linux-kernel - Re: [PATCH] USB: add USB test and measurement class driver

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Message-Id: <200808282329.34780.oliver@neukum.org>
Date:	Thu, 28 Aug 2008 23:29:33 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Greg KH <greg@...ah.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	USB list <linux-usb@...r.kernel.org>,
	Stefan Kopp <stefan_kopp@...lent.com>,
	Marcel Janssen <korgull@...e.nl>,
	Felipe Balbi <me@...ipebalbi.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] USB: add USB test and measurement class driver - round 2


> > > +		buffer[4] = (this_part - 12 - 3) & 255;
> > > +		buffer[5] = ((this_part - 12 - 3) >> 8) & 255;
> > > +		buffer[6] = ((this_part - 12 - 3) >> 16) & 255;
> > > +		buffer[7] = ((this_part - 12 - 3) >> 24) & 255;
> > 
> > We have excellent endianness conversion macros.
> 
> For splitting values up into the individual byte portions?  I think this
> is far more obvious as to exactly what is going on, don't you?

No. Kernel code does not exist to show how to do endianness conversion.
We have clearly labeled functions.

> > > +		buffer[8] = data->TermCharEnabled * 2;
> > > +		/* Use term character? */
> > 
> > smp_rmb(); /* we must make sure we don't read a stale terminator */
> 
> I'm not going to worry about races here, that's not a real issue.

There is no such thing as an ignorable race. On second thought
you can take the mutex in the sysfs handlers. That will also do the job.


> > This and usbtmc_read() need a test for disconnection. Open() and disconnect
> > are guarded in usbcore, read & write are not. By reference count you've made
> > sure you have a valid device descriptor, but the device may have been reprobed.
> 
> If so, then struct usb_device would be different, right?  Oh, I see,
> disconnect() using usbfs/sysfs.  Bah, is it really something that
> happens in the real world?  Oh well, I'll go fix this...

This is oopsable from user space.

> > If you ignore an error return, be open about it.
> 
> I'm not?  Should I print an error and then just continue on?  Would that
> be sufficient?

Yes.
 
> > > +static void usbtmc_disconnect(struct usb_interface *intf)
> > > +{
> > > +	struct usbtmc_device_data *data;
> > > +
> > > +	dev_dbg(&intf->dev, "usbtmc_disconnect called\n");
> > > +
> > 
> > You must set a flag for read, write and ioctl.
> 
> Will do.  Then I need to lock the flag with a mutex, right?

Yes. You can set the intf pointer to NULL. That's sort of idiomatic.
And you should NULL intfdata.


	Regards
		Oliver


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