linux-kernel - Re: [PATCH v4 0/9] Add Mule MFD support

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Date: Mon, 24 Jun 2024 18:13:49 +0200
From: Farouk Bouabid <farouk.bouabid@...rry.de>
To: Guenter Roeck <linux@...ck-us.net>,
 Quentin Schulz <quentin.schulz@...rry.de>, Jean Delvare <jdelvare@...e.com>,
 Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Lee Jones <lee@...nel.org>,
 Peter Rosin <peda@...ntia.se>, Heiko Stuebner <heiko@...ech.de>
Cc: linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH v4 0/9] Add Mule MFD support

Hi Guenter,

On 19.06.24 15:31, Guenter Roeck wrote:
> On 6/19/24 00:45, Farouk Bouabid wrote:
>
>>>
>>> If it is properly defined in devicetree, the emulated AMC6821 should be
>>> an i2c device, possibly sitting behind an i2c multiplexer, not a
>>> platform device.
>>
>>
>> The emulated AMC6821 and the Mule I2C mux are both reachable using 
>> I2C address (0x18), and hence the use of MFD as the mux only uses one 
>> I2C register that is not used by AMC6821.
>>
>
> Whatever you do, the amc chip is still an i2c driver and needs to 
> remain one.
> Modeling it as platform driver is simply wrong, and I won't accept 
> those patches.
>

The issue that we have cannot be handled by an I2C mux because in that 
case both the mux and its child would have the same address which is not 
supported in the I2C subsystem:


i2c-mux@18 {

         compatible = "tsd,mule-i2c-mux";

         reg = <0x18>;

         #address-cells = <1>;

         #size-cells = <0>;


         i2c10: i2c@0 {

                 reg = <0x0>;

                 #address-cells = <1>;

                 #size-cells = <0>;


                 fan: fan@18 {

                         compatible = "ti, amc6821";

                         reg = <0x18>;

                 };

         };

};


The I2C maintainer rejected supporting this use case and suggested that 
an MFD could probably be more suitable.


On one hand, the MFD looks indeed more appropriate and a lot of I2C 
devices are modeled through platform sub devices. On the other hand we 
are emulating the amc6821 in our device which requires us to have it 
modeled as platform:


       +--------+----------------+------------------------------+
       |  Mule         (MFD)                                    |
  0x18 |        +----------------+                              |
--------+----->|    amc6821     |                              |
       | |      +----------------+                              |
       | +----->|      Mux       |-----+                        |
       |        +----------------+     |                        |
       |                               V__          +---------+ |
       |                              |   \-------->| isl1208 | |
       |                              |   |         +---------+ |
  0x6f |                              | M |-------->| dev #1  | |
------------------------------------>| U |         +---------+ |
       |                              | X |-------->| dev #2  | |
       |                              |   |         +---------+ |
       |                              |   /-------->| dev #3  | |
       |                              |__/          +---------+ |
       +--------------------------------------------------------+


If we cannot proceed with that then we could add a compatible to the 
amc6821 driver to add the mux device (Basically the "tsd,mule" 
compatible in amc6821 compatible list would be a combo driver with mux 
logic + amc6821). Do you think that is more appropriate ?


Cheers,

Farouk