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From: Marek Vasut <marek.vasut-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
To: David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>
Cc: Frank Rowand
	<frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Devicetree Compiler
	<devicetree-compiler-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"devicetree-spec-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-spec-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Jon Loeliger <jdl-CYoMK+44s/E@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Pantelis Antoniou
	<pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>,
	"Pantelis
	Antomarek.vasut-Re5JQEeQqe/2sr8fMPgRzw@public.gmane.org"
	<panto-wVdstyuyKrO8r51toPun2/C9HSW9iNxf@public.gmane.org>,
	Grant Likely
	<grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>,
	Tom Rini <trini-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>,
	Kyle Evans <kevans-h+KGxgPPiopAfugRpC6u6w@public.gmane.org>,
	Geert Uytterhoeven
	<geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>,
	Alan Tull <atull-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>
Subject: Re: [RFC] devicetree: new FDT format version
Date: Sat, 27 Jan 2018 21:30:11 +0100	[thread overview]
Message-ID: <5a943937-3b59-514b-3939-df25daea5470@gmail.com> (raw)
In-Reply-To: <20180125123734.GC2099@umbus>

On 01/25/2018 01:37 PM, David Gibson wrote:
> On Thu, Jan 25, 2018 at 10:14:38AM +0100, Marek Vasut wrote:
>> On 01/23/2018 10:17 PM, Frank Rowand wrote:
>> [...]
>>> My knowledge of bootloader use of the FDT is nearly non-existent, so
>>> I'm on thin ice here.  But I am guessing that a lot of bootloader
>>> processing of FDT data could be removed if the kernel would unflatten
>>> a chained FDT and overlay FDT(s).
>>>
>>> Comments from the bootloader folks on this topic would be much
>>> appreciated.
>>
>> U-Boot, when using DT as a hardware-description (rather than just
>> passing it to Linux) usually operates on the FDT, unless explicitly
>> configured to unflatten the tree. That later option is not enabled by
>> default.
>>
>> The reason for that is so that the DT can be accessed very early on, at
>> which point there may not be any other memory available than a very
>> limited stack and the FDT blob can still be located in some read-only
>> memory.
> 
> Right, this is exactly the reason that the fdt format is designed so
> you can read thing from it directly.  If you're doing more than very
> minor modifications, however, it does become worth unflattening.

Right

> Note that while unflatenning requires an allocator of sorts, it can be
> a trivial one.  A bump allocator in a fixed buffer of reasonable size
> would do fine.  You don't realy need a free(), since the bootloader
> has limited lifetime you can just leak nodes until you throw the lot
> away.

There's a regular mallocator in later stages of u-boot and a small
allocator for the early stages if needed. It's only the early one that
doesn't implement free() and it's only used for a very short time when
the system is still running from flash.

Thus far, I don't see a usecase for unflattening the tree that early,
but maybe one will come in the future.In the later stages, the design is
that you do unflatten only if you have a system which benefits from it
and has the resources to do that. Modern systems usually do, but there
are systems with weird limitations.

Since you can do a lot more than just booting with u-boot, leaking any
memory is a really bad idea.

> You can also use pointers into the flattened tree blob to avoid
> allocating space for the actual property values (except for ones you
> add later, obviously).  And if you make extensive enough changes that
> leaked pieces really do become a problem, you could do a
> flatten/unflatten pass to recompact everything.

-- 
Best regards,
Marek Vasut

  reply	other threads:[~2018-01-27 20:30 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-22  8:09 Frank Rowand
     [not found] ` <b96829f9-2e8b-fdc5-5090-58591e2260cf-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-22  8:14   ` Frank Rowand
     [not found]     ` <ec68cfb3-4702-ba00-e926-3ad63b20b199-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-22 20:08       ` Frank Rowand
2018-01-22 20:08   ` Frank Rowand
     [not found]     ` <bc44f051-835d-1c8d-a928-be0fd4ef80b5-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25  0:27       ` Frank Rowand
     [not found]         ` <b5a72db1-45b2-f21f-9afd-0991b288840e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-27  8:48           ` David Gibson
2018-01-29  8:08             ` Frank Rowand
     [not found]               ` <2c352396-154e-ea75-c50b-0f2bfafd19da-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29 10:56                 ` David Gibson
2018-01-30  1:29                   ` Frank Rowand
2018-01-22 21:01   ` Rob Herring
     [not found]     ` <CAL_JsqJR2y7bMw_-9TBAGWZ_kf7_sZo5qvqvRowJ8jiy=4G0Bg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-23 12:42       ` David Gibson
2018-01-23 21:17         ` Frank Rowand
     [not found]           ` <20328477-e511-e875-7dc4-253640f2219e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-24 15:47             ` Rob Herring
     [not found]               ` <CAL_Jsq+fvFrGhqO0zbEUE_i23FkU=G4Z1-e0vnXHi4KbS2oK0A-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-24 21:16                 ` Frank Rowand
     [not found]                   ` <e1bec77d-4dac-200b-34be-23573bf738f0-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-24 22:27                     ` Alan Tull
2018-01-25  0:22                     ` Frank Rowand
     [not found]                       ` <e986435b-481b-2629-7600-10d9e21ac58e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25 12:29                         ` David Gibson
2018-01-25 20:01                           ` Frank Rowand
     [not found]                             ` <72d30756-a963-92c9-1838-3e3f80c57e39-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29 18:32                               ` Grant Likely
     [not found]                                 ` <CACxGe6tUB3NhNhOtqzoJfThx=RE1_=TSDQz+wQUm=sdE5JirZQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-29 23:15                                   ` David Gibson
2018-01-26  8:56                           ` Geert Uytterhoeven
     [not found]                             ` <CAMuHMdV0jdj90uzqMx_wtvz=-KaagJG2_UQTm1DW3gzt6cNG6A-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-26  8:59                               ` Geert Uytterhoeven
2018-01-26 22:08                               ` Frank Rowand
     [not found]                                 ` <83008da0-7383-ba2d-a239-e11ad7d1327d-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-27  9:00                                   ` David Gibson
2018-01-27  8:56                               ` David Gibson
2018-01-25 23:11                     ` Frank Rowand
2018-01-25 12:22                 ` David Gibson
2018-01-25  9:14             ` Marek Vasut
     [not found]               ` <90983180-ae3b-5a31-9dc0-b62b978a0fba-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25 12:37                 ` David Gibson
2018-01-27 20:30                   ` Marek Vasut [this message]
     [not found]                     ` <5a943937-3b59-514b-3939-df25daea5470-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29  0:53                       ` David Gibson
2018-01-23 12:05   ` David Gibson
2018-01-23 21:28     ` Frank Rowand

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