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Date:	Mon, 1 Oct 2012 12:03:37 +0530
From:	anish singh <anish198519851985@...il.com>
To:	Arun MURTHY <arun.murthy@...ricsson.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCHv4 1/4] modem_shm: Add Modem Access Framework

On Mon, Oct 1, 2012 at 11:00 AM, Arun MURTHY <arun.murthy@...ricsson.com> wrote:
>> On Fri, Sep 28, 2012 at 01:35:01PM +0530, Arun Murthy wrote:
>> > +#include <linux/module.h>
>> > +#include <linux/slab.h>
>> > +#include <linux/err.h>
>> > +#include <linux/printk.h>
>> > +#include <linux/modem_shm/modem.h>
>> > +
>> > +static struct class *modem_class;
>>
>> What's wrong with a bus_type instead?
>
> Can I know the advantage of using bus_type over class?
>
>>
>> > +static int __modem_is_requested(struct device *dev, void *data) {
>> > +   struct modem_desc *mdesc = (struct modem_desc *)data;
>> > +
>> > +   if (!mdesc->mclients) {
>> > +           printk(KERN_ERR "modem_access: modem description is
>> NULL\n");
>> > +           return 0;
>> > +   }
>> > +   return atomic_read(&mdesc->mclients->cnt);
>> > +}
>> > +
>> > +int modem_is_requested(struct modem_desc *mdesc) {
>> > +   return class_for_each_device(modem_class, NULL, (void *)mdesc,
>> > +__modem_is_requested); }
>>
>> Where is the documentation for your public api functions like this?
>
> Sure will include this in the next patchset.
>
>>
>> > +
>> > +int modem_release(struct modem_desc *mdesc) {
>> > +   if (!mdesc->release)
>> > +           return -EFAULT;
>> > +
>> > +   if (modem_is_requested(mdesc)) {
>> > +           atomic_dec(&mdesc->mclients->cnt);
>> > +           if (atomic_read(&mdesc->use_cnt) == 1) {
>> > +                   mdesc->release(mdesc);
>> > +                   atomic_dec(&mdesc->use_cnt);
>> > +           }
>>
>> Eeek, why aren't you using the built-in reference counting that the struct
>> device provided to you, and instead are rolling your own?  This happens in
>> many places, why?
>
> My usage of counters over here is for each modem there are many clients.
> Each of the clients will have a ref to modem_desc. Each of them use this for
> requesting and releasing the modem. One counter for tracking the request
> and release for each client which is done by variable 'cnt' in struct clients.
> The counter use_cnt is used for tracking the modem request/release irrespective
> of the clients and counter cli_cnt is used for restricting the modem_get to
> the no of clients defined in no_clients.
>
> So totally 3 counter one for restricting the usage of modem_get by clients,
> second for restricting modem request/release at top level, and 3rd for
> restricting modem release/request for per client per modem basis.
>
> Can you let me know if the same can be achieved by using built-in ref
> counting?
Is this your model:
You have a modem device which can be requested by many clients.This clients
can register for a particular service which this modem provides and then after
that if it client doesn't need this service then it will call un-register.
This can happen for many clients.
So what you need is a way to track clients and once no client is in picture, you
want to de-allocate all the memory and resource associated with modem device.

If this is your model then read on otherwise please skip.
What you can do is this:
On each modem_register
 list_add(&modm_dev->entry, &modm_dev_list);
and once you de-register, remove the device from the modem_dev_list.

Have this in your modem_register function
modem->dev->release = modem_dev_release;
This will be called once all the device references have been released
and you need to remove all the memory/resources associated with your
modem device.So you will do the final cleanup
modem_cleanup(edev, true); //this will be "false" when the client just does
the modem_unregister.

Something as below:
void modem_dev_unregister(struct modem_dev *edev)
{
    modem_cleanup(edev, false);
}

static void modem_dev_release(struct device *dev)
{
    struct modem_dev *edev = (struct modem_dev *) dev_get_drvdata(dev);

    modem_cleanup(edev, true);
}

static void modem_cleanup(struct modem_dev *edev, bool skip)
{
    mutex_lock(&modem_dev_list_lock);
    list_del(&modem->entry);
    mutex_unlock(&modem_dev_list_lock);

    if (!skip && get_device(modem->dev)) {
//do the cleanup here
        }

        device_unregister(modem->dev);
        put_device(modem->dev);
    }

    kfree(modem->dev);
}

>
> Thanks and Regards,
> Arun R Murthy
> ------------------
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