeprom = bin_attr_wo_root_nvmem;
> +		else
> +			return -EPERM;
> +	} else if (nvmem->reg_read && nvmem->reg_write && !nvmem->read_only) {
>  		if (config->root_only)
>  			nvmem->eeprom = bin_attr_rw_root_nvmem;
>  		else
>  			nvmem->eeprom = bin_attr_rw_nvmem;
> -	}
> +	} else
> +		return -EPERM;
> +
>  	nvmem->eeprom.attr.name = "eeprom";
>  	nvmem->eeprom.size = nvmem->size;
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> -- 
> 2.25.1
>