Re: proper struct device selection for dev_printk()

[Dmitry Torokhov]

On Thu, May 03, 2012 at 09:09:37AM -0700, Greg KH wrote:
> Hi Kay,
> 
> I've been working on removing the old err() and dbg() functions in usb.h
> that have been there since the 2.2 kernel and replace them with calls to
> dev_err() and dev_dbg(), as that's what we want to have, especially with
> your dev_printk() reworks.
> 
> In some recent changes in the input drivers, Dmitry noted that I was
> picking the "wrong" struct device to pass to these functions.  I was
> using the "farthest down the tree" struct device that I could get to, in
> the USB input driver's case, the struct device for the input device, a
> "class" device.
> 
> But that seems to produce an output that is less than helpful.  Dmitry
> used this as an example output to show this for a serio device:
> 	dev_warn(&input_dev->dev, "warning using input device\n");
> 	dev_warn(&serio->dev, "warning using parent serio device\n");
> 
> Produces:
> 	[    1.903608] input input6: warning using input device
> 	[    1.903612] psmouse serio1: warning using parent serio device
> 
> Here it seems that the "one up from the lowest struct device" works
> best.
> 
> So I tried this out with a usb to serial device, and got the following
> results.  With the code:
> 	dev_err(&port->dev, "dev_err port->dev output\n");
> 	dev_err(&serial->dev->dev, "dev_err serial->dev->dev output\n");
> 	dev_err(&serial->interface->dev, "dev_err serial->interface->dev output\n");
> 	dev_err(port->port.tty->dev, "dev_err port->port.tty->dev output\n");
> 
> I get:
> 	[   68.519639] pl2303 ttyUSB0: dev_err port->dev output
> 	[   68.519645] usb 2-1.2: dev_err serial->dev->dev output
> 	[   68.519649] pl2303 2-1.2:1.0: dev_err serial->interface->dev output
> 	[   68.519653] tty ttyUSB0: dev_err port->port.tty->dev output
> 
> All of these "describe" the device being operated on in one fashion or
> the other, as they are struct devices that are easily accessable from
> the driver.
> 
> My question is, what is the "best" thing to be doing here?
> 
> I still think the "lowest" struct device would be best (in this case,
> the last line above from the port->port.tty->dev pointer), but what do
> you think is best for userspace to have here?

My $.02:

In general, drivers should emit messages for the devices they bind to.
This way driver like psmouse (which is serio subsystem driver) will emit
messages using serio port it is bound (or attempting to bind to):

	[    1.903612] psmouse serio1: warning using parent serio device

and drivers like wacom which bind to a USB interface will emit messages
using USB intraface, like:

	[    1.234567] wacom 2-1.2:1.0: some error happened

The benefit of using the device we are binding to is that it allows us
to crearly identify the driver that emits the error and we are using the
same device throughout (leaf device might not have been created yet and
we already need to emit an error or a warning).

Thanks.

-- 
Dmitry
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