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

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Message-Id: <6A3D5EFF-AC8B-4039-AFF8-652687EB8EDA@holtmann.org>
Date:	Thu, 18 Aug 2016 13:01:24 +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.

Regards

Marcel