[U-Boot] [PATCH v3 01/11] dm: serial: Update binding for PL01x serial UART

[Rob Herring]

On Thu, Aug 13, 2015 at 2:04 PM, Ian Lepore <ian at freebsd.org> wrote:
> On Thu, 2015-08-13 at 14:13 -0400, Tom Rini wrote:
>> On Thu, Aug 13, 2015 at 10:02:58AM -0600, Stephen Warren wrote:
>> > On 08/13/2015 09:59 AM, Simon Glass wrote:
>> > >Hi Linus,
>> > >
>> > >On 11 August 2015 at 07:00, Linus Walleij <linus.walleij at linaro.org> wrote:
>> > >>On Fri, Aug 7, 2015 at 3:42 PM, Simon Glass <sjg at chromium.org> wrote:
>> > >>
>> > >>>This binding differs from that of Linux. Update it and change existing
>> > >>>users.
>> > >>>
>> > >>>Signed-off-by: Simon Glass <sjg at chromium.org>
>> > >>(...)
>> > >>>  doc/device-tree-bindings/serial/pl01x.txt | 55 ++++++++++++++++++++++++++++---
>> > >>
>> > >>So why does U-Boot have its own copy of any bindings at all?
>> > >>
>> > >>This is forking the ontology of who gets to define bindings I fear.
>> > >>It's a bit like have two bibles both claiming to be the word of god.
>> > >>(OK maybe a hyperbolic statement, but you see what I mean.)
>> > >>
>> > >>Can't we just have the bindings in the Linux kernel tree please?
>> > >
>> > >Is there any plan to move them out of Linux and put them in a separate place?
>> > >
>> > >We should make an effort to sync the device tree files with Linux periodically.
>> > >
>> > >I quite like having the bindings in U-Boot since it makes people think
>> > >about what they are adding. Are you worried that the bindings in
>> > >U-Boot might evolve separately? Certainly there has been some of that.
>> > >
>> > >However I recently sent a series to add a few things for Raspberry Pi
>> > >("arm: rpi: Device tree modifications for U-Boot") and I don't yet see
>> > >a willingness to add what some see as 'U-Boot things' to the binding.
>> > >How do we address that?
>> >
>> > DT isn't supposed to contain "U-Boot things", but "an OS-agnostic
>> > description of the hardware". So, I imagine the solution is not to
>> > attempt to do that:-)
>>
>> This always makes me ask if the FreeBSD folks or VxWorks folks have
>> adopted the "Linux" bindings or if the DT files continue to be
>> "OS-agnostic" and "only functional with Linux".  It was a while ago last
>> I looked but it made my head hurt a little doing a quick translation for
>> an SoC that I was familiar with.
>>
>
> As the FreeBSD person who got our first SoC (imx6, only partially
> supported) converted to use the Linux DT files rather than our own
> homebrew mess we started with, I would say that my opinion of the
> existing DT information is that it is an extension of Linux device
> drivers written in a different language.

Sadly, we see little non-Linux participation on DT lists. We've tried
to separate core (devicetree-spec) and driver lists, but pretty much
everything still goes to one list and it is a fire hose. I'm open for
ideas on how to get more non-Linux participation.

> The information available is in no way independent of the Linux device
> drivers, it is exactly the information those drivers need.  It is often
> not the information needed in another OS with independently written
> drivers.  And especially it is not ordered and structured in a way that
> works well with the device enumeration and instantiation models used by
> another OS.
>
> A great case in point would be i2c eeproms.  What a perfect opportunity
> DT would be to describe everything about the eeprom parts (total
> capacity, page read/write size, whether the page address bits extend
> into the bus-slave address bits, etc).  It seems to me that anything
> claiming to be an independent description of the hardware would have to
> include such things.  Instead, all the bindings define is the compatible
> string.  That's crazy.  Why?  Well, when I went and looked at the Linux
> eeprom drivers it became clear why:  that's all they need to know,
> because everything else is hard-coded in tables in the driver source.

Think of compatible as a PID/VID pair like USB or PCI. It is simply a
unique identifier. You don't get any more information with PCI or
non-class USB devices. DT was originally only solving the problem of
finding the non-probeable h/w. It's grown to be a lot more with
today's SOCs.

The more we put into DT the more chance it can be wrong and then we
have to work around not h/w quirks, but DT quirks.

That said we can always add to the bindings. It would have to be
worded something like "optional, but required for new dts's". You
would have to have a new DTB if the OS is dependent on the new
properties.

> So if I want to write a FreeBSD i2c eeprom driver that uses DT data,
> what are my choices?  I have exactly one:  make my driver essentially a
> clone of the Linux driver, with all the same data hard-coded in source.

Don't you already have these drivers w/o using DT? If you did, you
would have this information already in the drivers. It wouldn't be a
question of the binding being Linux specific, but rather can we move
more of the data out of drivers and into DT. That is fundamentally a
different issue than is a binding Linux specific.

Rob