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Date:   Wed, 23 Feb 2022 14:39:27 +0100
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Subject: Re: [RFC 10/10] net: sfp: add support for fwnode
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To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        =?UTF-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= <clement.leger@bootlin.com>,
        Daniel Scally <djrscally@gmail.com>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Wolfram Sang <wsa@kernel.org>, Peter Rosin <peda@axentia.se>,
        Andrew Lunn <andrew@lunn.ch>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, linux-kernel@vger.kernel.org,
        linux-acpi@vger.kernel.org, linux-i2c@vger.kernel.org,
        netdev@vger.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>
References: <20220221162652.103834-1-clement.leger@bootlin.com>
 <20220221162652.103834-11-clement.leger@bootlin.com>
 <YhPSkz8+BIcdb72R@smile.fi.intel.com> <20220222142513.026ad98c@fixe.home>
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 <888f9f1a-ca5a-1250-1423-6c012ec773e2@redhat.com>
 <YhYriwvHJKjrDQRf@shell.armlinux.org.uk>
From:   Hans de Goede <hdegoede@redhat.com>
In-Reply-To: <YhYriwvHJKjrDQRf@shell.armlinux.org.uk>
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Hi,

On 2/23/22 13:41, Russell King (Oracle) wrote:
> On Wed, Feb 23, 2022 at 01:02:23PM +0100, Hans de Goede wrote:
>> Hi,
>>
>> On 2/23/22 12:22, Andy Shevchenko wrote:
>>> On Tue, Feb 22, 2022 at 02:25:13PM +0100, Clément Léger wrote:
>>>> Le Mon, 21 Feb 2022 19:57:39 +0200,
>>>> Andy Shevchenko <andriy.shevchenko@linux.intel.com> a écrit :
>>>>
>>>>> On Mon, Feb 21, 2022 at 05:26:52PM +0100, Clément Léger wrote:
>>>>>> Add support to retrieve a i2c bus in sfp with a fwnode. This support
>>>>>> is using the fwnode API which also works with device-tree and ACPI.
>>>>>> For this purpose, the device-tree and ACPI code handling the i2c
>>>>>> adapter retrieval was factorized with the new code. This also allows
>>>>>> i2c devices using a software_node description to be used by sfp code.  
>>>>>
>>>>> If I'm not mistaken this patch can even go separately right now, since all used
>>>>> APIs are already available.
>>>>
>>>> This patches uses fwnode_find_i2c_adapter_by_node() which is introduced
>>>> by "i2c: fwnode: add fwnode_find_i2c_adapter_by_node()" but they can
>>>> probably be contributed both in a separate series.
>>>
>>> I summon Hans into the discussion since I remember he recently refactored
>>> a bit I2C (ACPI/fwnode) APIs. Also he might have an idea about entire big
>>> picture approach with this series based on his ACPI experience.
>>
>> If I understand this series correctly then this is about a PCI-E card
>> which has an I2C controller on the card and behind that I2C-controller
>> there are a couple if I2C muxes + I2C clients.
> 
> That is what I gathered as well.
> 
>> Assuming I did understand the above correctly. One alternative would be
>> to simply manually instantiate the I2C muxes + clients using
>> i2c_new_client_device(). But I'm not sure if i2c_new_client_device()
>> will work for the muxes without adding some software_nodes which
>> brings us back to something like this patch-set.
> 
> That assumes that an I2C device is always present, which is not always
> the case - there are hot-pluggable devices on I2C buses.
> 
> Specifically, this series includes pluggable SFP modules, which fall
> into this category of "hot-pluggable I2C devices" - spanning several
> bus addresses (0x50, 0x51, 0x56). 0x50 is EEPROM like, but not quite
> as the top 128 bytes is paged and sometimes buggy in terms of access
> behaviour. 0x51 contains a bunch of monitoring and other controls
> for the module which again can be paged. At 0x56, there may possibly
> be some kind of device that translates I2C accesses to MDIO accesses
> to access a PHY onboard.
> 
> Consequently, the SFP driver and MDIO translation layer wants access to
> the I2C bus, rather than a device.
> 
> Now, before ARM was converted to DT, we had ways to cope with
> non-firmware described setups like this by using platform devices and
> platform data. Much of that ended up deprecated, because - hey - DT
> is great and more modern and the old way is disgusting and we want to
> get rid of it.
> 
> However, that approach locks us into describing stuff in firmware,
> which is unsuitable when something like this comes along.
> 
> I think what we need is both approaches. We need a way for the SFP
> driver (which is a platform_driver) to be used _without_ needing
> descriptions in firmware. I think we have that for GPIOs, but for an
> I2C bus, We have i2c_get_adapter() for I2C buses, but that needs the
> bus number - we could either pass the i2c_adapter or the adapter
> number through platform data to the SFP driver.
> 
> Or is there another solution to being able to reuse multi-driver
> based infrastructure that we have developed based on DT descriptions
> in situations such as an add-in PCI card?

The use of software fwnode-s as proposed in this patch-set is another
way to deal with this. There has been work to abstract ACPI vs
of/dt firmware-nodes into a generic fwnode concept and software-nodes
are a third way to define fwnode-s for "struct device" devices.

Software nodes currently are mainly used as so called secondary
fwnodes which means they can e.g. add extra properties to cover
for the firmware description missing some info (which at least
on ACPI happens more often then we would like).

But a software-node can also be used as the primary fwnode for
a device. So what this patch-set does is move the i2c of/dt
enumeration code over to the fwnode abstraction (1). This allows
the driver for the SPF card to attach a software fwnode to the
device for the i2c-controller which describes the hotplug pins +
any other always present hw in the same way as it would be done
in a devicetree fwnode and then the existing of/dt based SPF
code can be re-used as is.

At least that is my understanding of this patch-set.

Regards,

Hans


1) This should result in no functional changes for existing
devicetree use cases.