linux-kernel - Re: [PATCH RFC 0/3] UART slave device support

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Date:	Sat, 6 Jun 2015 15:09:41 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Mark Rutland <mark.rutland@....com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Reichel <sre@...nel.org>,
	NeilBrown <neil@...wn.name>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Marek Belisko <marek@...delico.com>,
	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	List for communicating with real GTA04 owners 
	<gta04-owner@...delico.com>
Subject: Re: [PATCH RFC 0/3] UART slave device support

On Wed 2015-06-03 13:49:30, Dr. H. Nikolaus Schaller wrote:
> Hi all,
> this patch series is our proposal to add hooks so that the driver for a device connected to an UART can
> monitor modem control lines and data activity of the connected chip.
> 
> It contains an example for such a device driver which needs such sophisticated power control: wi2wi,w2sg0004
> 
> A remote device connected to a RS232 interface is usually power controlled by the DTR line.
> The DTR line is managed automatically by the UART driver for open() and close() syscalls
> and on demand by tcsetattr().
> 
> With embedded devices, the serial peripheral might be directly and always connected to the UART
> and there might be no physical DTR line involved. Power control (on/off) has to be done by some
> chip specific device driver (which we call "UART slave") through some mechanisms (I2C, GPIOs etc.)
> not related to the serial interface. Some devices do not tell their power state except by sending
> or not sending data to the UART. In such a case the device driver must be able to monitor data
> activity. The role of the device driver is to encapsulate such power control in a single place.
> 
> This patch series allows to support such UART slave drivers by providing:
> * a mechanism that a slave driver can identify the UART instance it is connected to
> * a mechanism that UART slave drivers can register to be notified
> * notfications for DTR (and other modem control) state changes
> * notifications that the device has sent some data to the UART
> 
> A slave device tree entry simply adds a phandle reference to the UART it is connected to, e.g.
> 
> 	gps {
> 		compatible = "wi2wi,w2sg0004";
> 		uart = <&uart1>;
> 	};

Device tree maintainers repeatedly pointed out this is not a way to go.

NAK.

										Pavel

> 
> The slave driver calls devm_serial_get_uart_by_phandle() to identify the uart driver.
> devm_serial_get_uart_by_phandle() follows the concept of devm_usb_get_phy_by_phandle().
> 
> A slave device driver registers itself with serial_register_slave() to receive notifications.
> Notification handlers can be registered by serial_register_mctrl_notification() and
> serial_register_rx_notification(). If an UART has a NULL slave or a NULL handler registered,
> no notifications are sent.
> 
> RX notification handlers can define a ktermios setup and modify or decide to throw away the
> character that is passed upwards.
> 
> This all is a follow-up to the w2sg0004 driver submitted in 2014 that did want to add an optional
> GPIO to DTR in omap-serial.c and required the w2sg0004 driver to present itself as a "virtual
> GPIO". The idea of a "virtual GPIO"  is not compatible with the concept that DT must
> describe hardware (and not virtual hardware). So in this new solution DT only describes that
> the w2sg0004 is connected to some UART and how the power state signalling works is left
> to the driver implementations.
> 
> The rx data notification also removes the concept of having two different pinmux states
> and make the w2sg0004 driver intercept rx activities by switching the rx line to a GPIO
> interrupt. This was very OMAP3 specific. The new solution is generic and might even be
> extensible that the chip driver could filter or consume the rx data before it is passed
> to the tty layer.
> 
> This patch works on linux-next as intended except one major weakness: we have to call 
> uart_change_speed() each time we open the tty. This is the opposite of what we would like
> to have: that the slave initializes the uart speed through some termios and the tty level just uses
> this setting. We have not yet completely understood how to make this work and are happy
> about help in this area.
> 
> And of course we would like to see general comments about the whole implementation
> approach.
> 
> 
> Signed-off-by: H. Nikolaus Schaller <hns@...delico.com>
> 

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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