Re: [PATCH v2 1/5] dt-bindings: at24: consistently document the compatible property

[Javier Martinez Canillas]

Hello Peter,

On Fri, Dec 22, 2017 at 12:07 AM, Peter Rosin <peda@xxxxxxxxxx> wrote:
> On 2017-12-21 21:27, Javier Martinez Canillas wrote:
>> Hello Peter,
>>
>> On Thu, Dec 21, 2017 at 5:20 PM, Peter Rosin <peda@xxxxxxxxxx> wrote:
>>> On 2017-12-21 14:48, Bartosz Golaszewski wrote:
>>>> Current description of the compatible property for at24 is quite vague.
>>>>
>>>> Specify an exact list of accepted compatibles and document the - now
>>>> deprecated - strings which were previously used in device tree files.
>>>
>>> Why is it suddenly deprecated to correctly specify what hardware you
>>> have, e.g. "nxp,24c32". In this case the manufacturer is nxp, damnit.
>>
>> Sorry but I disagree.
>>
>> When you specify a compatible string, you are not specifying a
>> particular hardware but a device programming model.
>
> That's not what it says in https://elinux.org/Device_Tree_Usage

I think the most up-to-date DT reference is at:

https://www.devicetree.org/

> in the "Understanding the compatible Property" section. I quote:
>
>         compatible is a list of strings. The first string in the
>         list specifies the exact device that the node represents
>         in the form "<manufacturer>,<model>". The following strings
>         represent other devices that the device is compatible with.
>
> Pretty clearly talks about devices and not programming models. But
> maybe I shouldn't trust that reference? What should I be reading
> instead?
>

For example the latest spec draft
(https://github.com/devicetree-org/devicetree-specification/releases/download/v0.2-rc1/devicetree-specification-v0.2-rc1.pdf)
says:

"The compatible property value consists of one or more strings that
define the specific programming model for the device. This list of
strings should be used by a client program for device driver
selection. The property value consists of a concatenated list of null
terminated strings, from most specific to most general. They allow a
device to express its compatibility with a family of similar devices,
potentially allowing a single device driver to match against several
devices."

The recommended format is "manufacturer,model", where manufacturer is
a string describing the name of the manufacturer (such as a stock
ticker symbol), and model specifies the model number."

Example:

compatible = "fsl,mpc8641", "ns16550";

In this example, an operating system would first try to locate a
device driver that supported fsl,mpc8641. If a driver was not found,
it would then try to locate a driver that supported the more general
ns16550 device type."

>> It's very common to use a compatible string that doesn't match exactly
>> the specific hardware used. That's why it's called _compatible_ BTW.
>
> That's not how I read the above.
>

That's not how I read it nor my experience with DT, but of course I
may be wrong on this.

>> For example when a DTS define a UART node with an ns16550 compatible,
>> they don't really mean that have a UART IC manufactured by National
>> Semiconductor.
>
> That just tells me that most people are a little bit lazy and ready
> to cut a corner or two when they can get away with it. Or that there
> is some form of misunderstanding at work...
>

For example, I usually see that different SoC families from the same
vendor use the same compatible string for integrated peripherals just
because are the same from a programming model point of view.

TI am33xx SoCs use a lot of omap3 compatible strings on their nodes
and its similar on Exynos SoCs which are the two ARM SoCs I'm most
familiar with. Following your logic that's wrong and instead a new
compatible string should be added for the GPIO or pinctrl drivers even
when are the same because refer to different devices.

>>> Sure, it happens to be compatible with "atmel,24c32", but that is
>>> supposed to be written with a fallback as
>>>
>>>         "nxp,24c32", "atmel,24c32"
>>
>> This isn't a requirement really, systems integrators are free to use
>> more than one <manufacturer,model> tuple in the compatible string if
>> they want the DTB to be future proof, just in case there's a need for
>> a more specific driver or if the programming model happened to not be
>> the same at the end. This is usually done for the boards compatible
>> string as an example, even when there isn't a struct machine_desc for
>> the specific board compatible and only for the SoC family.
>>
>> So it's OK if you want to define the compatible as "nxp,24c32",
>> "atmel,24c32", but that's a general OF concept and not something
>> related to the at24 eeprom driver so I'm not sure if it should be
>> mentioned in the at24 DT binding doc.
>
> One problem is that if "nxp,24c32" (or "nxp,24c02" as in the example
> below) is not a valid compatible, the tooling will be correct to
> complain about it. Currently, it is just a checkpatch deficiency that
> it complains like this:
>
> $ scripts/checkpatch.pl -f arch/arm/boot/dts/at91-tse850-3.dts
> WARNING: DT compatible string "nxp,24c02" appears un-documented -- check ./Documentation/devicetree/bindings/
> #249: FILE: arch/arm/boot/dts/at91-tse850-3.dts:249:
> +               compatible = "nxp,24c02", "atmel,24c02";
>

But isn't that a bug in checkpatch? as long as there's a valid
<manufacturer,model> tuple in the compatible string, it shouldn't
complain.

>>> if I understand correctly. So, why is that deprecated in this case?
>>>
>>> What if (a few years down the line) it is discovered that some weird
>>> quirk is needed that is only appropriate for nxp chips?
>>>
>>> nxp is of course just an example, pick any manufacturer of eeproms
>>> (supposedly) compatible with the atmel interface.
>>>
>>
>> That's indeed a possibility and the reason why most DT nodes have a
>> more specific <manufacturer,model> before the generic one matched by
>> drivers. I really doubt that in the future it will be found that a
>> more specific compatible string is needed for one manufacturer in this
>> case, specially since the driver didn't even care about the
>> manufacturers until recently.
>
> I'm not talking about the driver. But if what you say is true, that
> change would have broken a number of existing DTBs. Is that really the
> case?
>

Sorry, I didn't get which change you meant.

>> So I think that makes no sense for a driver to support all possible
>> manufacturers as possible compatible strings if all the devices have
>> the same exact programming model.
>
> That's what the fallback is for. With that in place the driver can
> start to care when there is a need but until then it can work just
> as it always has. Without knowing the specific device, the driver has
> less chance to adapt when the unexpected is discovered. But again,
> I'm not really discussing driver details, I'm complaining about the
> proposed changes to the bindings. I simply don't see how they make
> sense.
>

My point is that saying in a DT binding that you could have a more
specific <manufacturer,model> followed by a generic one is redundant.
That's already something one can do to make the DT future proof and
latter if is found that the more specific pair is needed, then
compatible can be added to the OF device ID table making the driver to
match that an the <manufacturer,model> added to the DT binding making
it formal.

That's my understanding at least, but I could be completely wrong too
since I'm not a DT expert.

> Cheers,
> Peter

Best regards,
Javier