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

Hello Peter,

On Fri, Dec 22, 2017 at 12:07 AM, Peter Rosin [off-list ref] wrote:

On 2017-12-21 21:27, Javier Martinez Canillas wrote:

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Hello Peter,

On Thu, Dec 21, 2017 at 5:20 PM, Peter Rosin [off-list ref] wrote:

quoted

On 2017-12-21 14:48, Bartosz Golaszewski wrote:

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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."

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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.

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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.

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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.

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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.

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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