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Date:   Fri, 20 Sep 2019 08:10:06 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Murali Nalajala <mnalajal@codeaurora.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: <20190920061006.GC473898@kroah.com>
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On Thu, Sep 19, 2019 at 08:36:51PM -0700, Bjorn Andersson wrote:
> On Thu 19 Sep 15:45 PDT 2019, Greg KH wrote:
> 
> > On Thu, Sep 19, 2019 at 03:40:17PM -0700, Bjorn Andersson wrote:
> > > On Thu 19 Sep 15:25 PDT 2019, Greg KH wrote:
> > > 
> > > > On Thu, Sep 19, 2019 at 03:14:56PM -0700, Bjorn Andersson wrote:
> > > > > On Thu 19 Sep 14:58 PDT 2019, 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?
> > > > > > 
> > > > > 
> > > > > The custom attributes that these drivers want to add to the common ones
> > > > > are known in advance, so I presume we could have them passed into
> > > > > soc_device_register() and registered together with the common
> > > > > attributes...
> > > > > 
> > > > > It sounds like it's worth a prototype.
> > > > 
> > > > Do you have an in-kernel example I can look at to get an idea of what is
> > > > needed here?
> > > > 
> > > 
> > > realview_soc_probe(), in drivers/soc/versatile/soc-realview.c,
> > > implements the current mechanism of acquiring the soc's struct device
> > > and then issuing a few device_create_file calls on that.
> > 
> > That looks to be a trivial driver to fix up.  Look at 6d03c140db2e
> > ("USB: phy: fsl-usb: convert platform driver to use dev_groups") as an
> > example of how to do this.
> > 
> 
> The difference between the two cases is that in the fsl-usb case it's
> attributes of the device itself, while in the soc case the realview-soc
> driver (or the others doing this) calls soc_device_register() to
> register a new (dangling) soc device, which it then adds its attributes
> onto.

That sounds really really odd.  Why can't the soc device do the creation
"automatically" when the device is registered?  The soc core should
handle this for the soc "drivers", that's what it is there for.

> We can't use dev_groups, because the soc_device (soc.c) isn't actually a
> driver and the list of attributes is a combination of things from soc.c
> and e.g. soc-realview.c.
> 
> But if we pass a struct attribute_group into soc_device_register() and
> then have that register both groups using dev.groups, this should be
> much cleaner at least.

Don't you have a structure you can store these in as well?

thanks,

greg k-h