[PATCH 2/3] console: Add simplified 'serdev' framework from Linux kernel

[Sascha Hauer]

On Mon, Apr 09, 2018 at 09:40:46AM -0700, Andrey Smirnov wrote:
> On Mon, Apr 2, 2018 at 11:54 PM, Sascha Hauer <s.hauer at pengutronix.de> wrote:
> > Hi Andrey,
> >
> > Some comments inside.
> >
> >
> > On Mon, Mar 26, 2018 at 06:09:14AM -0700, Andrey Smirnov wrote:
> >> Port 'serdev' UART-slave deivce framework found in recent Linux
> >> kernels (post 4.13) in order to be able to port 'serdev' slave drivers
> >> from Linux.
> >>
> >> Signed-off-by: Andrey Smirnov <andrew.smirnov at gmail.com>
> >> @@ -323,6 +324,17 @@ int console_register(struct console_device *newcdev)
> >>               dev->parent = newcdev->dev;
> >>       platform_device_register(dev);
> >>
> >> +     newcdev->open_count = 0;
> >> +
> >> +     /*
> >> +      * If our console deive is a serdev, we skip the creation of
> >
> > s/deive/device/
> 
> Will fix in v2.
> 
> >
> >> +      * corresponding entry in /dev as well as registration in
> >> +      * console_list and just go straigh to populating child
> >
> > s/straigh/straight/
> 
> Ditto.
> 
> >
> >> +      * devices.
> >> +      */
> >> +     if (serdev_node)
> >> +             return of_platform_populate(serdev_node, NULL, dev);
> >
> > How is this going to be used? A serdev driver binds to the serdev_node
> > and then it probably needs to get a pointer to the console device,
> > right? How does the driver accomplish this?
> >
> 
> Serdev slave driver doesn't hold explicit pointer to console device,
> instead accessing it via point to serdev_device. The latter could
> obtained by calling to_serdev_device(dev->parent), where dev is
> device_d given to slave driver's probe function.
> 
> 
> >> +/**
> >> + * struct serdev_device - Basic representation of an serdev device
> >> + *
> >> + * @dev:             Corresponding device
> >> + * @fifo:            Circular buffer used for console draining
> >> + * @buf:             Buffer used to pass Rx data to consumers
> >> + * @poller           Async poller used to poll this serdev
> >> + * @polling_interval:        Async poller periodicity
> >> + * @polling_window:  Duration of a single busy loop poll
> >> + * @receive_buf:     Function called with data received from device;
> >> + *                   returns number of bytes accepted;
> >> + */
> >> +struct serdev_device {
> >> +     struct device_d *dev;
> >> +     struct kfifo *fifo;
> >> +     unsigned char *buf;
> >> +     struct poller_async poller;
> >> +     uint64_t polling_interval;
> >> +     uint64_t polling_window;
> >> +
> >> +     int (*receive_buf)(struct serdev_device *, const unsigned char *,
> >> +                        size_t);
> >> +};
> >> +
> >> +int serdev_device_open(struct serdev_device *);
> >> +unsigned int serdev_device_set_baudrate(struct serdev_device *, unsigned int);
> >> +int serdev_device_write(struct serdev_device *, const unsigned char *,
> >> +                     size_t, unsigned long);
> >
> > So a serdev driver uses serdev_device_write() to send characters out. To
> > receive characters it has to implement serdev_device->receive_buf,
> > right?
> 
> Right. I tried to implement exactly the same API that Linux's serdev
> API provides.
> 
> > What kind of devices did you implement this for?
> 
> I ported serdev in support of porting the driver for RAVE SP which is
> a small microcontroller device found many ZII board including RDU2. It
> implement a whole bunch of various functionality including watchdog,
> parameter EEPROM, sensor access, backlight control, button input event
> generation, etc.
> 
> > For devices which send data without request (GPS?) this seems the way to go. For
> > others a synchronous receive function might be good, no?
> >
> 
> I didn't implement anything like that mostly because Linux serdev API
> doesn't and any ported driver wouldn't have any need for those
> functions. But in general, I am not sure how useful synchronous
> receive function would be. In my experience, devices like that usually
> implement some binary transport protocol with packetization/escape
> sequences on top of UART, which usually requires a state machine
> operating with byte granularity as the data comes in to parse
> responses correctly and synchronous APIs are not extremely useful for
> that kind of a use-case.
> 
> FWIW, since serdev API is integrated into poller infrastructure it is
> pretty trivial to write blocking code with it. Here's how I use it in
> my driver to implement request-response type of a function:
> 
> rave_sp_write(sp, data, data_size);
> /*
> * is_timeout will implicitly poll serdev via poller
> * infrastructure
> */
> while (!is_timeout(start, SECOND) && !reply.received)
>    ;

I understand that synchronous receiving might not be that useful. Given
how simple it is we could add a synchronous receive wrapper function
just for the sake of completeness, even if it only provides an example
how the code can be used.

Sascha

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