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 12:26:14 -0600	[thread overview]
Message-ID: <98217e0b-e937-f5b3-fa16-57c79b6ae3f6@gmail.com> (raw)
In-Reply-To: <5e552b57-4e8b-6774-577d-4fa7a8d440ba@gmail.com>

Hi Uwe,

On 1/26/21 12:03 PM, Frank Rowand wrote:
> +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:.

Unfortunately I have not been on irc recently (now rectified).  Do you
perchance have a copy of the irc conversation that you can send me?
(No need to edit out unrelated messages, a simple cut and paste from
the start of the conversation to the end is fine.)

-Frank

>>
>> 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
>>
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>
> 