linux-kernel - Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

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Date:   Tue, 21 Dec 2021 07:53:32 +0100
From:   Rafał Miłecki <zajec5@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH 2/2] nvmem: expose NVMEM cells in sysfs

On 21.12.2021 07:45, Greg Kroah-Hartman wrote:
> On Tue, Dec 21, 2021 at 07:39:24AM +0100, Rafał Miłecki wrote:
>> On 21.12.2021 07:33, Greg Kroah-Hartman wrote:
>>> On Mon, Dec 20, 2021 at 09:39:43PM +0100, Rafał Miłecki wrote:
>>>> Hi Greg,
>>>>
>>>> On 20.12.2021 09:00, Greg Kroah-Hartman wrote:
>>>>> On Mon, Dec 20, 2021 at 07:47:30AM +0100, Rafał Miłecki wrote:
>>>>>>     static void nvmem_cell_entry_add(struct nvmem_cell_entry *cell)
>>>>>>     {
>>>>>> +	struct device *dev = &cell->nvmem->dev;
>>>>>> +	int err;
>>>>>> +
>>>>>>     	mutex_lock(&nvmem_mutex);
>>>>>>     	list_add_tail(&cell->node, &cell->nvmem->cells);
>>>>>>     	mutex_unlock(&nvmem_mutex);
>>>>>> +
>>>>>> +	sysfs_attr_init(&cell->battr.attr);
>>>>>> +	cell->battr.attr.name = cell->name;
>>>>>> +	cell->battr.attr.mode = 0400;
>>>>>> +	cell->battr.read = nvmem_cell_attr_read;
>>>>>> +	err = sysfs_add_bin_file_to_group(&dev->kobj, &cell->battr,
>>>>>> +					  nvmem_cells_group.name);
>>>>>
>>>>> Why not just use the is_bin_visible attribute instead to determine if
>>>>> the attribute should be shown or not instead of having to add it
>>>>> after-the-fact which will race with userspace and loose?
>>>>
>>>> I'm sorry I really don't see how you suggest to get it done.
>>>>
>>>> I can use .is_bin_visible() callback indeed to respect nvmem->root_only.
>>>
>>> Great.
>>>
>>>> I don't understand addig-after-the-fact part. How is .is_bin_visible()
>>>> related to adding attributes for newly created cells?
>>>
>>> You are adding a sysfs attribute to a device that is already registered
>>> in the driver core, and so the creation of that attribute is never seen
>>> by userspace.  The attribute needs to be attached to the device _BEFORE_
>>> it is registered.
>>>
>>> Also, huge hint, if a driver has to call as sysfs_*() call, something is
>>> wrong.
>>>
>>>> Do you mean I can
>>>> avoid calling sysfs_add_bin_file_to_group()?
>>>
>>> Yes.
>>>
>>>> Do you recall any existing example of such solution?
>>>
>>> Loads.
>>>
>>> Just add this attribute group to your driver as a default attribute
>>> group and the driver core will create it for you if needed.
>>>
>>> Or if you always need it, no need to mess sith is_bin_visible() at all,
>>> I can't really understand what you are trying to do here at all.
>>
>> Thanks a lot! In nvmem_register() first there is a call to the
>> device_register() and only later cells get added. I suppose I just have
>> to rework nvmem_register() order so that:
>> 1. Cells are collected earlier. For each cell I allocate group attribute
> 
> No, add all of the attributes to the device at the beginning before you
> register it, there's no need to allocate anything.

If you mean static structures I can't do that, because cells almost
never are static. They are not known in advance. nvmem allows cells to
be:
1. Specified in OF
2. Submitted as list while registering a NVMEM device

So every cells gets its own structure allocated dynamically. My plan is
to put bin_attribute in that struct and then create a group collecting
all those cells.


>> 2. device_register() gets called
> 
> Yes.