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

[H. Nikolaus Schaller]

Hi Alan,

Am 18.01.2016 um 12:19 schrieb One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>:

>> I have not counted how often I have explained that, but I am happy to do it
>> again.
>> 
>> * the GTA04 device is an open hackable smartphone platform where power saving is highest priority
> 
> Yep I did real the manual before replying last time.

That is good to know, because it allows to refer to it if necessary.

> 
>> * the wi2wi,w2sg0084 chip is a GPS chip (it does not understand HCI protocol, it just sends NMEA records if powered on)
>> * we want to provide NMEA data through /dev/ttyO1 (it uses an OMAP UART) because a tty port is the most common interface for GPS devices (e.g. a bluetooth GPS mouse is also presented as a tty)
>> * we want the chip to automatically power up as soon (but not before) as any gps client opens /dev/ttyO1 (or activates the DTR mctrl)
>> * we want the chip to automatically power down if no process uses /dev/ttyO1 any more
>> The standard logic of GPS daemons and applications is to receive
>> NMEA records through some serial /dev/tty.
>> 
>> Please tell me how power on/off management can be done without intercepting
>> somewhere in the kernel that /dev/ttyO1 is opened/closed (which is not the same
>> as suspend/resume).
> 
> Your user space can do it (as most Android does).

How can it do it in automatically in a standardized way without need for daemon support?

> 
>> Secondly, the chip has a very special logic that it may end up in the opposite
>> power state than the kernel driver thinks. Especially after boot it simply can not
>> know the state. The chip might be powered up/send records or might not.
>> 
>> A driver can only detect such a discrepancy if it thinks the GPS chip is powered off,
>> but there is still data coming through the UART.
> 
> To play devils advocate a moment: so can user space.

* how can it be done without permanently running a daemon that monitors RX data (/dev/tty*)
and attaches to some /sysfs control?
* how can the daemon present another /dev/tty so that applications expecting such
an interface can attach to it (maybe through pty - but isn't that an overkill?)
* Who makes sure that this daemon is installed and running right after boot up on *any* Linux system
so that it can always react in case that the chip did start when it should be off?

More generally: what is a kernel good for? Why do we need kernel drivers?

You can do almost everything in user space if you want to: we can expose
every wire to /sys/class/gpio and clever user space daemons can bitbang on
them to implement any protocol.

Well, only in theory: this is too slow and needs too much energy because the CPU
runs at 100%.

Just think about waking up the daemon process if a character is received. This is
much more costly than calling a notification function in the kernel driver which might
have to execute just 4 or 5 assembler instructions to decide what to do.

So kernel drivers are sometimes the best solution and more efficient and controllable
than a user space daemon.

That is why we propose a kernel solution for this problem.

> 
>> Please tell me how this situation can be detected without monitoring the data stream
>> in the kernel going to /dev/ttyO1 - from the UART behind it - even if /dev/ttyO1 is closed.
>> 
>> This are *our* requirements.
> 
> The Linux kernel runs on billions of devices. If we put kernel hooks in
> random glue layers for every weird little platform corner case it would
> collapse in a heap. Those are *our* requirements. Thus it is good to push
> back on stuff that can be done in user space just as well.

I understand that. But what is Linux good for? For it's own sake or for
users and platforms using it? Isn't it that we take it from the community
and contribute improvements back to it?

And it appears that this is not a little platform corner case but there is some
more general need for drivers to access the UART layer and not a higher level
abstraction. The networking stack also has mechanisms to access all layers
because not everything can be done on every layer (some functions of
lower layers are hidden when accessing higher layers - e.g. you can't
get the Ethernet MAC from the TCP/socket layer) and it might be more
performant to directly go to a lower layer.

So to me it appears that such a kernel feature is missing. Therefore we
are discussing it.

> 
>> Other people think that our approach helps to solve their driver architecture as well
>> and have added their requirements on top. This is why I attempt to make the API
>> more general than just for our own use-cases.
> 
> If it's going to be generic then it needs to be dealing with this at the
> tty/tty_port level not uart. uart isn't a general serial abstraction,
> tty_port and tty are.

well I think most driver projects that have expressed that they want to have
such a solution just want an UART abstraction and not a general tty/serial
interface with all bells and whistles.

> uart is just a helper library for some types of
> port.

Yes, this is the type of port, our peer devices are directly connected to.

> In practise that ought to be a small distinction. If you have to bind
> some kind of device logic to the port activation/deactivation then bind
> it to tty_port not uart. uart open/close is basically an implementation
> of the tty port->ops->activate() and port->ops->shutdown() method.

I am not sure if that still exists for UART based tty_ports (but I am not
understanding everything of the tty layer and have not followed recent
changes since I have focussed on the uart_port and serial-core):

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/tty/serial/serial_core.c?id=9e31364fc3272073ec8c5fac18a3e01d4f013418

Independently of that, doing the port open/close could work that way, if
it were the only topic to be solved.

But we need several components to make everything we need work.

All of them need to be present.

Another question if we discuss moving the hooks into the tty_port layer:

is it possible from tty layer to activate the UART without a user-space open()
and keep it activated after a close()?

> 
>> 
>>> 
>>> The 8686 is already working in serial mode  with no kernel hackery on
>>> other boards.
>> 
>> You appear to mix the chips we are talking about. We have the w2sg0084 gps
>> chip and a w2cbw003, which is a combo of an 8686 and a CSR BT serial device.
>> 
>> Both chips need somehow to be powered on or off if not used. Ideally automatically
>> at the moment no user space client is using them any more. For the bluetooth side
>> the moment to power off is when a hciattach is killed.
>> 
>> Other 8686 boards appear not to have such critical power restrictions and then they
>> just leave power on.
> 
> The ones I am familiar with either have the userspace managing it via
> sysfs (which has some latency advantages when doing clever stuff) or
> wired the power control to the carrier signal (or that is declared the
> gpio that controls it to be the carrier).

How many of these special drivers are in mainline? Our target is to get full support
by mainline and not run our own kernel branch forever. Because we are not
yet-another-android-thing-that-just-needs-to-work-for-6-months-and-nobody-cares-
about-updates-and-GPL.

In all mainlining attempts I know, the user-space control approach was rejected
because it was said that the kernel should take care of and not a /sysfs or some
other artificial protocol.

Especially as the information when to power on the chip is already known inside the
kernel and does not need a new side-band control mechanism. Other similar (GPS)
devices don't need it as well. This makes our devices unnecessarily a big exception
on user-side, especially as we have proven that it can be solved inside the kernel.

We just have not found an architecture that is accepted by mainline.

Some years ago all this started with this proposal:
* make the driver expose an gpio
* use mctrl-gpios for the DTR line
* connect them in device tree

http://neil.brown.name/blog/20120724060722

This was rejected because a chip driver is not a gpio controller and virtual gpios
are not gpios. In other words: sysfs managing and its related device tree representation
was rejected for mainlining as well.

But anyway, it would not solve the device RX data stream monitoring problem.
It just could be a solution to know when to turn the chip on or off.

Neil's proposal to monitor the RX line was rejected because it was doing
nasty tricks with switching pinmux states and setting a temporary interrupt
on the UART RX line.

Therefore we now want to monitor the RX line "behind" the UART, i.e. on the
byte stream coming from the UART shift registers.

This did lead to tty/uart slaves concept and everybody wanted it. Now as we
have code proposals it should go back to become a user space daemon...

> 
>> No, the opposite: it can be any open source application, which by principle
>> could be changed in any way. But we can't because we as the hardware
>> platform+kernel developers have no (and don't want to have) control over
>> the the user space. So we have to fit into the standards of the user space.
> 
> Well the standard of user space today *is* managing via sysfs or the
> carrier trick. Take a look at all the Android devices out there - they
> all tackle it that way. Some of them do very aggressive power management
> as you can imagine. But yes that's ugly 8)

And these solutions are not mainline. Or is any of these?

> 
>> Here, we need to solve the problem to power down the chip if NO
>> tty port is open any more.
> 
> port->ops->shutdown in the tty layer.
> 
>>> You can just report EBUSY in your open method. You don't need to touch
>>> the serial core layer. It's quite sufficient to do
>>> 
>>> 	if (busy)
>>> 		return -EBUSY;
>>> 
>>> at the top of your uart open method.
>> 
>> Hm. I am not writing a new UART driver or touch them. I am using the existing
>> ones (omap-serial). They all call this uart_add_one_port() in their probe() function.
> 
> At the moment. But they may not, or they may get folded together.

Are they? If you have a clear plan for changes, we can update our hooks to what
you have planned.

Otherwise we are digging in the dark.

> 
>> If I would modify just omap-serial, people would for sure complain that the solution
>> is not generic enough.
> 
> I would say two things
> 
> 1. If you modify omap-serial it's not that generic, but it doesn't mess
> with library code it shouldn't.

And tomorrow comes someone who connects the same chip to an i.MX.
Next week to a Samsung SoC. Every time we add the same patches
to the device specific UART driver.

> That is better than messing with uart
> layer code. It also solves your problem and localises the solution. That
> to me is a win.


Neil had proposed that a while ago and there was code in the kernel, but it
was removed last year, because it is not general enough and we did not
get the driver into mainline that would have used it: 985bfd54

> 
> 2. If not then hook tty_port_shutdown() and tty_port_open() because those
> are the right abstraction point. Everything in the kernel that is a tty
> is a tty_port.

What is in your view the right abstraction point for a peer device driver to get
notified about rx characters (even if the tty is currently not open)?

> 
>>> The uart layer is part of the tty layer. It's just a glue library to make
>>> writing some tty drivers a bit easier.
>> 
>> Yes, and exactly that is helpful to solve this problem. We attach only to the
>> glue layer and everything is done.
> 
> It's the wrong place - it's not the abstraction.

Maybe we think about different levels of abstractions.

I think on a level of abstraction of all the different UART drivers. Which is sufficient
to access the UART by the peer driver. This is why I call it UART-peer (and
not serial or tty peer).

I understand the peer device driver requirements that were mentioned so far,
that they all will be happy with basic UART functionality (receive characters,
send characters, activate/deactivate, set baud rate).

You want to abstract from UART and add all tty bells and whistles. We need them
for the GPS chip's data stream (because our unknown GPS applications might
use them) - but other low level UART peer drivers do not even need them.

> 
> What I am trying to say is that if you do this generically then add the
> needed method calls into tty_port_open and tty_port_shutdown, make them
> run after port->ops->activate and before port->ops->shutdown so there is
> a sensible ordering if you need to do something to the port itself, and
> also so on open it only runs if port->ops->activate succeeded.
> 
> Something like
> 
>       if (!test_bit(ASYNCB_INITIALIZED, &port->flags)) {
>                clear_bit(TTY_IO_ERROR, &tty->flags);
>                if (port->ops->activate) {
>                        int retval = port->ops->activate(port, tty);
>                        if (retval) {
>                                mutex_unlock(&port->mutex);
>                                return retval;
>                        }
>                }
> 		/* Wake the device if we have one tied to us */
> 		if (port->ops->activate_slave)
> 			port->ops->activate_slave(port, tty);
>                set_bit(ASYNCB_INITIALIZED, &port->flags);
> 	}
> 
> Then all you need is the (possibly device specific) small patches to check
> the device tree for the bindings on init, and if so set the port->ops
> methods according to the binding.

For me it appears to only solves a small part of the whole problem.

1. it must be possible to start the UART before any user-space open()
2. the UART must be kept running as long as the peer driver wants to
monitor rx data
3. we need a hook to monitor: that (and which) data is incoming

Another aspect is that on a physical RS232 the DTR line is usually
used to power on/off a remote device (the DCE). Therefore I prefer
to mimic that in software by intercepting the mctrl changes. This
additionally allows a client to turn off power of the chip through a "standard"
protocol, even while the tty file is kept open.

> 
>>> It's tied deeply to the tty_port
>>> implementation. One day it might even cease to exist replaced by more
>>> generic tty_port helpers.
>> 
>> Is there a concrete plan to change that? Is anyone working on it now?
> 
> Nobody wants to lose the ability to do so or to move stuff around.

Indeed. But why would you loose this ability at all?

> 
>> If not, I would not worry about this, because it is not specific to the problem
>> we want to solve today (to be precise: since 3 years).
> 
> You solve it today you block other things for the next 20 years. The
> kernel has to deal in long terms.

i understand what you mean but I don't think we are blocking anything.

This is a too black&white view for me. Especially since we would probably still
have all the APIs of ca. Linux 1.3 (1996) if things were blocked for 20 years
by any new piece of hardware. I am quite sure that a lot of things has been
completely turned upside down in the past 20 years.

If significant architectural changes are to be done, you can deprecate this
API and ask all users (driver authors) to replace it with something new.
And if they aren't adjusted within some time or no new interface is proposed,
they get removed. The *I* have the problem back on my table and not you :)

Isn't this a standard way to handle such situations?

Things are gradually changed every now and then. And other parts have to
follow. BTW: most of my work to update a production kernel to a new -rc1 is
to adjust non-mainline (or not-yet) drivers to such API changes.

So this is daily life for someone who maintains a working kernel for a specific
device and therefore does not come unexpected.

So it is more you blocking an adequate solution for our devices than we blocking
Linux development...

> 
>>> The tty layer is also an *abstract* concept. There is no real world tie
>>> between physical collections of shift registers that dribble bits to one
>>> another and tty devices in the kernel. You only need a tty driver for
>>> certain types of user interaction.
>> 
>> And we need it to process the GPS data by standard applications and tools.
> 
> Ok - no problem with that, and for what you've explained that bit makes
> sense.
> 
> Alan

BR,
Nikolaus