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Re: [Qemu-ppc] [Qemu-devel] [PATCH] PPC: E500: Generate device tree on r

From: Scott Wood
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH] PPC: E500: Generate device tree on reset
Date: Tue, 23 Jul 2013 16:55:21 -0500

On 07/23/2013 04:44:02 PM, Alexander Graf wrote:

Am 23.07.2013 um 23:19 schrieb Scott Wood <address@hidden>:

> On 07/23/2013 04:15:59 PM, Alexander Graf wrote:
>> On 23.07.2013, at 21:38, Scott Wood wrote:
>> > On 07/22/2013 10:28:17 AM, Alexander Graf wrote:
>> >> Today we generate the device tree once on machine initialization and then
>> >> store the finalized blob in memory to reload it on reset.
>> >> This is bad for 2 reasons. First we potentially waste a bunch of RAM for no >> >> good reason, as we have all information required to regenerate the device
>> >> tree available anyways.
>> >> The second reason is even more important. On machine init when we generate >> >> the device tree for the first time, we don't have all of the devices fully >> >> initialized yet. But the device tree needs to potentially walk devices to
>> >> put information about them into the device tree.
>> >
>> > If you can't produce the entire device tree at init time, how can you calculate its size with a dry run?
>> >
>> > Device trees are generally pretty small; couldn't we just set a maximum size and allocate that much space? >> It's what we do, unless we load it from the disk. In that case we take the fdt size from disk.
> So why do we need the dry run stuff?

Because dumpdtb otherwise generates a halfway complete dtb on the first dry pass as device realization is yet incomplete :).

What I mean is why have a first pass at all?


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