Re: [PATCH v3 10/12] libfdt: Add overlay application function

From: David Gibson <david@gibson.dropbear.id.au>
To: Frank Rowand <frowand.list@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
	u-boot@lists.denx.de,
	Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
	Tom Rini <trini@konsulko.com>,
	Alexander Kaplan <alex@nextthing.co>,
	devicetree-compiler@vger.kernel.org
Subject: Re: [PATCH v3 10/12] libfdt: Add overlay application function
Date: Thu, 30 Jun 2016 15:08:57 +1000	[thread overview]
Message-ID: <20160630050857.GR8885@voom.fritz.box> (raw)
In-Reply-To: <5774854E.2020008@gmail.com>

[-- Attachment #1.1: Type: text/plain, Size: 1975 bytes --]

On Wed, Jun 29, 2016 at 07:34:54PM -0700, Frank Rowand wrote:
> On 06/27/16 20:12, David Gibson wrote:
> > On Mon, Jun 27, 2016 at 01:40:00PM +0200, Maxime Ripard wrote:
> >> Hi David,
> >>
> >> On Mon, Jun 27, 2016 at 03:26:07PM +1000, David Gibson wrote:
> >>>> +static uint32_t overlay_get_target_phandle(const void *fdto, int fragment)
> >>>> +{
> >>>> +	const uint32_t *val;
> >>>> +	int len;
> >>>> +
> >>>> +	val = fdt_getprop(fdto, fragment, "target", &len);
> >>>> +	if (!val || (*val == 0xffffffff) || (len != sizeof(*val)))
> >>>> +		return 0;
> >>>
> >>> This doesn't distinguish between a missing property (which may
> >>> indicate a valid overlay using a target-path or some other method)
> >>> and a badly formatted 'target' property, which is definitely an error
> >>> in the overlay.
> >>>
> >>> I think those should be treated differently.
> >>
> >> AFAIK, phandles can have any 32 bits values but 0xffffffff. In order
> >> to cover the two cases, we would need to have some error code, but
> >> that doesn't really work with returning a uint32_t.
> > 
> > Actually phandles can have any value except 0xffffffff *or* 0.  So you
> > can use 0 for "couldn't find" and -1 for "badly formatted".
> 
> < snip >
> 
> Hi David,
> 
> I would like to capture this for the specification.
> 
> It seems like I could say that a value of 0 in the FDT is not allowed.
> 
> Then thinking of what Pantelis is doing with overlays, it seems like a
> value of 0xffffffff is allowed in the FDT, but it means not a valid
> phandle, so do not try to de-reference it.
> 
> Does that sound good?

That should be ok.  Basically both 0 and -1 are invalid phandle
values, so it's up to us if we want to assign them specific "error"
meanings.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

[-- Attachment #1.2: signature.asc --]
[-- Type: application/pgp-signature, Size: 819 bytes --]

[-- Attachment #2: Type: text/plain, Size: 134 bytes --]

_______________________________________________
U-Boot mailing list
U-Boot@lists.denx.de
http://lists.denx.de/mailman/listinfo/u-boot