Date:   Mon, 9 Nov 2020 12:26:55 +0000
From:   Aisheng Dong <aisheng.dong@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>
Subject: RE: [PATCH RESEND] driver core: export device_is_bound() to fix build
 failure

> From: Greg Kroah-Hartman <gregkh@...uxfoundation.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@...uxfoundation.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@...uxfoundation.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@...uxfoundation.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@...il.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;
}

const char *dev_driver_string(const struct device *dev)
{
        struct device_driver *drv;
                                         
        /* dev->driver can change to NULL underneath us because of unbinding,
         * so be careful about accessing it.  dev->bus and dev->class should
         * never change once they are set, so they don't need special care.
         */
        drv = READ_ONCE(dev->driver);
        return drv ? drv->name :
                        (dev->bus ? dev->bus->name :
                        (dev->class ? dev->class->name : ""));
}       
EXPORT_SYMBOL(dev_driver_string);

Anyway, I will continue to investigate device_link according to your suggestions.

Regards
Aisheng

> thanks,
> 
> greg k-h

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