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Date:   Fri, 20 Sep 2019 00:46:24 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     mnalajal@codeaurora.org
Cc:     Bjorn Andersson <bjorn.andersson@linaro.org>, rafael@kernel.org,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH] base: soc: Export soc_device_to_device API
Message-ID: <20190919224624.GB447028@kroah.com>
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On Thu, Sep 19, 2019 at 03:27:45PM -0700, mnalajal@codeaurora.org wrote:
> On 2019-09-19 14:58, Greg KH wrote:
> > On Thu, Sep 19, 2019 at 02:53:00PM -0700, Bjorn Andersson wrote:
> > > On Thu 19 Sep 14:32 PDT 2019, Greg KH wrote:
> > > 
> > > > On Thu, Sep 19, 2019 at 02:13:44PM -0700, Murali Nalajala wrote:
> > > > > If the soc drivers want to add custom sysfs entries it needs to
> > > > > access "dev" field in "struct soc_device". This can be achieved
> > > > > by "soc_device_to_device" API. Soc drivers which are built as a
> > > > > module they need above API to be exported. Otherwise one can
> > > > > observe compilation issues.
> > > > >
> > > > > Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>
> > > > > ---
> > > > >  drivers/base/soc.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/drivers/base/soc.c b/drivers/base/soc.c
> > > > > index 7c0c5ca..4ad52f6 100644
> > > > > --- a/drivers/base/soc.c
> > > > > +++ b/drivers/base/soc.c
> > > > > @@ -41,6 +41,7 @@ struct device *soc_device_to_device(struct soc_device *soc_dev)
> > > > >  {
> > > > >  	return &soc_dev->dev;
> > > > >  }
> > > > > +EXPORT_SYMBOL_GPL(soc_device_to_device);
> > > > >
> > > > >  static umode_t soc_attribute_mode(struct kobject *kobj,
> > > > >  				struct attribute *attr,
> > > >
> > > > What in-kernel driver needs this?
> > > >
> > > 
> > > Half of the drivers interacting with the soc driver calls this API,
> > > several of these I see no reason for being builtin (e.g.
> > > ux500 andversatile). So I think this patch makes sense to allow us to
> > > build these as modules.
> > > 
> > > > Is linux-next breaking without this?
> > > >
> > > 
> > > No, we postponed the addition of any sysfs attributes in the Qualcomm
> > > socinfo driver.
> > > 
> > > > We don't export things unless we have a user of the export.
> > > >
> > > > Also, adding "custom" sysfs attributes is almost always not the correct
> > > > thing to do at all.  The driver should be doing it, by setting up the
> > > > attribute group properly so that the driver core can do it automatically
> > > > for it.
> > > >
> > > > No driver should be doing individual add/remove of sysfs files.  If it
> > > > does so, it is almost guaranteed to be doing it incorrectly and racing
> > > > userspace.
> > > >
> > > 
> > > The problem here is that the attributes are expected to be attached to
> > > the soc driver, which is separate from the platform-specific
> > > drivers. So
> > > there's no way to do platform specific attributes the right way.
> > > 
> > > > And yes, there's loads of in-kernel examples of doing this wrong, I've
> > > > been working on fixing that up, look at the patches now in Linus's tree
> > > > for platform and USB drivers that do this as examples of how to do it
> > > > right.
> > > >
> > > 
> > > Agreed, this patch should not be used as an approval for any crazy
> > > attributes; but it's necessary in order to extend the soc device's
> > > attributes, per the current design.
> > 
> > Wait, no, let's not let the "current design" remain if it is broken!
> > 
> > Why can't the soc driver handle the attributes properly so that the
> > individual driver doesn't have to do the create/remove?
> > 
> > thanks,
> > 
> > greg k-h
> 
> Current soc framework is allowing the soc info drivers to create very
> limited
> soc information as sysfs entries like machine name, family, revision and
> soc_id.
> Sometimes it become a limited information for the clients to know more about
> soc
> information. In this scenario soc info drivers are adding their own sysfs
> entries
> on top of what soc framework is doing like hw_platform, hw_platform_subtype
> etc.
> I believe this will be an issue for the other soc vendors as well. It is
> good that
> if we could add these into the soc framework rather than individual clients
> defining
> and adding them as per their requirement.

It's fine to add sysfs files (as long as you document them in
Documentation/ABI/) but don't do so file-by-file in a driver as that is
racy and will not work.  See 6d03c140db2e ("USB: phy: fsl-usb: convert
platform driver to use dev_groups") as an example of how to do this in a
simple way to have the driver core do all of the work for you
automatically.

thanks,

greg k-h