netdev - RE: [PATCHv4 1/4] modem_shm: Add Modem Access Framework

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Date:	Mon, 1 Oct 2012 11:12:24 +0200
From:	Arun MURTHY <arun.murthy@...ricsson.com>
To:	anish singh <anish198519851985@...il.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

> >> >> > +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. That’s 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?
>

Yes, you are right, so will add a check in modem_put and modem_unregister
to check if modem is requested and if so will release first and then go ahead.
But actually clients are not suppose to call modem_put/modem_unregister
unless modem is released but yes, in MAF we can take this precaution.

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