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

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Date:	Thu, 18 Aug 2016 13:51:41 +0200
From:	Marcel Holtmann <marcel@...tmann.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	"H. Nikolaus Schaller" <hns@...delico.com>,
	Rob Herring <robh@...nel.org>, Jiri Slaby <jslaby@...e.com>,
	Sebastian Reichel <sre@...nel.org>,
	Pavel Machek <pavel@....cz>,
	Peter Hurley <peter@...leysoftware.com>,
	NeilBrown <neil@...wn.name>, Arnd Bergmann <arnd@...db.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	"open list:BLUETOOTH DRIVERS" <linux-bluetooth@...r.kernel.org>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] UART slave device bus

Hi Greg,

>>>>>> Currently, devices attached via a UART are not well supported in the
>>>>>> kernel. The problem is the device support is done in tty line disciplines,
>>>>>> various platform drivers to handle some sideband, and in userspace with
>>>>>> utilities such as hciattach.
>>>>>> 
>>>>>> There have been several attempts to improve support, but they suffer from
>>>>>> still being tied into the tty layer and/or abusing the platform bus. This
>>>>>> is a prototype to show creating a proper UART bus for UART devices. It is
>>>>>> tied into the serial core (really struct uart_port) below the tty layer
>>>>>> in order to use existing serial drivers.
>>>>>> 
>>>>>> This is functional with minimal testing using the loopback driver and
>>>>>> pl011 (w/o DMA) UART under QEMU (modified to add a DT node for the slave
>>>>>> device). It still needs lots of work and polish.
>>>>>> 
>>>>>> TODOs:
>>>>>> - Figure out the port locking. mutex plus spinlock plus refcounting? I'm
>>>>>> hoping all that complexity is from the tty layer and not needed here.
>>>>>> - Split out the controller for uart_ports into separate driver. Do we see
>>>>>> a need for controller drivers that are not standard serial drivers?
>>>>>> - Implement/test the removal paths
>>>>>> - Fix the receive callbacks for more than character at a time (i.e. DMA)
>>>>>> - Need better receive buffering than just a simple circular buffer or
>>>>>> perhaps a different receive interface (e.g. direct to client buffer)?
>>>>>> - Test with other UART drivers
>>>>>> - Convert a real driver/line discipline over to UART bus.
>>>>>> 
>>>>>> Before I spend more time on this, I'm looking mainly for feedback on the
>>>>>> general direction and structure (the interface with the existing serial
>>>>>> drivers in particular).
>>>>> 
>>>>> Some quick comments (can't do any real life tests in the next weeks) from my (biased) view:
>>>>> 
>>>>> * tieing the solution into uart_port is the same as we had done. The difference seems to
>>>>> me that you completely bypass serial_core (and tty) while we want to integrate it with standard tty operation.
>>>>> 
>>>>> We have tapped the tty layer only because it can not be 100% avoided if we use serial_core.
>>>>> 
>>>>> * one feedback I had received was that there may be uart device drivers not using serial_core. I am not sure if your approach addresses that.
>>>>> 
>>>>> * what I don't see is how we can implement our GPS device power control driver:
>>>>> - the device should still present itself as a tty device (so that cat /dev/ttyO1 reports NMEA records) and should
>>>>>  not be completely hidden from user space or represented by a new interface type invented just for this device
>>>>>  (while the majority of other GPS receivers are still simple tty devices).
>>>>> - how we can detect that the device is sending data to the UART while no user space process has the uart port open
>>>>>  i.e. when does the driver know when to start/stop the UART.
>>>> 
>>>> I am actually not convinced that GPS should be represented as
>>>> /dev/ttyS0 or similar TTY. It think they deserve their own driver
>>>> exposing them as simple character devices. That way we can have a
>>>> proper DEVTYPE and userspace can find them correctly. We can also
>>>> annotate them if needed for special settings.
>>> 
>>> I would _love_ to see that happen, but what about the GPS line
>>> discipline that we have today?  How would that match up with a char
>>> device driver?
>> 
>> we have a GPS line discipline? What is that one doing? As far as I
>> know all GPS implementations are fully userspace.
> 
> Hm, for some reason I thought that was what n_gsm.c was being used for,
> but I could be wrong, I've never seen the hardware that uses that
> code...

the n_gsm.c is for 3GPP TS 07.10. Which is a TTY multiplexer. Has nothing to do with GPS.

Regards

Marcel