netdev - Re: [PATCH v2 01/29] nvmem: add support for cell lookups

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Message-ID: <626e6122-2e58-3317-89b2-77959c9310f7@linaro.org>
Date:   Tue, 28 Aug 2018 15:48:21 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Boris Brezillon <boris.brezillon@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        linux-doc <linux-doc@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Kevin Hilman <khilman@...nel.org>,
        Richard Weinberger <richard@....at>,
        Russell King <linux@...linux.org.uk>,
        Marek Vasut <marek.vasut@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        David Lechner <david@...hnology.com>,
        Arnd Bergmann <arnd@...db.de>,
        Sven Van Asbroeck <svendev@...x.com>,
        "open list:MEMORY TECHNOLOGY..." <linux-mtd@...ts.infradead.org>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>, Naren <naren.kernel@...il.com>,
        netdev <netdev@...r.kernel.org>, Alban Bedel <albeu@...e.fr>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2 01/29] nvmem: add support for cell lookups



On 28/08/18 15:41, Bartosz Golaszewski wrote:
> 2018-08-28 15:45 GMT+02:00 Srinivas Kandagatla <srinivas.kandagatla@...aro.org>:
>>
>>
...
>>> I would like to support an additional use case here: the provider is
>>> generic and is not aware of its cells at all. Since the only way of
>>> defining nvmem cells is through DT or nvmem_config, we lack a way to
>>> allow machine code to define cells without the provider code being
>>> aware.
>>
>>
>> machine driver should be able to do
>> nvmem_device_get()
>> nvmem_add_cells()
>>
> 
> Indeed, I missed the fact that you can retrieve the nvmem device by
> name. Except that we cannot know that the nvmem provider has been
> registered yet when calling nvmem_device_get(). This could potentially
> be solved by my other patch that adds notifiers to nvmem, but it would
> require much more boilerplate code in every board file. I think that
> removing nvmem_cell_info from nvmem_config and having external cell
> definitions would be cleaner.

Yes, notifiers would work!

...
>>>
>>> Yes, I would like to rework nvmem a bit. I don't see any non-DT users
>>> defining nvmem-cells using nvmem_config. I think that what we need is
>>> a way of specifying cell config outside of nvmem providers in some
>>> kind of structures. These tables would reference the provider by name
>>> and define the cells. Then we would have an additional lookup
>>> structure which would associate the consumer (by dev_id and con_id,
>>> where dev_id could optionally be NULL and where we would fall back to
>>> using con_id only) and the nvmem provider + cell together. Similarly
>>> to how GPIO consumers are associated with the gpiochip and hwnum. How
>>> does it sound?
>>
>> Yes, sounds good.
>>
>> Correct me if am wrong!
>> You should be able to add the new cells using struct nvmem_cell_info and add
>> them to particular provider using nvmem_add_cells().
>>
>> Sounds like thats exactly what nvmem_add_lookup_table() would look like.
>>
>> We should add new nvmem_device_cell_get(nvmem, conn_id) which would return
>> nvmem cell which is specific to the provider. This cell can be used by the
>> machine driver to read/write.
> 
> Except that we could do it lazily - when the nvmem provider actually
> gets registered instead of doing it right away and risking that the
> device isn't even there yet.
> 
Yes, it makes more sense to do it once the provider is actually present!

>>
>>>>>
>>>>> BTW: of_nvmem_cell_get() seems to always allocate an nvmem_cell
>>>>> instance even if the cell for this node was already added to the nvmem
>>>>> device.
>>>>
>>>>
>>>> I hope you got the reason why of_nvmem_cell_get() always allocates new
>>>> instance for every get!!
>>>
>>>
>>>
>>> I admit I didn't test it, but just from reading the code it seems like
>>> in nvmem_cell_get() for DT-users we'll always get to
>>> of_nvmem_cell_get() and in there we always end up calling line 873:
>>> cell = kzalloc(sizeof(*cell), GFP_KERNEL);
>>>
>> That is correct, this cell is created when we do a get and release when we
>> do a put().
>>
> 
> Shouldn't we add the cell to the list, and check first if it's there
> and only create it if not?
Yes I agree, duplicate entry checks are missing!

--srini
> 
> Bart
>