Re: [PATCH 1/3] of: dynamic: add of_property_alloc() and of_property_free()

From: "Clément Léger" <clement.leger@bootlin.com>
To: Rob Herring <robh@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Nathan Lynch <nathanl@linux.ibm.com>,
	Laurent Dufour <ldufour@linux.ibm.com>,
	Daniel Henrique Barboza <danielhb413@gmail.com>,
	David Gibson <david@gibson.dropbear.id.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Ohhoon Kwon <ohoono.kwon@samsung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	YueHaibing <yuehaibing@huawei.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	devicetree@vger.kernel.org,
	Allan Nielsen <allan.nielsen@microchip.com>,
	Horatiu Vultur <horatiu.vultur@microchip.com>,
	Steen Hegelund <steen.hegelund@microchip.com>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Subject: Re: [PATCH 1/3] of: dynamic: add of_property_alloc() and of_property_free()
Date: Fri, 6 May 2022 09:49:05 +0200	[thread overview]
Message-ID: <20220506094905.27bc99aa@fixe.home> (raw)
In-Reply-To: <YnQnayouXw9/jp/E@robh.at.kernel.org>

Le Thu, 5 May 2022 14:37:15 -0500,
Rob Herring <robh@kernel.org> a écrit :

> > +
> > +/**
> > + * of_property_alloc - Allocate a property dynamically.
> > + * @name:	Name of the new property
> > + * @value:	Value that will be copied into the new property value
> > + * @value_len:	length of @value to be copied into the new property value
> > + * @len:	Length of new property value, must be greater than @value_len  
> 
> What's the usecase for the lengths being different? That doesn't seem 
> like a common case, so perhaps handle it with a NULL value and 
> non-zero length. Then the caller has to deal with populating 
> prop->value.

That was actually something used by powerpc code but agreed, letting
the user recopy it's values seems fine to me and the usage will be more
clear.

> >  	/*
> > -	 * NOTE: There is no check for zero length value.
> > -	 * In case of a boolean property, this will allocate a value
> > -	 * of zero bytes. We do this to work around the use
> > -	 * of of_get_property() calls on boolean values.
> > +	 * Even if the property has no value, it must be set to a
> > +	 * non-null value since of_get_property() is used to check
> > +	 * some values that might or not have a values (ranges for
> > +	 * instance). Moreover, when the node is released, prop->value
> > +	 * is kfreed so the memory must come from kmalloc.  
> 
> Allowing for NULL value didn't turn out well...
> 
> We know that we can do the kfree because OF_DYNAMIC is set IIRC...
> 
> If we do 1 allocation for prop and value, then we can test 
> for "prop->value == prop + 1" to determine if we need to free or not.

Sounds like a good idea.

-- 
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com