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

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Date:	Wed, 26 Jun 2013 10:32:09 +0200
From:	Oliver Schinagl <oliver+list@...inagl.nl>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Maxime Ripard <maxime.ripard@...e-electrons.com>, arnd@...db.de,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	andy.shevchenko@...il.com, linux@....linux.org.uk,
	linus.walleij@...aro.org, linux-sunxi@...glegroups.com,
	Oliver Schinagl <oliver@...inagl.nl>
Subject: Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

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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