linux-kernel - Re: [PATCHv3 1/1] [RFC] uartclk from serial

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Date:	Fri, 17 Aug 2012 22:25:52 +0200
From:	Tomas Hlavacek <tmshlvck@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Marek Vasut <marek.vasut@...il.com>, alan@...ux.intel.com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 1/1] [RFC] uartclk from serial_core exposed to sysfs

Hello!

On Fri, Aug 17, 2012 at 9:01 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Fri, Aug 17, 2012 at 08:44:14PM +0200, Marek Vasut wrote:
>> Dear Greg KH,
>>
>> > On Fri, Aug 17, 2012 at 06:30:36PM +0200, Tomas Hlavacek wrote:
>> > > Hello Greg!
>> > >
>> > > On Fri, Aug 17, 2012 at 5:06 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> > > >> @@ -2355,6 +2373,14 @@ int uart_add_one_port(struct uart_driver *drv,
>> > > >> struct uart_port *uport)
>> > > >>
>> > > >>       }
>> > > >>
>> > > >>       /*
>> > > >>
>> > > >> +      * Expose uartclk in sysfs. Use driverdata of the tty device for
>> > > >> +      * referencing the UART port.
>> > > >> +      */
>> > > >> +     dev_set_drvdata(tty_dev, port);
>> > > >> +     if (device_create_file(tty_dev, &dev_attr_uartclk) < 0)
>> > > >> +             dev_err(tty_dev, "Failed to add uartclk attr\n");
>> > > >
>> > > > I think you just raced with userspace in creating the file after the
>> > > > device was announced to userspace.  Are you sure it's ok?
>> > > >
>> > > > If not (hint, I don't think so), please make it a default attribute of
>> > > > the device, which will then cause the file to be created before it is
>> > > > announced to userspace.  It will also be less code as you don't have to
>> > > > clean it up by hand :)
>> > >
>> > > Do you mean I should modify the tty_register_device() function not to
>> > > use device_create() but it should rather do the device initialization
>> > > on it's own.
>> >
>> > No, not at all.
>> >
>> > > And I should add add the attribute (via struct attribute_group) to
>> > > struct device in between device_initialize() and device_add() calls.
>> > > Did I get it right?
>> >
>> > No, make this a driver attribute, that way when the device is
>> > registered, it adds the attribute automagically to the device that is
>> > bound to it.
>>
>> (hint, DEVICE_ATTR), right ?
>
> No, that's just a macro that creates the structure for the attribute.
> You need to take that structure and tie it to the driver itself, using
> the struct device_driver->groups; field.

Please forgive me my ignorance, but I am lost in this. I tried to read
through the serial_core and tty_io.c over and over to figure out how
the drivers are registered and where could I hook the driver
initialization for all UART ports, but I do not get it.

And there is another thing I am confused of: Should I use macro
DEVICE_ATTR or DRIVER_ATTR? There is a different signature of
callbacks in case of DRIVER_ATTR and I do not know how to find out
from struct device_driver which particular UART port it is associated
to? (Is it indeed associated with one particular port?) Value uartclk
could be, AFAIK, different on each port of the same chip and therefore
I need to know which particular TTY device I want to operate on. More
precisely, I have to get to proper struct uart_state associated with
the particular port.

Thanks,
Tomas

-- 
Tomáš Hlaváček <tmshlvck@...il.com>
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