linux-kernel - Re: [PATCHv3] drivers/misc: Altera active serial implementation

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Message-ID: <4CE10B32.8090705@suse.cz>
Date:	Mon, 15 Nov 2010 11:28:02 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Baruch Siach <baruch@...s.co.il>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Indan Zupancic <indan@....nu>, Greg KH <greg@...ah.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alex Gershgorin <agersh@...bler.ru>
Subject: Re: [PATCHv3] drivers/misc: Altera active serial implementation

On 11/15/2010 11:08 AM, Baruch Siach wrote:
> Hi Jiri,
> 
> Thanks for reviewing. See my response to your comments below.
> 
> On Mon, Nov 15, 2010 at 10:33:37AM +0100, Jiri Slaby wrote:
>> On 11/15/2010 09:23 AM, Baruch Siach wrote:
>>> --- /dev/null
>>> +++ b/drivers/misc/altera_as.c
>>> @@ -0,0 +1,349 @@
>> ...
>>> +struct altera_as_device {
>>> +	unsigned id;
>>> +	struct device *dev;
>>> +	struct miscdevice miscdev;
>>> +	struct mutex open_lock;
>>> +	struct gpio gpios[AS_GPIO_NUM];
>>> +};
>>> +
>>> +/*
>>> + * The only functions updating this array are .probe/.remove, which are
>>> + * serialized. Hence, no locking is needed here.
>>> + */
>>> +static struct {
>>> +	int minor;
>>
>> Why you need minor here? It's in drvdata->miscdev.minor.
> 
> I use the minor field to lookup the drvdata pointer in get_as_dev(), which I 
> use is altera_as_open().  I do this because I have no access to the 'struct 
> device' pointer from the .open method. Do you know a better way?
> 
>>> +	struct altera_as_device *drvdata;
>>> +} altera_as_devs[AS_MAX_DEVS];
>>
>> You don't need the struct at all.
>> static struct altera_as_device *drvdata[AS_MAX_DEVS];
>> should be enough.
> 
> See above.

The answer to you previous question is here. You can just have a global
array of struct altera_as_device.

>>> +static int altera_as_open(struct inode *inode, struct file *file)
>>> +{
>>> +	int ret;
>>> +	struct altera_as_device *drvdata = get_as_dev(iminor(inode));
>>> +
>>> +	if (drvdata == NULL)
>>> +		return -ENODEV;
>>> +	file->private_data = drvdata;
>>> +
>>> +	ret = mutex_trylock(&drvdata->open_lock);
>>> +	if (ret == 0)
>>> +		return -EBUSY;
>>> +
>>> +	ret = gpio_request_array(drvdata->gpios, ARRAY_SIZE(drvdata->gpios));
>>> +	if (ret < 0) {
>>> +		mutex_unlock(&drvdata->open_lock);
>>> +		return ret;
>>> +	}
>>
>> So leaving to userspace with mutex held. This is *wrong*. use
>> test_and_set_bit or alike instead.
> 
> Can you explain why this is wrong? I don't mind doing test_and_set_bit 
> instead, I'm just curious.

The mutex has an owner which it expects to unlock it. For example if you
fork a process which already opened the device, you have a problem. This
is a technical point. Another point is that it's ugly to leave to
userspace with any lock.

>>> +static int __init altera_as_probe(struct platform_device *pdev)
>>> +{
>> ...
>>> +	drvdata->miscdev.minor = MISC_DYNAMIC_MINOR;
>>> +	drvdata->miscdev.name = kasprintf(GFP_KERNEL, "altera_as%d",
>>> +			pdata->id);
>>> +	if (drvdata->miscdev.name == NULL)
>>> +		return -ENOMEM;
>>> +	drvdata->miscdev.fops = &altera_as_fops;
>>> +	if (misc_register(&drvdata->miscdev) < 0) {
>>
>> Hmm, how many devices can be in the system?
> 
> Up to AS_MAX_DEVS, currently 16.
> 
>> This doesn't look like the right way.
> 
> What is the right way then?

Ok, so for that count it definitely deserves its own major to not eat up
misc device space.

>>> +		kfree(drvdata->miscdev.name);
>>> +		return -ENODEV;
>>> +	}
>>> +	altera_as_devs[pdata->id].minor = drvdata->miscdev.minor;
>>> +	altera_as_devs[pdata->id].drvdata = drvdata;
>>
>> So now the device is usable without the mutex and gpios inited?
> 
> I was thinking that the device lock which is being held during the .probe run, 
> prevents the device from being open. After all I can still return -EGOAWAY at 
> this point. Am I wrong?
> 
> If so, I'll reorder this code.

This cannot be done easily. You need to set drvdata prior to minor and
after all the assignments here. The former because in get_as_dev you
test minor and return drvdata. The latter because you use open_lock &
gpios after playing with minor & drvdata.

Technically, it can be done with a use of barriers, but I don't
recommend it as drivers should not use barriers at all. You should
introduce some locking here (it introduces barriers on its own).

So move the assignment to altera_as_devs below the mutex_init & gpios
setup and lock it appropriately. Then add a lock to get_as_dev.

>>> +	mutex_init(&drvdata->open_lock);
>>> +
>>> +	drvdata->id = pdata->id;
>>> +
>>> +	drvdata->gpios[ASIO_DATA].gpio		= pdata->data;
>>> +	drvdata->gpios[ASIO_DATA].flags		= GPIOF_IN;
>>> +	drvdata->gpios[ASIO_DATA].label		= "as_data";
>> ...
>>> +	drvdata->gpios[ASIO_NCE].gpio		= pdata->nce;
>>> +	drvdata->gpios[ASIO_NCE].flags		= GPIOF_OUT_INIT_HIGH;
>>> +	drvdata->gpios[ASIO_NCE].label		= "as_nce";
>>> +
>>> +	dev_info(drvdata->dev, "Altera Cyclone Active Serial driver\n");
>>> +
>>> +	return 0;
>>> +}

regards,
-- 
js
suse labs
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