linux-kernel - Re: [Gta04-owner] [PATCH 0/3] tty slave device support

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Message-Id: <72268D51-F993-497A-B4F9-707C8DB7155C@goldelico.com>
Date:	Fri, 20 Mar 2015 14:57:12 +0100
From:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>
To:	Sebastian Reichel <sre@...nel.org>
Cc:	NeilBrown <neilb@...e.de>, Mark Rutland <mark.rutland@....com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Peter Hurley <peter@...leysoftware.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pavel Machek <pavel@....cz>,
	Grant Likely <grant.likely@...aro.org>,
	Jiri Slaby <jslaby@...e.cz>, devicetree@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>,
	Belisko Marek <marek.belisko@...il.com>,
	List for communicating with real GTA04 owners 
	<gta04-owner@...delico.com>
Subject: Re: [Gta04-owner] [PATCH 0/3] tty slave device support - version 3.

Hi,

Am 20.03.2015 um 14:08 schrieb Sebastian Reichel <sre@...nel.org>:

> Hi,
> 
> On Fri, Mar 20, 2015 at 09:54:21AM +0100, Dr. H. Nikolaus Schaller wrote:
>> [...]
>> And we do not write a driver for the w2sg0004 but the regulator inside that
>> chip.
> 
> DT describes the hardware structure and not the Linux driver
> structure.

I am not advocating for describing the Linux driver structure in DT.
Rather I suggest that the DT should describe hardware and there
is a regulator inside that chip… Linux drivers can follow that or mix
everything into a single driver if that is better.

> 
>> You also won’t expect that the omap3 driver hides everything. You
>> expect that the twl4030 provides some regulators that can be enabled
>> by subsystems inside the omap3. And if I remember correctly there are
>> regulators inside the omap3 which have explicit regulator nodes in the DT.
> 
> so let's have a look at twl regulators. They can be found below
> the twl node, so they will look similar to
> 
> / -> omap3 -> i2c -> twl -> regulator
> 
> So properly mapped to the w2sg0004 device this would put your
> regulator to
> 
> / -> omap3 -> serial -> w2sg0004 -> regulator

Yes. Indeed.

Currently it is

omap3 -> serial -> serial-power-manager (trying to cover a multitude
of different chips).

> 
> This will gain you nothing as far as I can tell.

And because of that I think Neil’s argument isn’t for or against
anything.

> 
> Please note that subdevices under the serial node are pretty useful,
> since then the kernel can e.g. automatically setup correct line
> disciplines for serial attached bluetooth chips and make bluetooth
> work out of the box.

I don’t doubt that such subnodes are useful. I just object mixing
different chips and controls into a single subnode driver.

And as soon as you want to control line disciplines from such
a subnode, you indeed need a driver for each variant.

So a GPS chip needing some line discipline could be

> / -> omap3 -> serial -> w2sg0004 -> line-discipline
> / -> omap3 -> serial -> w2sg0004 -> regulator

And if omap3 -> serial can directly control a regulator, it can
easily be the w2sg0004 -> regulator

> 
> I am aware, that the Linux kernel has no such thing for serial
> attached GPS devices, but that's Linux specific and irrelevant
> for the DT description.
> 
>> The w2sg0004-regulator approach just follows this principle.
>> Anyone willing to control the power of the w2sg0004 can use this
>> regulator interface.
> 
> From a HW perspective your regulator "hides" the information, that
> there is a device attached to the serial port and instead claims,
> that a regulator is needed to make the serial port work.

Yes, without this regulator the serial port does not work. It is IMHO
more important than stating the obvious that a serial device is
connected to a serial port.

And if there is nothing to hide (the obvious serial interface wiring), why
describe it at all?

Anyways, in my proposal there will be a subnode of the uart, the
one where it is specified that it should control the regulator of the
w2sg.

Basically, Neil’s proposal already covers this case. His bluetooth
subnode just controls &vaux4 which turns on power for the w2cbw003
chip.

So for my view it is not really understandable why one uart subnode
can control a regulator, and the other can’t or shouldn’t and why
this regulator function can’t be divided from the serial management
into a regulator driver.

> 
> Apart from that this interface is limited in its features. I'm
> currently working on N900's bluetooth chip. This one needs to set a
> gpio before data is sent data to the chip, has a reset gpio and
> a gpio to wakeup the OMAP SoC from idle states to avoid data loss
> on the UART.

Yes, that looks equally complicated to solve if not even more than
the w2sg chip.

But what would you prefer:

* extend the drivers/tty/slave/serial-power-manager.c to also cover your
  special case

* instantiate a drivers/misc/n900-bluetooth-regulator.c and hook it up
  exactly like the vaux4 of the gta04 bluetooth chip? Just referencing
  a different regulator node?

And as said I don’t mind if the regulator is a subnode of the omap3 serial.

BR,
Nikolaus

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