linux-kernel - Re: [PATCH 0/4] UART slave device support

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Date:	Wed, 12 Aug 2015 09:20:59 +1000
From:	NeilBrown <neil@...wn.name>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	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>,
	Sebastian Reichel <sre@...nel.org>,
	Rob Herring <robherring2@...il.com>,
	Pavel Machek <pavel@....cz>,
	Grant Likely <grant.likely@...aro.org>,
	Jiri Slaby <jslaby@...e.cz>,
	GTA04 owners <gta04-owner@...delico.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] UART slave device support - version 4

On Fri, 7 Aug 2015 15:01:47 +0200 Linus Walleij
<linus.walleij@...aro.org> wrote:

> Hi Neil,
> 
> first, this is a *VERY* interesting and much needed patch series,
> I intend to look closer at it, and if possible test it with some
> (heh) board file device. Would be happy of you put me on CC for these.
> 
> On Mon, May 11, 2015 at 3:56 AM, NeilBrown <neil@...wn.name> wrote:
> 
> >  When a device is connected to a UART via RS-232 (or similar), there
> >  is a DTR line that can be used for power management, and other "modem
> >  control" lines.
> >
> >  On an embedded board, it is very likely that there is no "DTR", and
> >  any power management need to be done using some completely separate
> >  mechanism.
> >
> >  So these "slaves" are really just for devices permanently attached to
> >  UARTs without a full "RS-232" (or similar) connection.  The driver
> >  does all the extra control beyond Tx/Rx.
> 
> What is usually happening (and I have seen it in a few places) is that
> the SoC has *one* fully featured RS232 with CTS/RTS and even
> DTS,DCD,RI and other esoterica, which is intended to be connected to a
> host serial port or so, for example if this SoC is to act as a modem
> or a fax machine, or if it is to drive one.
> 
> Then they often have a few more UART blocks, usually identical, which
> only have RxD+TxD available, so they are "just" UARTs.
> 
> To complicate things further, you may wonder what happened with
> the CTS/RTS (etc) signals from the other blocks. Usually they are there
> in the silicon but just routed to dead ends.
> 
> To complicate it even further, usually all these pins are placed under
> pin control multiplexing, so in an actual electronic design, the
> system will mux out CTS/RTS (etc) from the fully featured RS232
> blocks and only use them as UARTs anyways.
> 
> Then there are those who created real simple RxD/TxD-only UARTs
> ("yeah lets dump this RS232 legacy crap" / "yeah yeah")
> and then realized they want to drive modems ("oh crap, it seemed
> like a good idea at the time"). Then they usually take
> two GPIO pins for CTS/RTS and drive them as GPIOs using
> software and you have a cheap 4-line modem line. This is what
> drivers/tty/serial/serial_mctrl_gpio.c is for if you wondered.
> 
> > I've tested this set and it seems to work ... except that something
> >  is sadly broken with bluetooth support in 4.1-rc1 so I've only really
> >  tested the GPS driver.  I guess it is time to rebase to -rc3.
> 
> You have a hardware taget I see. Which one?

GTA04 (www.gta04.org - openmoko successor).

3 uarts on OMAP3 are wired: one as RS-232 for console, one to bluetooth
half of a wifi/bluetooth module, and one to a GPS.

For the GPS, I just want to power on/off when the TTY is opened/closed,
but the power-on sequence is non-trivial as both "turn on" and
"turn-off' toggle the same line, so I need to be able to detect current
state.

For the bluetooth, the power is a (shared) regulator.  As well as
power-on when the TTY is opened, I'd like regulator to be turned of
when I "hciconfig down" - even though the TTY is still open.
I did a patch a while ago which hooked in to hci_uart_{open,close} to
make this work, but it isn't a really good patch.

It would be nice to hide the TTY from user-space in the bluetooth case,
and have the "hciattach" happen in the kernel, but I think hciattach
does extra initialisation...

NeilBrown


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