linux-kernel - Re: [Gta04-owner] [PATCH 0/4] UART slave device support

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Message-ID: <56992959.2020204@hurleysoftware.com>
Date:	Fri, 15 Jan 2016 09:16:09 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	"H. Nikolaus Schaller" <hns@...delico.com>,
	Andrey Vostrikov <andrey.vostrikov@...entembedded.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	List for communicating with real GTA04 owners 
	<gta04-owner@...delico.com>, tomeu@...euvizoso.net,
	NeilBrown <neil@...wn.name>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robherring2@...il.com>,
	Pavel Machek <pavel@....cz>, linux-serial@...r.kernel.org,
	Grant Likely <grant.likely@...aro.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Marek Belisko <marek@...delico.com>
Subject: Re: [Gta04-owner] [PATCH 0/4] UART slave device support - version 4

On 01/15/2016 08:08 AM, H. Nikolaus Schaller wrote:
> Hi Andrey,
> ah that is fine to learn about another project that needs some solution (however it will look like).
> 
> Am 15.01.2016 um 16:43 schrieb Andrey Vostrikov <andrey.vostrikov@...entembedded.com>:
> 
>> Hi Nikolaus,
>>
>> H. Nikolaus Schaller wrote:
>>> And IMHO nobody has described that he/she needs a solution to model the*data*  relationship
>>> for devices connected behind a tty port.
>>
>> I am not sure if my case fits *data* relationship or not in this case. Some time ago I asked about state of your patches.
>> In my case I have supervising microcontroller unit (MCU) that is connected to one of UARTs on SoC.
>>
>> This MCU implements several functions that will be implemented as MFD driver:
>> - watchdog and system reset
>> - NVMEM EEPROM
>> - HWMON sensors
>> - Input/power button
>> - and similar low level functions
>>
>> So in my case DTS binding looks like:
>>
>> &uart3 {
>> 	mcu {
>> 		line-speed = <baud rate>;
>> 		watchdog {
>> 			timeout = <ms>;
>> 			...other params...
>> 		};
>> 		eeprom {
>> 			#address-cells
>> 			#size-cells
>> 			cell1 : cell@1 {
>> 				reg = <1 2>;
>> 			};
>> 			cell2 : cell@2 {
>> 				reg = <2 1>;
>> 			};
>> 		};
>> 		hwmon {
>> 			sensors-list = "voltage", "current", etc...;
>> 		}
>> 	}
>> }
> 
> With my proposal it would just become
> 
> / {
> 	themcu: mcu {
> 		uart = <&uart3>;
> 		line-speed = <baud rate>;
> 		watchdog {
> 			timeout = <ms>;
> 			...other params...
> 		};
> 		eeprom {
> 			#address-cells
> 			#size-cells
> 			cell1 : cell@1 {
> 				reg = <1 2>;
> 			};
> 			cell2 : cell@2 {
> 				reg = <2 1>;
> 			};
> 		};
> 		hwmon {
> 			sensors-list = "voltage", "current", etc...;
> 		}
> 	}
> };
> 
> Which is almost the same. Except that it allows to move your mcu node whereever you like and easily allows to change the interface to connect to a different device by
> 
> &themcu {
> 	uart = <&uart1>;
> };
> 
> With the subnode style you would need some tricks to get the driver instance for uart3 disabled, although it is possible (everything is possible - just easier or more difficult).
> 
>>
>> This MCU receives commands and notifies MFD driver about events via UART protocol.
>> It looks like not really a slave though, more like a partnership from data flow point of view.
> 
> Yes!. That is why I started to question the term "slave".
> 
> And yes, this is the second use case I am aware of: a device that just *uses" the UART to do its works and there is no /dev/tty involved.
> 
>>
>> There is no user space code involved in this case as whole interactions are between drivers (just a kick to open /dev/ttyXXX using sys_open, as there is no way to start probe on uart_slave bus and assign line discipline).
> 
> Exactly this is what we want to provide as API for the drivers by our patches to serial-core.c. 
> 
> We want to allow such a "partner" device to take a line-speed property e.g. from its DT node (or a 9600 constant as for our GPS chip) and ask the UART driver to set the required clocks. Or to get the driver notified that someone has opened the /dev/tty* etc. So make it possible to use some UART from another driver.
> 
> In the long run it should be possible to use the UART even if there is no /dev/tty client or interface in user-space but that is something not perfectly working (there is some initialization race in the tty/serial subsystem we have not yet understood).
> 
> As you see, I have a driver-specific standpoint (and not coming from user space).
> 
> Thanks for sharing this example.


I'd like to see the exemplar slave driver be something more complicated than
trivial on-off, before hacking in junk into the serial core.

As it stands, this gps could be supported on any uart driver that implements
mctrl gpios (which is trivial with the serial mctrl gpio helpers).

Not that I'm against uart slave device support, just that I don't think hacks
is the way to go about it.

What I'd like to see is a split of the serial core into a tty driver and a
standalone device abstraction. Anything else is just workarounds.

Regards,
Peter Hurley