linux-kernel - Re: RFC: UIO null parent for __uio_register_device and uio_device

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Date:	Wed, 19 Jan 2011 08:30:05 -0800
From:	Greg KH <greg@...ah.com>
To:	Earl Chew <echew@...acom.com>
Cc:	"Hans J. Koch" <hjk@...sjkoch.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Hans J. Koch" <hjk@...utronix.de>
Subject: Re: RFC: UIO  null parent for __uio_register_device and
 uio_device_name()

On Wed, Jan 19, 2011 at 07:42:12AM -0800, Earl Chew wrote:
> Hans,
> 
> Thanks for the quick response.
> 
> >> o Allow a null parent during uio_register_device. We've had
> >>   situations where there was no convenient parent. We could
> >>   concoct a parent, but it seemed to make sense to allow
> >>   for a null.
> > 
> > Hmm. I saw many UIO drivers over the years but we never had that
> > problem. In the probe() function of your UIO driver you get a
> > suitable device as parameter. Why don't you just use it?
> 
> We build custom hardware, and UIO has served us well by providing
> a way out of kernel space.
> 
> [ I'm calling the driver we write the "UIO client driver" to
>   avoid confusion with the Linux UIO driver. ]
> 
> In some implementations, the HW binds directly to the CPU
> physical address space. In particular, it is not accessed via
> PCI space. Thus there is no probe() function required at the
> UIO client driver.
> 
> The physical address assignments for the device are fixed
> at system design time, and the UIO client driver (for the custom hardware)
> has those addresses built into it.
> 
> No probing is required in this scenario.

Great, then create a platform device for your "hardware device" and you
should be fine, right?

> > Yes, we intentionally defined struct uio_device in uio.c and not in
> > a header file to prevent driver authors from using it ;-)
> 
> Yes. This is a good pattern. I like using it too  ;-)
> 
> >> +const char *uio_device_name(struct uio_info *info)
> >> +{
> >> +       return dev_name(info->uio_dev->dev);
> > 
> > No NULL checks? Note that info->uio_dev can be NULL if uio_register_device()
> > failed...
> 
> Apologies. I've tightened up the code.  See below.
> 
> > The function above returns strings like "uio0", "uio1", and so on. What's
> > the value of that? To identify a certain driver or distinguish several
> > instances of it, the "name" member of struct uio_info should be used.
> > 
> > But maybe I just miss the point. What's your use case?
> 
> This is how our code currently works.
> 
> 1. UIO client driver registers with Linux UIO driver.
> 2. UIO client driver advertises the name of the Linux UIO driver
>    instance to which it is bound (eg "uio2") to our custom
>    application
> 3. Our custom application queries for the name of the driver
>    to use (eg "uio2")
> 4. Our custom application issues open("/dev/uio2") to access hardware.
> 
> Working on the implementation of our new system last night I realise
> that our new configuration runs very lean, and I now do not have the
> use of udev to populate /dev/uio[0-9].

Then what populates your other device nodes?

> This new configuration runs so lean that I don't have access to sysfs.

I really find this hard to believe.  Why would you not configure sysfs
in your system?

> I've asked about including sysfs, but there are good reasons not to
> include it.

What are they?

> This means that /sys/class/uio[0-9]/... will not be available.

Nor will lots of other things that you will end up needing.

> The result is that I need to figure out the major/minor device number in
> order to access the Linux UIO device. I will advertise this through /proc/
> entries specific to the UIO client driver. We can then use this information
> to mknod /dev/uio[0-9].

Heh, you have /proc/ but not /sys/?  Someone needs to move out of the
90's :)

> I've added uio_device_chrdev() alongside uio_device_name() to allow
> us to figure out the coordinates of the Linux UIO device.

Ick, no, please use the sysfs files that are alredy present for such a
thing, don't reinvent the wheel, I will not accept such a change.

thanks,

greg k-h
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