linux-kernel - Re: [PATCH v0] nvmem: core: Export nvmem cell info to userspace

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Date:   Mon, 25 Mar 2019 10:58:38 +0000
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Gaurav Kohli <gkohli@...eaurora.org>, linux-kernel@...r.kernel.org
Cc:     linux-arm-msm@...r.kernel.org,
        Shiraz Hashim <shashim@...eaurora.org>
Subject: Re: [PATCH v0] nvmem: core: Export nvmem cell info to userspace



On 24/03/2019 15:25, Gaurav Kohli wrote:
> 
> On 3/22/2019 8:53 PM, Srinivas Kandagatla wrote:
>>
>>
>> On 20/03/2019 05:53, Gaurav Kohli wrote:
>>> From: Shiraz Hashim <shashim@...eaurora.org>
>>>
>>> Existing nvmem framework export full register space
>>> as nvmem binary, but not exporting child node of nvmem
>>> which is nvmem cell. Kernel can read the specific cell
>>> by using nvmem_cell_read but userspace don't have such
>>> provision.
>>>
>>> Add framework to export nvmem cell as well, So
>>> userspace can use it directly.
>>>
>>> Signed-off-by: Shiraz Hashim <shashim@...eaurora.org>
>>> Signed-off-by: Gaurav Kohli <gkohli@...eaurora.org>
>>> Co-developed-by: Gaurav Kohli <gkohli@...eaurora.org>
>>>
>>> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
>>
>> Thankyou for the patch.
>>
>> Why do you need such provision when the userspace can just get the 
>> cell values using correct offset and size.
>> This will also bring over head of managing entries dynamically + 
>> confusing userspace abi.
>>
>> Unless you have a valid reason or usecase I don't see the need for this.
> 
> 
> Hi Srinivas,
> 
> 
> This is mainly for user space convenience, In existing implementation 
> they have to do manipulation according

> 
> to offset and bit, And with present patch, they just have to do cat for 
> cell name and which can also be easily maintainable
Yes, that is expected I guess!

> 
> for different soc. But with current, it is difficult to maintain users 
> space code as each time we have to change user space code according to bit.

Which user space code/application are you referring to here? Are these 
open source?

> 
> 
> This would also help to expose certain bit only as per the bit parameter 
> mentioned in dt node, which would also help to protect exposing of
> 
NVMEM is not just limited for DT users, non dt users use this f/w too.
So the problem is not as simple as it sounds.

If your issue is just about DT, you could easily parse active device 
tree via  /proc/device-tree and get cell offset, length and names from 
it, use this information to read from nvmem.

There are other concerns about the userspace ABI w.r.t udev events.
udev events would race with the creation on this cell entries resulting 
in a behavior where user-space applications would not see the entries 
after udev events.

In worst case if we decide to go with adding cells to nvmem then we 
should do it before the device is even probed using group attributes.
And this would mean that we can not support cells that are dynamically 
defined. And there might be some memory freeing issues in this method too!

--srini


> other bits to user space.
> 
>>
>> thanks,
>> srini
>