[Letux-kernel] [RFC 0/3] misc: new serdev based drivers for w2sg00x4 GPS module and w2cbw003 wifi/bluetooth

H. Nikolaus Schaller hns at goldelico.com
Tue May 23 14:48:03 CEST 2017

Hi Rob,

> Am 23.05.2017 um 14:28 schrieb Rob Herring <robh+dt at kernel.org>:
> On Tue, May 23, 2017 at 12:43 AM, H. Nikolaus Schaller
> <hns at goldelico.com> wrote:
>> Hi Rob,
>>> Am 23.05.2017 um 04:26 schrieb Rob Herring <robh+dt at kernel.org>:
>>> On Sun, May 21, 2017 at 5:44 AM, H. Nikolaus Schaller <hns at goldelico.com> wrote:
>>>> Since our proposed API was not acceptable and the new serdev API has arrived in 4.11 kernels,
>>>> we finally took the challenge to update the w2sg and w2cbw drivers to use the serdev API.
>>>> The approach is to write a "man in the middle" driver which is on one side a serdev client
>>>> which directly controls the UART where the device is connected to and on the other side
>>>> presents a new tty port so that user-space software can talk to the chips as if they would
>>>> directly talk to the UART of the SoC (e.g. ttyO1). This is similar to connecting to a remote
>>>> serial device e.g. through USB (ttyACM) or Bluetooth UART profiles.
>>>> For example gpsd or hciattach expect a /dev/tty they can control (flow control, baud rate
>>>> etc.).
>>> I understand from the prior discussion why you want to pass the data
>>> thru for gps, but why do you need to do that for BT?
>> Because we otherwise can't turn on power when /dev/ttyBT0 is opened and turn off when it
>> is closed. I.e. it should not be powered unless someone does a hciattach /dev/ttyBT0. And it
>> should be turned off by a killall hciattach.
> Still, you can do power control within BT HCI drivers.

We do not use any driver for bluetooth. We just start hciattach on demand.
And afaik there is no plugin mechanism for adding power control to hciattach.

Or do you have a link to what you think about?

> You wouldn't be
> limited to just open/close, but can handle suspend/resume as well.

Well, it does not look as if we need more than open/close since suspend/resume
is already handled by the regulator driver. We just need to keep it powered off
if there is no user-space client.

>> Basically we would like to have a power control automatic like it exists for many other devices.
>> Since the BT chip is described as a serdev by DT, we see no other means than to pass data
>> through the serdev driver.
> We could have a blacklist if we need to have serdev not create a
> device and create a tty device instead.

Interesting idea.

>> We had looked into the line discipline approach but it makes a lot of problems. The first one
>> is that registering a new system-wide ldesc number is required. Next we do not see how to make
>> a serdev driver (as it seems to be required by the DT) to register a different ldesc.
>>>> Here is the result of our first hack which is working as a demo on GTA04 devices (and the
>>>> w2cbw driver can also be used to control a GTA04 variant with WL1837).
>>>> Since it is just a demo hack, the code is not yet cleaned up, nor does it completely pass
>>>> check-patch, nor follows 100% the coding styles. And certainly has some bugs.
>>>> The most significant issue is that calling tty_port_register_device() inside of the
>>>> serdev probe() function makes the serdev probe() function to be entered a second
>>>> time. This does not lead to big problems since we currently have minor = 0
>>>> and this makes the second call assume the device is not available.
>>>> But we have no idea why this happens and how it can be prevented.
>>> Johan's fixes may help there, but it is intended to be temporary to
>>> have a separate API for registering tty ports with or without serdev.
>> Ah, would that mean something like a tty_port_register_device_without_serdev()?
> Yes, but other way around. The old function doesn't register with
> serdev and there's a new function that will.

Ah, ok.

>> Do you have a reference to his fixes?
> They are in Greg's tty-linus branch if not Linus' tree now.

Have found them. Not yet in Linus' tree but likely in one of the next rc

Looks good. This would mean that this problem will simply go away soon.

BR and thanks,

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