linux-kernel - Re: [PATCH 1/4] of: Add of_parse_phandle_with_opt

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Date:	Wed, 30 Sep 2015 16:39:30 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	Marc Zyngier <marc.zyngier@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Cc:	Mark Rutland <Mark.Rutland@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rob Herring <robh+dt@...nel.org>,
	Jason Cooper <jason@...edaemon.net>
Subject: Re: [PATCH 1/4] of: Add of_parse_phandle_with_opt_args() helper
 function

Hi Marc,

On 22/09/15 18:52, Marc Zyngier wrote:
> of_parse_phandle_with_args() is slightly inflexible as it doesn't
> allow the (unusual) case where the #*-cells property is not defined.
> In order to support this, introduce of_parse_phandle_with_opt_args()
> which assumes that #*-cells is zero when it is not defined,
> as required by the msi-parent binding
>
> This is done by turning __of_parse_phandle_with_args into an even
> bigger monster, which is a bit frightening.

A monster indeed; I can't quite figure out the exact effect this change 
has on of_count_phandle_with_args(), but I have a lingering doubt it may 
be something undesirable, since AFAICS that's now going to proceed from 
where it would have errored out before, with a count of -2.

I think it might be nicer to implement this by passing an extra "assume 
zero if #cells not found" boolean to __of_parse_phandle_with_args().

Alternatively, what's the actual likelihood of legacy bindings being 
mixed in with new ones? Could we not simply mandate that anyone adding 
an MSI controller with #msi-cells to a DT must ensure any existing nodes 
are also updated with #msi-cells = 0, and keep the legacy workaround 
self-contained in the MSI layer? e.g. paraphrasing from patch 2/2:

msi_np = of_parse_phandle(np, "msi-parent", 0);
if (!of_property_read_bool(msi_np, "#msi-cells"))
	return parse_this_thing(...);
else
	while (!of_parse_phandle_with_opt_args(np, "msi-parent"...
		if (parse_this_thing(...))
			return;

Robin.

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