linux-kernel - Re: [PATCH] include: sysfs: Add macro to assign show for RO attributes

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Date:   Wed, 27 Jan 2021 20:51:26 +0800
From:   Orson Zhai <orsonzhai@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jonathan Corbet <corbet@....net>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Joe Perches <joe@...ches.com>,
        Denis Efremov <efremov@...ux.com>,
        "David S. Miller" <davem@...emloft.net>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Orson Zhai <orson.zhai@...soc.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] include: sysfs: Add macro to assign show for RO
 attributes

On Wed, Jan 27, 2021 at 08:50:28AM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 27, 2021 at 12:19:22PM +0800, Orson Zhai wrote:
> > In some circumstances, multiple __ATTR_RO attributes need to be assigned
> > with a single show function.
> > 
> > Add this macro to make life easier with simple code.
> > 
> > Signed-off-by: Orson Zhai <orsonzhai@...il.com>
> > ---
> >  Documentation/filesystems/sysfs.rst | 2 ++
> >  include/linux/sysfs.h               | 5 +++++
> >  2 files changed, 7 insertions(+)
> > 
> > diff --git a/Documentation/filesystems/sysfs.rst b/Documentation/filesystems/sysfs.rst
> > index 004d490..0e2274a 100644
> > --- a/Documentation/filesystems/sysfs.rst
> > +++ b/Documentation/filesystems/sysfs.rst
> > @@ -141,6 +141,8 @@ __ATTR_RO_MODE(name, mode):
> >  	         fore more restrictive RO access currently
> >                   only use case is the EFI System Resource Table
> >                   (see drivers/firmware/efi/esrt.c)
> > +__ATTR_RO_SHOW(name, show):
> > +		 assumes default mode 0444 with specified show.
> >  __ATTR_RW(name):
> >  	         assumes default name_show, name_store and setting
> >                   mode to 0644.
> > diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> > index 2caa34c..c851592 100644
> > --- a/include/linux/sysfs.h
> > +++ b/include/linux/sysfs.h
> > @@ -117,6 +117,11 @@ struct attribute_group {
> >  	.show	= _name##_show,						\
> >  }
> >  
> > +#define __ATTR_RO_SHOW(_name, _show) {					\
> > +	.attr	= { .name = __stringify(_name), .mode = 0444 },		\
> > +	.show	= _show,						\
> > +}
> 
> Do you have a real user for this?  Using "raw" kobject attributes is

Yes, I have found at least one user in current kernel code.

Please refer to [1].

The author implemented a similar marcro __ATRR_MRO as mine, plus an
__ATRR_MWO with specified restore.

If this patch merged, I'd to replace his marcro with mine.

> rare and should not be used often, so who needs this?

Agree. But for some device drivers it might be useful without side effect.

Another example is from Android increment-fs code out there.
That driver has 3 sysfs attributes which shared with same show function
which only prints "support" to userland.

Best Regards,
Orson

> 
> thanks,
> 
> greg k-h

[1] https://elixir.bootlin.com/linux/v5.11-rc5/source/drivers/media/pci/ddbridge/ddbridge-core.c#L2735