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

["Rafał Miłecki" <zajec5@gmail.com>]

On 21.12.2021 13:56, Greg Kroah-Hartman wrote:
> On Tue, Dec 21, 2021 at 01:24:00PM +0100, Rafał Miłecki wrote:
>> On 21.12.2021 08:13, Greg Kroah-Hartman wrote:
>>> On Tue, Dec 21, 2021 at 07:53:32AM +0100, Rafał Miłecki wrote:
>>>> 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.
>>>
>>> A device has a driver associated with it, and that driver has default
>>> groups associated with it.  Use that, I am not saying to use static
>>> structures, that is not how the driver model works at all.
>>
>> I'm helpless on dealing with attributes.
>>
>> I tried building a list of attributes dynamically but that of course
>> fails:
>>
>> drivers/nvmem/core.c: In function ‘nvmem_register’:
>> drivers/nvmem/core.c:930:31: error: assignment of member ‘bin_attrs’ in read-only object
>>    930 |   nvmem_cells_group.bin_attrs = nvmem->cells_bin_attrs;
>>        |                               ^
>>
>>
>> What I'm trying to achieve is having
>> /sys/bus/nvmem/devices/*/cells/*
>> with each file being an attribute.
> 
> What is the full path here that you are looking to add these attributes
> to?  Where is the struct device in play?  What .c file should I look at?
> 
>> Please kindly point me to a single example of "struct attribute_group"
>> that has a variable list of attributes with each attribute having
>> runtime set name.
> 
> Why would you ever want each attribute have a runtime-set name?  That's
> not what attributes are for.  Think of them as "key/value" pairs.  The
> "key" part is the name (i.e. filename), that is well known to everyone,
> unique to that struct device type, and documented in Documentation/ABI/.
> The "value" part is the value you read from the file (or write to it.)
> 
> That's it, it's not all that complex.
> 
>> Almost all cases in kernel look like:
>> static const struct attribute_group foo_attr_group = {
>> 	.attrs = foo_attrs,
>> };
>> with "foo_attrs" being a list of attributes with *predefined* names.
> 
> Yes, because that is what you really want.
> 
> Why do you feel this device is somehow unique to deserve attributes that
> are not predefined?  And if they are not predefined, how are you going
> to define them when you create them in the code and document them?   :)
> 
>> Every example of dynamic attributes (runtime created) I see in a kernel
>> (e.g. drivers/base/node.c) uses sysfs_*().
> 
> drivers/base/* is not the best place to look at for how to implement
> bus/driver logic, look at existing busses and drivers instead please.
> We have a few hundred to choose from :)
> 
> So, let's break it down, what exactly are you wanting your device on
> your bus to look like?  What will be the attributes you want to expose,
> and what are the values of those attributes?  You have to start with
> that, as Documentation/ABI/ is going to require you to write them down.

This patch subject / body is a basic summary. It's about
drivers/nvmem/core.c .


Let me explain it with more details:

NVMEM is a data blob.
NVMEM consists of entries called cells.


Example:

U-Boot environment variables is NVMEM. Example:
00000000  c4 09 30 54 62 6f 6f 74  63 6d 64 3d 74 66 74 70  |..0Tbootcmd=tftp|
00000010  00 62 6f 6f 74 64 65 6c  61 79 3d 35 00 65 74 68  |.bootdelay=5.eth|

A single environment variable is NVMEM cell. Example:
bootcmd=tftp
bootdelay=5


Every NVMEM device is exposed in sysfs as:
/sys/bus/nvmem/devices/*/

You can read NVMEM blob doing
cat /sys/bus/nvmem/devices/*/nvmem | hexdump -C


What I'm trying to do is to expose NVMEM cells in sysfs as:
/sys/bus/nvmem/devices/cells/*

Example:
$ cat /sys/bus/nvmem/devices/foo/cells/bootcmd
tftp
$ cat /sys/bus/nvmem/devices/foo/cells/bootdelay
5

As you can see above NVMEM cells are not known at compilation time.


So I believe the question is: how can I expose cells in sysfs?