linux-kernel - Re: [PATCH] of/lib: Export fdt routines to modules

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Date:	Fri, 18 Oct 2013 12:32:29 -0700
From:	Michael Bohan <mbohan@...eaurora.org>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Rob Herring <robherring2@...il.com>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Guenter Roeck <linux@...ck-us.net>,
	David Gibson <david@...son.dropbear.id.au>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	rob.herring@...xeda.com, ralf@...ux-mips.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	david.daney@...ium.com, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] of/lib: Export fdt routines to modules

On Fri, Oct 18, 2013 at 09:30:32AM -0700, David Daney wrote:
> On 10/18/2013 08:57 AM, Rob Herring wrote:
> [...]
> >
> >Unflattening is definitely the right
> >direction to go here.
> >
> 
> I wonder if that is really true.
> 
> The device tree in question is very short lived, and used to control
> the configuration of some hardware device when loading the driver.
> 
> The use of it is completely contained within a single driver (at
> least that is my understanding), it is not information that needs to
> be shared system wide.

That's correct.

> Given that it is a driver implementation issue, rather than making
> things work nicely system wide, I don't think it really matters what
> is done.
> 
> It may be that the overhead of unflattening the tree and then
> freeing it, is much greater than just extracting a few things from
> the FDT.

Yes, this was my original thought as well. On the other hand,
having libfdt in the kernel does add a little extra bloat, and so
it seems a tradeoff from one-time runtime overhead to footprint.

> That said, I don't really have a preference for what is done.  My
> original questions were targeted at understanding this particular
> use case.

My preference is probably straight libfdt calls, but if others
think that unpacking is a better solution, I'm able to go that
route as well. My only concern there is that we provide a means
to detect invalid dtb image (ex. handle error codes) and also
free the device_node allocations once the device is released.

Thanks,
Mike

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