linux-kernel - Re: [PATCH v4 10/14] gunyah: sysfs: Add node to describe supported features

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Date:   Sun, 02 Oct 2022 16:58:59 -0700
From:   Joe Perches <joe@...ches.com>
To:     Jeff Johnson <quic_jjohnson@...cinc.com>,
        Elliot Berman <quic_eberman@...cinc.com>,
        Bjorn Andersson <quic_bjorande@...cinc.com>
Cc:     Murali Nalajala <quic_mnalajal@...cinc.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Marc Zyngier <maz@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/14] gunyah: sysfs: Add node to describe supported
 features

On Sun, 2022-10-02 at 16:30 -0700, Jeff Johnson wrote:
> On 9/29/2022 12:36 AM, Joe Perches wrote:
> > On Wed, 2022-09-28 at 12:56 -0700, Elliot Berman wrote:
> > > Add a sysfs node to list the features that the Gunyah hypervisor and
> > > Linux supports. For now, Linux support cspace (capability IDs) and
> > > message queues, so only report those..
> > []
> > > diff --git a/drivers/virt/gunyah/sysfs.c b/drivers/virt/gunyah/sysfs.c
> > []
> > > @@ -25,9 +25,24 @@ static ssize_t variant_show(struct kobject *kobj, struct kobj_attribute *attr, c
> > >   }
> > >   static struct kobj_attribute variant_attr = __ATTR_RO(variant);
> > >   
> > > +static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr, char *buffer)
> > > +{
> > > +	int len = 0;
> > > +
> > > +	if (GH_IDENTIFY_PARTITION_CSPACE(gunyah_api.flags))
> > > +		len += sysfs_emit_at(buffer, len, "cspace ");
> > > +	if (GH_IDENTIFY_MSGQUEUE(gunyah_api.flags))
> > > +		len += sysfs_emit_at(buffer, len, "message-queue ");
> > > +
> > > +	len += sysfs_emit_at(buffer, len, "\n");
> > > +	return len;
> > > +}
> > 
> > It's generally nicer to avoid unnecessary output spaces.
> > 
> > Perhaps:
> > 
> > {
> > 	int len = 0;
> > 
> > 	if (GH_IDENTIFY_PARTITION_CSPACE(gunyah_api.flags))
> > 		len += sysfs_emit_at(buffer, len, "cspace");
> > 	if (GH_IDENTIFY_MSGQUEUE(gunyah_api.flags)) {
> > 		if (len)
> > 			len += sysfs_emit_at(buffer, len, " ");
> > 		len += sysfs_emit_at(buffer, len, "message-queue");
> > 	}
> > 
> > 	len += sysfs_emit_at(buffer, len, "\n");
> > 
> > 	return len;
> > }
> > 
> 
> that approach seems ok for 2 features, but imo doesn't scale for more.
> I like the original code with one exception:
> 
> 	if (GH_IDENTIFY_PARTITION_CSPACE(gunyah_api.flags))
> 		len += sysfs_emit_at(buffer, len, "cspace ");
> 	if (GH_IDENTIFY_MSGQUEUE(gunyah_api.flags))
> 		len += sysfs_emit_at(buffer, len, "message-queue ");
> 
> 	/* overwrite last trailing space */
> 	if (len)
> 		len--;
> 
> 	len += sysfs_emit_at(buffer, len, "\n");
> 	return len;
> 

That's fine as long as every formatted output uses a trailing space.

A trivial negative would be that the linker would generally not be
able to deduplicate these output strings with trailing spaces across
the entire codebase.