RE: [PATCH RESEND] driver core: export device_is_bound() to fix build failure

[Aisheng Dong]

> From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Sent: Wednesday, November 18, 2020 6:46 PM
> 
> On Wed, Nov 18, 2020 at 10:23:42AM +0000, Aisheng Dong wrote:
> > Hi Greg,
> >
> > > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Sent: Monday, November 9, 2020 8:48 PM
> > >
> > > On Mon, Nov 09, 2020 at 12:26:55PM +0000, Aisheng Dong wrote:
> > > > > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > > 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@xxxxxxxxxxxxxxxxxxx>
> > > > > > > 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@xxxxxxxxxxxxxxxxxxx>
> > > > > > > > > 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> > > > > > > > > > > > ---
> > > > > > > > > > > >
> > > > > > > > > > > > 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.
> >
> > I tried to understand how to use devlink for this case by diving deep
> > into the devlink code, however, it looked to me there're a few limitations on
> the current devlink usage.
> 
> Adding Saravana, who wrote that code to help explain it.
> 
> > We can't create driver presence dependency link in consumer's probe
> > function while the supplier driver is still not probed or even not created yet.
> > (we're the later case that supplier device scu-pd may be created after scu-clk
> device).
> 
> Sounds like your DT is set up backwards?

Yes, the dts is like below:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/freescale/imx8qxp.dtsi?h=v5.10-rc4
	scu {
		compatible = "fsl,imx-scu";
		mbox-names = "tx0",
			     "rx0",
			     "gip3";
		mboxes = <&lsio_mu1 0 0
			  &lsio_mu1 1 0
			  &lsio_mu1 3 3>;

		clk: clock-controller {
			compatible = "fsl,imx8qxp-clk";
			#clock-cells = <1>;
			clocks = <&xtal32k &xtal24m>;
			clock-names = "xtal_32KHz", "xtal_24Mhz";
		};

		pd: imx8qx-pd {
			compatible = "fsl,imx8qxp-scu-pd";
			#power-domain-cells = <1>;
		};
		...

Clk and pd devices under scu node are propagated by scu driver in DT order.
If moving pd node before clk node can also address this issue.

> 
> > The kernel doc Documentation/driver-api/device_link.rst also mentioned
> > this limitation and the suggested solution is:
> > "The onus is thus on the consumer to check presence of the supplier
> > after adding the link, and defer probing on non-presence."
> >
> > Then the question is , back again, , how to check the presense of
> > other device driver if we can't use device_is_bound() API in module?
> 
> Your driver should not care, as you can't get direct access to it, so don't try to
> ask the driver core about it as that is racy and not viable.
> 
> If your driver needs resources that it can not get at this point in time, just return
> from probe with a defer error.  That way it will be called again after other
> drivers load.
> 

Referring to above dts snippet, you can see the problem here is that there're no power
domain property in clock-controller node which cause the clock driver unable to use the
Normal/standard way to acquire PD resources and return a EPROBE_DEFFER if failed like
Other resources, e.g. irq, gpio and etc.
That's the main reason we use device_is_bound() in clock driver to check if PD driver
has been probed.

And of_genpd_add_device() we used in scu clk driver does not support EPROBE_DEFER
and in order to maintainer the backwards compatibility of imx_clk_scu_alloc_dev() API,
We also can't return PROBE_DEFER. All those are special reasons which made things complicated
and un-easy to address in standard way.

Maybe the simplest solution is justing move scu pd node above scu clk node which is also make
Sense because it can save a huge number of defer probes.

Anyway, thanks for the suggestion and detailed, patient explanation.
I could cook a patch to remove device_is_bound() checking code from clk driver
and moving scu pd node in dt to address it if no strong objections.

Regards
Aisheng

> > The dev_driver_string() seems be a quick and dirty solution for the
> > this build break issue as the driver name is less possible to be changed and
> under control.
> 
> No, driver names are not ever guaranteed to stay the same and are not to be
> used for this at all, sorry.
> 
> > How would you suggest for current situation?
> 
> defer your probe like all other drivers in this situation do it?  What makes this
> one driver so much different than the thousands of other ones we currently
> have?
> 
> thanks,
> 
> greg k-h