Date:	Fri, 15 Jan 2016 09:43:39 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	"H. Nikolaus Schaller" <hns@...delico.com>
Cc:	Andrey Vostrikov <andrey.vostrikov@...entembedded.com>,
	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 09:32 AM, H. Nikolaus Schaller wrote:
> Hi Peter,
> 
> Am 15.01.2016 um 18:16 schrieb Peter Hurley <peter@...leysoftware.com>:
> 
>> 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).
> 
> in the GPS case basic mctrl is not enough because the "partner" driver must get meta-data
> that there is data activity. This is something mctrl can't provide.

A binary state is hardly "meta-data". What is the purpose of the rx notification?


> And the GPS chip does not need a simple gpio state to power on/off but an on/off toggle impulse.

Genericity. If this chip needs such a state mechanism, then that should be reflected
generically in gpio support, and we're back to trivial mctrl.


> In our case there are no mctrl gpios (omap) but part of our driver proposal is just to
> forward changes of the mctrl bits to the partner driver.

Please feel free to submit patches for mctrl gpios for the omap-serial driver.


>> 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.

I think you misunderstand what I mean by "standalone device abstraction"; let me
be clearer: "standalone UART device abstraction".

Regards,
Peter Hurley

> Here (was rebased from what I had submitted to LKML a while ago):
> 
> 1. serial core (two patches add API for any such partner drivers)
> 
> http://git.goldelico.com/?p=gta04-kernel.git;a=commit;h=c75ab51483e56afe08f56de104b5ed3fa1d6b0e8
> http://git.goldelico.com/?p=gta04-kernel.git;a=commit;h=f910d951fcf816fce3261814d7f8c46ac6b35e68
> 
> 2. standalone driver example (using the new API)
> 
> http://git.goldelico.com/?p=gta04-kernel.git;a=commit;h=4fd1dbd4e915d741dddd264d6f87396e72351b3a
> 
> BR and thanks,
> Nikolaus
> 

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