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Date:   Mon, 9 Nov 2020 13:48:01 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Aisheng Dong <aisheng.dong@nxp.com>
Cc:     Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Shawn Guo <shawnguo@kernel.org>,
        Stephen Boyd <sboyd@kernel.org>
Subject: Re: [PATCH RESEND] driver core: export device_is_bound() to fix
 build failure
Message-ID: <20201109124801.GA1890488@kroah.com>
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On Mon, Nov 09, 2020 at 12:26:55PM +0000, Aisheng Dong wrote:
> > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Sent: Monday, November 9, 2020 8:05 PM
> > 
> > On Mon, Nov 09, 2020 at 11:55:46AM +0000, Aisheng Dong wrote:
> > > > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Sent: Monday, November 9, 2020 7:41 PM
> > > >
> > > > On Mon, Nov 09, 2020 at 10:57:05AM +0000, Aisheng Dong wrote:
> > > > > Hi Greg,
> > > > >
> > > > > > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > > > Sent: Monday, November 9, 2020 6:37 PM
> > > > > > Subject: Re: [PATCH RESEND] driver core: export
> > > > > > device_is_bound() to fix build failure
> > > > > >
> > > > > > On Mon, Nov 09, 2020 at 10:14:46AM +0000, Sudip Mukherjee wrote:
> > > > > > > Hi Greg,
> > > > > > >
> > > > > > > On Sun, Nov 8, 2020 at 8:23 AM Greg Kroah-Hartman
> > > > > > > <gregkh@linuxfoundation.org> wrote:
> > > > > > > >
> > > > > > > > On Sat, Nov 07, 2020 at 10:47:27PM +0000, Sudip Mukherjee wrote:
> > > > > > > > > When CONFIG_MXC_CLK_SCU is configured as 'm' the build
> > > > > > > > > fails as it is unable to find device_is_bound(). The error being:
> > > > > > > > > ERROR: modpost: "device_is_bound"
> > [drivers/clk/imx/clk-imx-scu.ko]
> > > > > > > > >       undefined!
> > > > > > > > >
> > > > > > > > > Export the symbol so that the module finds it.
> > > > > > > > >
> > > > > > > > > Fixes: 77d8f3068c63 ("clk: imx: scu: add two cells binding
> > > > > > > > > support")
> > > > > > > > > Signed-off-by: Sudip Mukherjee
> > > > > > > > > <sudipm.mukherjee@gmail.com>
> > > > > > > > > ---
> > > > > > > > >
> > > > > > > > > resending with the Fixes: tag.
> > > > > > > > >
> > > > > > > > >  drivers/base/dd.c | 1 +
> > > > > > > > >  1 file changed, 1 insertion(+)
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c index
> > > > > > > > > 148e81969e04..a796a57e5efb 100644
> > > > > > > > > --- a/drivers/base/dd.c
> > > > > > > > > +++ b/drivers/base/dd.c
> > > > > > > > > @@ -353,6 +353,7 @@ bool device_is_bound(struct device *dev)
> > {
> > > > > > > > >       return dev->p &&
> > > > > > > > > klist_node_attached(&dev->p->knode_driver);
> > > > > > > > >  }
> > > > > > > > > +EXPORT_SYMBOL(device_is_bound);
> > > > > > > >
> > > > > > > > EXPORT_SYMBOL_GPL() please, like all the other exports in this file.
> > > > > > > >
> > > > > > > > Also, wait, no, don't call this, are you sure you are
> > > > > > > > calling it in a race-free way?  And what branch/tree is the above
> > commit in?
> > > > > > >
> > > > > > > I have not checked fully but since it is being called from
> > > > > > > probe() I assume the lock will be held at that time.
> > > > > >
> > > > > > probe() should never call this function as it makes no sense at
> > > > > > all at that point in time.  The driver should be fixed.
> > > > >
> > > > > Would you suggest if any other API we can use to allow the driver
> > > > > to know whether another device has been probed?
> > > >
> > > > There is none, sorry, as that just opens up way too many problems.
> > > >
> > > > > For imx scu driver in question, it has a special requirement that
> > > > > it depends on scu power domain driver. However, there're a huge
> > > > > number
> > > > > (200+) of power domains for each device clock, we can't define
> > > > > them all in DT
> > > > for a single clock controller node.
> > > > >
> > > > > That's why we wanted to use device_is_bound() before to check if
> > > > > scu power domain is ready or not to support defer probe.
> > > >
> > > > Use the device link functionality for this type of thing, that is
> > > > what it was created for.
> > > >
> > >
> > > Thanks for the suggestion. I will check it how to use.
> > > BTW, I wonder if dev_driver_string() could be an optional solution
> > > which seems a more simple way?
> > 
> > Also, how do you really know you even have a valid pointer to that other device
> > structure?  How are you getting access to that?
> > 
> 
> The rough idea is as follows. Not sure if those APIs are safe enough as there're
> many users In kernel.
> 
> pd_np = of_find_compatible_node(NULL, NULL, "fsl,scu-pd");
> pd_dev = of_find_device_by_node(pd_np);
> if (!pd_dev || !dev_driver_string(&pd_dev->dev) ||
>    strcmp(dev_driver_string(&pd_dev->dev), "imx-scu-pd")) {
>         of_node_put(pd_np);
>         return -EPROBE_DEFER;
> }

Ick, again, no, don't do that, you can not guarantee "names" of devices
anywhere in the system, sorry.

greg k-h