linux-kernel - Re: [PATCH] driver core:Export the symbol device_is

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Date:   Thu, 18 Jun 2020 08:45:55 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Sandeep Maheswaram <sanm@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Felipe Balbi <balbi@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>, rafael@...nel.org,
        linux-arm-msm@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Manu Gautam <mgautam@...eaurora.org>
Subject: Re: [PATCH] driver core:Export the symbol device_is_bound

Hi Greg,

On Thu, Jun 18, 2020 at 10:14:43AM +0200, Greg Kroah-Hartman wrote:
> On Wed, Jun 03, 2020 at 12:09:52AM +0530, Sandeep Maheswaram wrote:
> > Export the symbol device_is_bound so that it can be used by the modules.
> 
> What modules need this?

drivers/usb/dwc3/dwc3-qcom.c (and probably other dwc3 'wrappers').

Short summary: QCOM dwc3 support is split in two drivers, the core dwc3
driver and the QCOM specific parts. dwc3-qcom is probed first (through
a DT entry or ACPI), dwc3_qcom_probe() then calls of_platform_populate()
to probe the core part. After a successful return from _populate() the
driver assumes that the core device is fully initialized. However the
latter is not correct, the driver core doesn't propagate errors from
probe() to platform_populate(). The dwc3-qcom driver would use
device_is_bound() to make sure the core device was probed successfully.

Related patches:

"usb: dwc3: qcom: Make sure core device is fully initialized before it is used"
https://lore.kernel.org/patchwork/patch/1257279/

"usb: dwc3: qcom: Add interconnect support in dwc3 driver"
https://patchwork.kernel.org/patch/11468647/

> > This change was suggested to solve the allmodconfig build error on adding
> > the patch https://lore.kernel.org/patchwork/patch/1218628/
> > 
> > Signed-off-by: Sandeep Maheswaram <sanm@...eaurora.org>
> > ---
> >  drivers/base/dd.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 9a1d940..65d16ce 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -337,6 +337,7 @@ bool device_is_bound(struct device *dev)
> >  {
> >  	return dev->p && klist_node_attached(&dev->p->knode_driver);
> >  }
> > +EXPORT_SYMBOL_GPL(device_is_bound);
> 
> If a driver needs to use this, something is really wrong with it.  What
> happens right after this, the state could have changed?
> 
> So, no, sorry, this is not a good idea.

Agreed that what some dwc3 'wrapper' drivers do is brittle, and that using
device_is_bound() is only a bandaid, that doesn't address the entire issue.

Do you have any suggestions on how this could be properly addressed?

Thanks

Matthias