From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Gibson Subject: Re: [PATCH v2 6/6] libfdt: Add overlay application function Date: Thu, 14 Jul 2016 18:30:58 +1000 Message-ID: <20160714083058.GN14615@voom.fritz.box> References: <20160711195623.12840-1-maxime.ripard@free-electrons.com> <20160711195623.12840-7-maxime.ripard@free-electrons.com> <20160712143404.GD16355@voom.fritz.box> <20160713083803.GD4761@lukather> <20160713150745.GG14615@voom.fritz.box> <20160713193757.GL4761@lukather> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="um2V5WpqCyd73IVb" Return-path: Content-Disposition: inline In-Reply-To: <20160713193757.GL4761@lukather> Sender: devicetree-compiler-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Maxime Ripard Cc: Phil Elwell , Pantelis Antoniou , Simon Glass , Boris Brezillon , Alexander Kaplan , Thomas Petazzoni , devicetree-compiler-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Antoine =?iso-8859-1?Q?T=E9nart?= , Stefan Agner , devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org --um2V5WpqCyd73IVb Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jul 13, 2016 at 09:37:57PM +0200, Maxime Ripard wrote: > On Thu, Jul 14, 2016 at 01:07:45AM +1000, David Gibson wrote: > > On Wed, Jul 13, 2016 at 10:38:03AM +0200, Maxime Ripard wrote: > > > Hi David, > > >=20 > > > On Wed, Jul 13, 2016 at 12:34:04AM +1000, David Gibson wrote: > > > > On Mon, Jul 11, 2016 at 09:20:44PM +0100, Phil Elwell wrote: > > > > > On 11/07/2016 20:56, Maxime Ripard wrote: > > > > [snip] > > > >=20 > > > > > > +static int overlay_merge(void *fdt, void *fdto) > > > > > > +{ > > > > > > + int fragment; > > > > > > + > > > > > > + fdt_for_each_subnode(fragment, fdto, 0) { > > > > > > + int overlay; > > > > > > + int target; > > > > > > + int ret; > > > > > > + > > > > > > + target =3D overlay_get_target(fdt, fdto, fragment); > > > > > > + if (target < 0) > > > > > > + continue; > > > > > > + > > > > > > + overlay =3D fdt_subnode_offset(fdto, fragment, "__overlay__"= ); > > > > > > + if (overlay < 0) > > > > > > + return overlay; > > > >=20 > > > > > Why does the absence of a target cause a fragment to be ignored b= ut > > > > > the absence of an "__overlay__" property cause the merging to be > > > > > abandoned with an error? Can't we just ignore fragments that aren= 't > > > > > recognised? > > > >=20 > > > > So, I had the same question. But fragments we can't make sense MUST > > > > cause failures, and not be silently ignored. > > > >=20 > > > > An incompletely applied overlay is almost certainly going to cause = you > > > > horrible grief at some point, so you absolutely want to know early = if > > > > your overlay is in a format your tool doesn't understand. > > >=20 > > > I'm not sure how we can achieve that without applying it once, and see > > > if it fails. The obvious things are easy to detect (like a missing > > > __overlay__ node), but some others really aren't (like a poorly > > > formatted phandle, or one that overflows) without applying it > > > entirely. And that seems difficult without malloc. > >=20 > > So, atomically applying either the whole overlay or nothing would be a > > nice property, but it is indeed infeasibly difficult to achieve > > without malloc(). Well.. we sort of could by making apply_overlay() > > take an output buffer separate from the base tree, but that's not what > > I'm suggesting. > >=20 > > I'm fine with the base tree being trashed with an incomplete > > application when apply_overlay() reports failure. WHat I'm not ok > > with is *silent* failure. If you ignore fragments you don't > > understand, then - if the overlay uses features that aren't supported > > by this version of the code - you'll end up with an incompletely > > applied overlay while the apply_overlay() function *reports success*. > > That is a recipe for disaster. >=20 > Ok, that makes sense. I'll return an error if the target is missing as > well then. >=20 > But then, I think we fall back to the discussion you had with > Pantelis: how do you identify an overlay node (that must have a > target) and some other "metadata" node that shouldn't be applied (and > will not have a target). In the first case, we need to report an error > if it's missing. In the second, we should just ignore the node > entirely. Right. I can see two obvious approaches: 1. All (top-level) nodes named fragment@* are assumed to be overlay fragments. 2. All top-evel nodes with a subnode named '__overlay__' are assumed to be overlay fragments (2) differs from looking for target properties because whatever target variants we add in future, they're still likely to want an __overlay__ node. Or at worst, we can add a dummy __overlay__ node to them. =20 > Would turning that code the other way around, and if it has an > __overlay__ subnode, target or target-path is mandatory, and if not > just ignore the node entirely, work for you? I'd prefer to pick a single defining factor for the overlay fragments, rather than a grab bag of options. --=20 David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. 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