linux-kernel - Re: [PATCH 1/3] of: dynamic: add of_property_alloc() and of_property

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Message-ID: <20220506094905.27bc99aa@fixe.home>
Date:   Fri, 6 May 2022 09:49:05 +0200
From:   Clément Léger <clement.leger@...tlin.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Frank Rowand <frowand.list@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Daniel Henrique Barboza <danielhb413@...il.com>,
        David Gibson <david@...son.dropbear.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Ohhoon Kwon <ohoono.kwon@...sung.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        YueHaibing <yuehaibing@...wei.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org,
        Allan Nielsen <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH 1/3] of: dynamic: add of_property_alloc() and
 of_property_free()

Le Thu, 5 May 2022 14:37:15 -0500,
Rob Herring <robh@...nel.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