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    SubjectRe: [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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