Re: [PATCH] base: soc: Export soc_device_to_device API

[mnalajal@codeaurora.org]

On 2019-09-23 21:50, Greg KH wrote:
> On Mon, Sep 23, 2019 at 02:35:33PM -0700, mnalajal@codeaurora.org 
> wrote:
>> On 2019-09-19 23:10, Greg KH wrote:
>> > 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.
>> >
>> Clients are registering to soc framework using 
>> "soce_device_register()"
>> with "soc_device_attribute". This attribute structure does not have 
>> all
>> the sysfs fields what client are interested. Hence clients are 
>> handling
>> their required sysfs fields in their drivers.
>> https://elixir.bootlin.com/linux/v5.3/source/drivers/base/soc.c#L114
> 
> Then you should fix that :)
If i understand, you are asking me to address additional sysfs entries 
from the client side using "default attribute" groups.
I saw your patches about "dev_groups" usage which might be part of 
5.4-rc1.
If i go with above approach, i end up seeing the soc information at two 
different sysfs paths i.e.
Is this my understanding correct?

1. /sys/devices/soc0/*
2. /sys/bus/platform/drivers/msm-socinfo/*

Couple of things which i can think of addressing this issue is:
1. Modify the soc framework APIs to pass the client side sysfs 
attributes. This will ensure all the soc information fall under 
/sys/devices/soc0/*
2. Modify "struct soc_device_attribute" and add more entries. So that we 
do not need to change any soc framework.
Problem here is others might have a different requirement which will not 
be full fill if i do this.

> 
>> > > 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?
>> At present client is populating entries one-by-one
>> https://android.googlesource.com/kernel/msm/+/android-7.1.0_r0.2/drivers/soc/qcom/socinfo.c#1254
> 
> And that is known to be broken and racy and will cause problems with
> userspace.  This should be fixed...
I saw your explanation here about race 
http://kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-correctly/
> 
> thanks,
> 
> greg k-h