linux-kernel - Re: [PATCH RFC 02/11 v4] gpio: Add sysfs support to block GPIO API

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Date:	Wed, 17 Oct 2012 10:39:25 +0200
From:	Roland Stigge <stigge@...com.de>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	grant.likely@...retlab.ca, linus.walleij@...aro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	w.sang@...gutronix.de, jbe@...gutronix.de, plagnioj@...osoft.com,
	highguy@...il.com, broonie@...nsource.wolfsonmicro.com,
	daniel-gl@....net, rmallon@...il.com
Subject: Re: [PATCH RFC 02/11 v4] gpio: Add sysfs support to block GPIO API

On 10/16/2012 06:43 PM, Greg KH wrote:
> On Tue, Oct 16, 2012 at 02:53:45PM +0200, Roland Stigge wrote:
>> On 10/16/2012 01:57 AM, Greg KH wrote:
>>> On Tue, Oct 16, 2012 at 01:31:18AM +0200, Roland Stigge wrote:
>>>> +int gpio_block_export(struct gpio_block *block)
>>>> +{
>>>> +	int		status;
>>>> +	struct device	*dev;
>>>> +
>>>> +	/* can't export until sysfs is available ... */
>>>> +	if (!gpio_class.p) {
>>>> +		pr_debug("%s: called too early!\n", __func__);
>>>> +		return -ENOENT;
>>>> +	}
>>>> +
>>>> +	mutex_lock(&sysfs_lock);
>>>> +	dev = device_create(&gpio_class, NULL, MKDEV(0, 0), block,
>>>> +			    block->name);
>>>> +	if (!IS_ERR(dev))
>>>> +		status = sysfs_create_group(&dev->kobj, &gpio_block_attr_group);
>>>> +	else
>>>> +		status = PTR_ERR(dev);
>>>> +	mutex_unlock(&sysfs_lock);
>>>
>>> You just raced with userspace telling it that the device was present,
>>> yet the attributes are not there.  Don't do that, use the default class
>>> attributes for the class and then the driver core will create them
>>> automagically without needing to this "by hand" at all.
>>
>> I guess you mean class attributes like gpio_class_attrs[] of gpio_class?
> 
> Yes.
> 
>> Aren't class attributes specific to a class only (i.e. only one
>> attribute at the root for all devices)? What I needed above are
>> attributes for the block itself (of which there can be several). So we
>> need device attributes for each block, not class attributes here.
> 
> Yes, that is what the dev_attrs field in 'struct class' is for.

That's what I was missing. Will update.

Thanks,

Roland
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