linux-kernel - Re: [PATCH] add new NRP power meter USB device driver

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Date:	Thu, 31 May 2012 11:21:40 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	linux-kernel@...r.kernel.org, greg@...ah.com,
	gregkh@...uxfoundation.org,
	thomas.braunstorfinger@...de-schwarz.com
Subject: Re: [PATCH] add new NRP power meter USB device driver

On Thu, 2012-05-31 at 10:20 +0200, Oliver Neukum wrote:

> > > 
> > > > +		if (arg) {
> > > > +			ret = wait_event_interruptible_timeout(
> > > > +				dev->out_running.wait,
> > > > +				list_empty(&dev->out_running.urb_list),
> > > > +				msecs_to_jiffies(arg));
> > > > +			if (!ret)
> > > > +				return -ETIMEDOUT;
> > > > +			if (ret < 0)
> > > > +				return ret;
> > > > +			return 0;
> > > > +		} else {
> > > > +			return wait_event_interruptible(
> > > > +				dev->out_running.wait,
> > > > +				list_empty(&dev->out_running.urb_list));
> > > > +		}
> > > > +		break;
> > > 
> > > This is very ugly. If you need fsync(), then implement it.
> > > 
> > 
> > fsync() did not meat the requirements, since i need in some case a
> > timeout for the device. poll() will also not help, since it signals only
> > that there is space to write.
> 
> Well, then implement fsync() with interruptible sleep and use a timer
> in user space.
> 

But this will not solve the problem of older software which is still
depending on this ioctl.


> Yes, but this seems to be buggy:
> 
> +	ret = usb_submit_urb(urb, GFP_KERNEL);
> +	if (ret) {
> +		usb_unanchor_urb(urb);
> +		urb_list_add_tail(&dev->read_lock, urb, &dev->in_avail);
> +		nrpz_err("Failed submitting read urb (error %d)", ret);
> +	}
> 
> You have already transfered the data to user space. It seems to me that you
> need to zero out the URB and need to handle the case of getting an URB
> without data.
> 

Okay, i understand what you mean. Zeroing out is not necessary since
usb_submit_urb will set urb->status to -EINPROGRESS. This behavior is
well documented.

I checked it again, and it will work perfectly in case of an error.

And in my opinion it is safe to reuse an urb without reinitialization,
this is a common practice in the callback handlers, so i see no reason
why this should not work in the read() function.

> > > > +static int nrpz_pre_reset(struct usb_interface *intf)
> > > > +{
> > > > +	struct usb_nrpz *dev = usb_get_intfdata(intf);
> > > > +
> > > > +	if (dev)
> > > > +		nrpz_draw_down(dev);
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static int nrpz_post_reset(struct usb_interface *intf)
> > > > +{
> > > > +	return 0;
> > > > +}
> > > 
> > > And you don't tell user space that the device has been reset?
> > > And what restarts reading?
> > > 
> > 
> > I have no idea how to do this. But i will have a look in the kernel
> > source and figure it out.
> 
> There probably is no generic answer. But I presume a reset will
> reinit the device and destroy anything you set up before, so I guess
> the next read() or write() after a reset has to return an error code that
> tells user space that it has to redo its setup.
> 

Is it okay to kick out the whole ..._reset() thing, since i have no idea
what it is good for.

Greetings,
Stefani


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