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From: Rob Herring <robh@kernel.org>
Date: Mon, 5 Dec 2016 11:27:07 -0600
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Subject: Re: [PATCH v2 2/2] eeprom: Add IDT 89HPESx driver bindings file
To: Serge Semin <fancer.lancer@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
        Andrew Lunn <andrew@lunn.ch>, Mark Rutland <mark.rutland@arm.com>,
        Sergey.Semin@t-platforms.ru,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
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On Mon, Dec 5, 2016 at 9:25 AM, Serge Semin <fancer.lancer@gmail.com> wrote:
> On Mon, Dec 05, 2016 at 08:46:21AM -0600, Rob Herring <robh@kernel.org> wrote:
>> On Tue, Nov 29, 2016 at 01:38:21AM +0300, Serge Semin wrote:
>> > See cover-letter for changelog
>> >
>> > Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
>> >
>> > ---
>> >  .../devicetree/bindings/misc/idt_89hpesx.txt       | 41 ++++++++++++++++++++++
>>
>> There's not a better location for this? I can't tell because you don't
>> describe what the device is.
>>
>
> The device is PCIe-switch EEPROM driver with additional debug-interface to
> access the switch CSRs. EEPROM is accesses via a separate i2c-slave
> interface of the switch.
>
> There might be another place to put the binding file in. There is a special
> location for EEPROM drivers bindings - Documentation/devicetree/bindings/eeprom/ .
> But as far as I understood from the files put in there, it's intended for
> pure EEPROM drivers only. On the other hand the directory I've chosen:
> Documentation/devicetree/bindings/misc/
> mostly intended for some unusual devices. My device isn't usual, since it
> has CSRs debug-interface as well. Additionally I've found
> eeprom-93xx46.txt binding file there, which describes EEPROM bindings.
>
> Anyway if you find the file should be placed in
> Documentation/devicetree/bindings/eeprom/ instead, I'll move it, it's not
> that a big problem.
>
>> >  1 file changed, 41 insertions(+)
>> >  create mode 100644 Documentation/devicetree/bindings/misc/idt_89hpesx.txt
>> >
>> > diff --git a/Documentation/devicetree/bindings/misc/idt_89hpesx.txt b/Documentation/devicetree/bindings/misc/idt_89hpesx.txt
>> > index 0000000..469cc93
>> > --- /dev/null
>> > +++ b/Documentation/devicetree/bindings/misc/idt_89hpesx.txt
>> > @@ -0,0 +1,41 @@
>> > +EEPROM / CSR SMBus-slave interface of IDT 89HPESx devices
>> > +
>> > +Required properties:
>> > +  - compatible : should be "<manufacturer>,<type>"
>> > +            Basically there is only one manufacturer: idt, but some
>> > +            compatible devices may be produced in future. Following devices
>> > +            are supported: 89hpes8nt2, 89hpes12nt3, 89hpes24nt6ag2,
>> > +            89hpes32nt8ag2, 89hpes32nt8bg2, 89hpes12nt12g2, 89hpes16nt16g2,
>> > +            89hpes24nt24g2, 89hpes32nt24ag2, 89hpes32nt24bg2;
>> > +            89hpes12n3, 89hpes12n3a, 89hpes24n3, 89hpes24n3a;
>> > +            89hpes32h8, 89hpes32h8g2, 89hpes48h12, 89hpes48h12g2,
>> > +            89hpes48h12ag2, 89hpes16h16, 89hpes22h16, 89hpes22h16g2,
>> > +            89hpes34h16, 89hpes34h16g2, 89hpes64h16, 89hpes64h16g2,
>> > +            89hpes64h16ag2;
>> > +            89hpes12t3g2, 89hpes24t3g2, 89hpes16t4, 89hpes4t4g2,
>> > +            89hpes10t4g2, 89hpes16t4g2, 89hpes16t4ag2, 89hpes5t5,
>> > +            89hpes6t5, 89hpes8t5, 89hpes8t5a, 89hpes24t6, 89hpes6t6g2,
>> > +            89hpes24t6g2, 89hpes16t7, 89hpes32t8, 89hpes32t8g2,
>> > +            89hpes48t12, 89hpes48t12g2.
>> > +            Current implementation of the driver doesn't have any device-
>>
>> Driver capabilties are irrelevant to bindings.
>>
>
> Why? I've told in the comment, that the devices actually differ by the CSRs
> map. Even though it's not reflected in the code at the moment, the CSRs
> read/write restrictions can be added by some concerned programmer in
> future. But If I left something like "compatible : idt,89hpesx" device
> only, it will be problematic to add that functionality.

Bindings describe the h/w, not what the Linux, FreeBSD, etc. driver
does. You don't want to be changing the binding doc when the driver
changes.

> Howbeit If you think it's not necessary and "compatible = idt,89hpesx" is
> ok, it's perfectly fine for me to make it this way. The property will be
> even simpler, than current approach.

NO! That's not at all what I'm suggesting. Specific compatible strings
are the right way to go for the reasons you give. You just don't need
to state why here (because it is true for all bindings).

>> > +            specific functionalities. But since each of them differs
>> > +            by registers mapping, CSRs read/write restrictions can be
>> > +            added in future.
>> > +  - reg :   I2C address of the IDT 89HPES device.
>> > +
>> > +Optional properties:
>> > +  - read-only :     Parameterless property disables writes to the EEPROM
>> > +  - idt,eesize : Size of EEPROM device connected to IDT 89HPES i2c-master bus
>> > +            (default value is 4096 bytes if option isn't specified)
>> > +  - idt,eeaddr : Custom address of EEPROM device
>> > +            (If not specified IDT 89HPESx device will try to communicate
>> > +             with EEPROM sited by default address - 0x50)
>>
>> Don't we already have standard EEPROM properties that could be used
>> here?
>>
>
> If we do, just tell me which one. There are standard options:

You can grep thru bindings as easily as I can. I can't do that for
everyone's binding.

> "compatible, reg, pagesize, read-only". There isn't any connected with
> EEPROM actual size.
> Why so? Because standard EEPROM-drivers determine the device size from the
> compatible-string name. Such approach won't work in this case, because
> PCIe-switch and it EEPROM are actually two different devices. Look at the
> chain of the usual platform board design:
> Host <--- i2c ----> i2c-slave iface |PCIe-switch| i2c-master iface <--- i2c ---> EEPROM
>
> As you cas see the Host reaches EEPROM through the set of PCIe-switch
> i2c-interfaces. In order to properly get data from it my driver needs actual
> EEPROM size and it address in the i2c-master bus of the PCIe-switch, in
> addition to the standard reg-field, which is address of PCIe-switch i2c-slave
> interface and read-only parameter if EEPROM-device has got WP pin asserted.

Ah, this needs to be much different than I thought. You need to model
(i.e. use the same binding) the EEPROM node just like it was directly
attached to the host. So this means you need the 2nd i2c bus modeled
which means you need the PCIe switch modeled. A rough outline of the
nodes would look like this:

host-i2c: i2c {
  compatible ="host-i2c"
};

pcie {
    pcie-switch {
        i2c-bus = <&host-i2c>;
        i2c-bus {
            eeprom@50 {
            };
        };
    };
};

So this models the PCIe switch as a PCIe device, it has a phandle back
to it's controller since it's not a child of the i2c controller. Then
the devices on switches i2c bus are modeled as children of the switch.

Alternatively, it could be described all as children of host-i2c node.
It's common for i2c devices to have downstream i2c buses. I2C muxes
are one example and there are bindings defined for all this. There's
also chips like mpu-6050 that have slave buses.

Rob