Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

[Oliver Schinagl]

On 24-06-13 23:46, Greg KH wrote:
> On Mon, Jun 24, 2013 at 11:21:16PM +0200, Oliver Schinagl wrote:
> > On 06/24/13 20:15, Greg KH wrote:
> > > On Mon, Jun 24, 2013 at 07:11:35PM +0200, Oliver Schinagl wrote:
> > > > Hey Greg,
> > > > On 06/24/13 18:04, Greg KH wrote:
> > > > > On Mon, Jun 24, 2013 at 11:29:42AM +0200, Maxime Ripard wrote:
> > > > > > Hi Greg,
> > > > > >
> > > > > > On Mon, Jun 17, 2013 at 03:58:47PM -0700, Greg KH wrote:
> > > > > > > On Mon, Jun 17, 2013 at 10:59:37PM +0200, Oliver Schinagl wrote:
> > > > > >
> > > > > > [..]
> > > > > >
> > > > > > > > +static int __init sunxi_sid_probe(struct platform_device *pdev)
> > > > > > > > +{
> > > > > > > > +	u8 entropy[SID_SIZE];
> > > > > > > > +	unsigned int i;
> > > > > > > > +	struct resource *res;
> > > > > > > > +	void __iomem *sid_reg_base;
> > > > > > > > +	int ret;
> > > > > > > > +
> > > > > > > > +	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > > > > > > +	sid_reg_base = devm_ioremap_resource(&pdev->dev, res);
> > > > > > > > +	if (IS_ERR(sid_reg_base))
> > > > > > > > +		return PTR_ERR(sid_reg_base);
> > > > > > > > +	platform_set_drvdata(pdev, sid_reg_base);
> > > > > > > > +
> > > > > > > > +	ret = device_create_bin_file(&pdev->dev, &sid_bin_attr);
> > > > > > > > +	if (ret)
> > > > > > > > +		return ret;
> > > > > > >
> > > > > > > You just raced with userspace, having the file show up after the device
> > > > > > > was announced to users that it was there.  Please use the proper device
> > > > > > > file api to add default attributes to prevent this from happening.
> > > > > >
> > > > > > Sorry if the question looks dumb, but what kind of race can we generate
> > > > > > here?
> > > > >
> > > > > Userspace gets told about the device from the driver core, udev runs and
> > > > > reads all of the attributes, then your probe function comes along and
> > > > > adds a new attribute.  Userspace will then not know about it at all.
> > > > >
> > > > > > The device_create_bin_file is the last call that we make (if we except
> > > > > > the entropy stuff, but it doesn't really matter here), so after we
> > > > > > created the file, we have everything properly initialised so that our
> > > > > > functions can be called, right?
> > > > > >
> > > > > > And another dumb question for you, what is the "proper device file API"
> > > > > > you are referring to ? :)
> > > > >
> > > > > Please read Documentation/driver_model/device.txt and see the section on
> > > > > Attributes for what to do.  If you have specific questions after reading
> > > > > that, please let me know.
> > > > Since Maxime kinda asked for me, I hope you don't mind me following up.
> > > >
> > > > That doc doesn't mention the binary interface at all. Initially I
> > > > had both devices up, the 'read' device as a textual representation
> > > > and added the binary one later. Maxime and I decided the binary one
> > > > made more sense, as the only textual user would be a human and they
> > > > don't poke that entry that often.
> > > >
> > > > So what default way exists for binary files or how would that be solved?
> > >
> > > The same interface should work just fine for binary files, have you
> > > tried it?
> > I'll just take the plunge and make myself look stupid ;)
> >
> > I tried to change things around, used DEVICE_ATTR(eeprom, S_IRUGO,
> > sid_read, NULL); So far so good I'd hope.
>
> Ick, no.
>
> > Of course now I'll have to change the function's parameters from
> >
> > static ssize_t sid_read(struct file *fd, struct kobject *kobj,
> > 			struct bin_attribute *attr, char *buf,
> > 			loff_t pos, size_t size)
> >
> > to
> >
> > static ssize_t sid_read(struct device *dev,
> > 			struct device_attribute *attr, char *buf)
>
> Which is what do you do not want, as you find out:
>
> > But now, I'm missing things like 'pos' and 'size', both which
> > determine the requested bytes. True, in this specific driver we are
> > talking about 'only' 16 bytes, but what if it weren't but a few MiB
> > and in sysfs we want to read some random byte, will we have to put
> > the entire blok into the buffer?
> >
> > So sorry for not understanding, but ... I don't understand :)
>
> Stick with a binary attribute, and attach that to the proper class
> structure and all should be fine.
>
> Ah crap, you're using a platform device.
>
> {sigh}
>
> Why?  Why not use a "real" device which has a "real" class, and then use
> the interfaces there?
Because, as I was told, this really is a platform device. If you have 
some example code I can look at and learn from that would be awesome. 
I'm still learning after all, and apparently I'm doing it wrong now :)
> greg k-h

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