linux-kernel - Re: [PATCH v2] driver core: Remove device link creation limitation

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Message-ID: <20190723075724.GB27243@kroah.com>
Date:   Tue, 23 Jul 2019 09:57:24 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Lukas Wunner <lukas@...ner.de>,
        Jon Hunter <jonathanh@...dia.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2] driver core: Remove device link creation limitation

On Tue, Jul 23, 2019 at 09:34:54AM +0200, Rafael J. Wysocki wrote:
> Hi Greg,
> 
> On Tue, Jul 16, 2019 at 5:21 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> >
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Subject: [PATCH] driver core: Remove device link creation limitation
> >
> > If device_link_add() is called for a consumer/supplier pair with an
> > existing device link between them and the existing link's type is
> > not in agreement with the flags passed to that function by its
> > caller, NULL will be returned.  That is seriously inconvenient,
> > because it forces the callers of device_link_add() to worry about
> > what others may or may not do even if that is not relevant to them
> > for any other reasons.
> >
> > It turns out, however, that this limitation can be made go away
> > relatively easily.
> >
> > The underlying observation is that if DL_FLAG_STATELESS has been
> > passed to device_link_add() in flags for the given consumer/supplier
> > pair at least once, calling either device_link_del() or
> > device_link_remove() to release the link returned by it should work,
> > but there are no other requirements associated with that flag.  In
> > turn, if at least one of the callers of device_link_add() for the
> > given consumer/supplier pair has not passed DL_FLAG_STATELESS to it
> > in flags, the driver core should track the status of the link and act
> > on it as appropriate (ie. the link should be treated as "managed").
> > This means that DL_FLAG_STATELESS needs to be set for managed device
> > links and it should be valid to call device_link_del() or
> > device_link_remove() to drop references to them in certain
> > sutiations.
> >
> > To allow that to happen, introduce a new (internal) device link flag
> > called DL_FLAG_MANAGED and make device_link_add() set it automatically
> > whenever DL_FLAG_STATELESS is not passed to it.  Also make it take
> > additional references to existing device links that were previously
> > stateless (that is, with DL_FLAG_STATELESS set and DL_FLAG_MANAGED
> > unset) and will need to be managed going forward and initialize
> > their status (which has been DL_STATE_NONE so far).
> >
> > Accordingly, when a managed device link is dropped automatically
> > by the driver core, make it clear DL_FLAG_MANAGED, reset the link's
> > status back to DL_STATE_NONE and drop the reference to it associated
> > with DL_FLAG_MANAGED instead of just deleting it right away (to
> > allow it to stay around in case it still needs to be released
> > explicitly by someone).
> >
> > With that, since setting DL_FLAG_STATELESS doesn't mean that the
> > device link in question is not managed any more, replace all of the
> > status-tracking checks against DL_FLAG_STATELESS with analogous
> > checks against DL_FLAG_MANAGED and update the documentation to
> > reflect these changes.
> >
> > While at it, make device_link_add() reject flags that it does not
> > recognize, including DL_FLAG_MANAGED.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Reviewed-by: Saravana Kannan <saravanak@...gle.com>
> > Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>
> 
> Do I need to resend this?
> 
> I know that posting new things during a merge window is not most
> convenient, sorry about that.

No, it's in my queue, give me a few days to dig out from my huge pending
patch list, no need to resend it.

thanks,

greg k-h