Date: Wed, 26 Jun 2013 10:51:44 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Oliver Schinagl <oliver+list@schinagl.nl>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>, arnd@arndb.de,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        andy.shevchenko@gmail.com, linux@arm.linux.org.uk,
        linus.walleij@linaro.org, linux-sunxi@googlegroups.com,
        Oliver Schinagl <oliver@schinagl.nl>
Subject: Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses
Message-ID: <20130626175144.GC2222@kroah.com>
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 <20130624092942.GG26008@lukather>
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 <51C87DC7.50005@schinagl.nl>
 <20130624181509.GA8847@kroah.com>
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On Wed, Jun 26, 2013 at 10:32:09AM +0200, Oliver Schinagl wrote:
> 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 :)

I was wrong, you can do this with a platform device just fine.  Set the
"groups" field in your platform device->device structure, and all will
work properly, right?

thanks,

greg k-h
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