Re: [PATCHv4 1/4] modem_shm: Add Modem Access Framework

[anish singh]

On Mon, Oct 1, 2012 at 12:27 PM, Arun MURTHY <arun.murthy@xxxxxxxxxxxxxx> wrote:
>> On Mon, Oct 1, 2012 at 11:00 AM, Arun MURTHY
>> <arun.murthy@xxxxxxxxxxxxxx> 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.
>
> Let me correct a bit over here:
> There are many clients, yes correct but the operations performed are only two,
> i.e modem request and modem release. This is something like waking up the
> modem and let modem to sleep.
> The traffic of this request and release is too high.
>
> So irrespective of the requests/releases made to the MAF framework, the MAF
> should perform the operation request/release only once.
>
> So each and every time handling list consumes time.
> Let me brief the context, this is a single chip modem and ape, basically used in
> mobile, tablets etc. So the traffic in ape-modem communication is too high and
> also time critical. If it bound to exceed the time, or delay might end up in buffer
> full. Thatâs the reason I have made it as simple as possible.

So let me put it this way:
           Modem                 Client1     Client2    Client3    Client4
State  turn-on                   request
State  no-state-change                     request
State  no-state-change                                   request
State  no-state-change
request
State  no-state-change      release
State  no-state-change                     release
State  no-state-change                                   release
State   turn-off
      release

So eventhough all the clients have requested the modem it is being
turned on only once and when all of them have released then it will be
turned-off.

In this case it makes sense to use the atomic variables to track the requests
and release but what will happen to sysfs incase the device is released when the
last call to release has come from client4?Won't it lead to kernel panic or some
unwanted behaviour?

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