Re: [PATCH] cmd_dtc: Enable generation of device tree symbols

From: Frank Rowand <frowand.list@gmail.com>
To: "Uwe Kleine-König" <uwe@kleine-koenig.org>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Frank Rowand" <frowand.list@gmail.com>
Cc: DTML <devicetree@vger.kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Arnd Bergmann <arnd@arndb.de>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Maxime Ripard <mripard@kernel.org>,
	cyril@debamax.com, Geert Uytterhoeven <geert@linux-m68k.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] cmd_dtc: Enable generation of device tree symbols
Date: Tue, 26 Jan 2021 12:03:14 -0600	[thread overview]
Message-ID: <5e552b57-4e8b-6774-577d-4fa7a8d440ba@gmail.com> (raw)
In-Reply-To: <9d9bb0f6-d4f4-b1b9-a4c4-786987578085@kleine-koenig.org>

+frank

On 1/26/21 1:20 AM, Uwe Kleine-König wrote:
> Hello Masahiro,
> 
> On 1/25/21 10:53 PM, Masahiro Yamada wrote:
>> On Mon, Jan 25, 2021 at 8:07 PM Uwe Kleine-König <uwe@kleine-koenig.org> wrote:
>>>
>>> Adding the -@ switch to dtc results in the binary devicetrees containing
>>> a list of symbolic references and their paths. This is necessary to
>>> apply device tree overlays e.g. on Raspberry Pi as described on
>>> https://www.raspberrypi.org/documentation/configuration/device-tree.md.
>>>
>>> Obviously the downside of this change is an increas of the size of the
>>> generated dtbs, for an arm out-of-tree build (multi_v7_defconfig):
>>>
>>>          $ du -s arch/arm/boot/dts*
>>>          101380  arch/arm/boot/dts-pre
>>>          114308  arch/arm/boot/dts-post
>>>
>>> so this is in average an increase of 12.8% in size.
>>>
>>> Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
>>
>>
>> (CCing DT ML.)
> 
> makes sense, thanks.
> 
>> https://www.spinics.net/lists/linux-kbuild/msg27904.html
>>
>> See Rob's comment:
>>
>> "We've already rejected doing that. Turning on '-@' can grow the dtb
>> size by a significant amount which could be problematic for some
>> boards."
> 
> The patch was created after some conversation on irc which continued
> after I sent the patch. I added the participating parties to Cc:.
> 
> The (relevant) followups were:
> 
> Geert suggested to always generate the symbols and provide a way to
> strip the symbols for installation if and when they are not needed.
> 
> Rob said: "I'm less concerned with the size increases, but rather that
> labels go from purely source syntax to an ABI. I'd rather see some
> decision as to which labels are enabled or not."
> 
> And then I learned with hints from Rob and Geert that symbols are not
> really necessary for overlays, you just cannot use named labels. But
> using
> 
>     target-path = "/soc/i2c@23473245";
> 
> or
> 
>     target = <&{/soc/i2c@23473245}>;
> 
> instead of
> 
>     target = <&i2c1>;
> 
> works fine. (And if you need to add a phandle the &{/path/to/node}
> construct should work, too (but I didn't test).) Using labels is a tad nicer, but the problem I wanted to address with my patch now has a known different solution.
> 
> Best regards
> Uwe
> 
> 
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