linux-kernel - Re: [PATCH] ARM: /proc/cpuinfo: Use DT machine name when possible

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Date:	Thu, 4 Dec 2014 12:10:30 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Sebastian Reichel <sre@...ian.org>,
	Pavel Machek <pavel@....cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH] ARM: /proc/cpuinfo: Use DT machine name when possible

On Thu, Dec 4, 2014 at 11:57 AM, Pali Rohár <pali.rohar@...il.com> wrote:
> On Thursday 04 December 2014 17:49:40 Rob Herring wrote:
>> On Wed, Dec 3, 2014 at 6:48 PM, Pali Rohár
> <pali.rohar@...il.com> wrote:
>> > On Thursday 04 December 2014 01:33:08 Rob Herring wrote:
>> >> On Mon, Nov 24, 2014 at 4:19 PM, Pali Rohár
>> >
>> > <pali.rohar@...il.com> wrote:
>> >> > On Wednesday 18 June 2014 22:46:19 Rob Herring wrote:
>> >> >> On Wed, Jun 18, 2014 at 2:22 PM, Pali Rohár
>> >> >
>> >> > <pali.rohar@...il.com> wrote:
>> >> >> > On Wednesday 18 June 2014 21:07:35 Rob Herring wrote:
>> >> >> >> On Wed, Jun 18, 2014 at 11:54 AM, Pali Rohár
>> >> >> >
>> >> >> > <pali.rohar@...il.com> wrote:
>> >> >> >> > Machine name from board description is some generic
>> >> >> >> > name on DT kernel. DT provides machine name
>> >> >> >> > property which is specific for board, so use it
>> >> >> >> > instead generic one when possible.
>> >>
>> >> [...]
>> >>
>> >> >> > Basically in DT kernel is missing Hardware, Revision
>> >> >> > and probably also Serial key. (Now I used only qemu
>> >> >> > for testing which set serial key to 0). All these
>> >> >> > informations is used by userspace applications which
>> >> >> > determinate how to behave.
>> >> >>
>> >> >> It is somewhat fragile to expect the name in the DT to
>> >> >> match the old name from the kernel. As your patch to
>> >> >> n900 dts shows, we'd probably have to update every dts
>> >> >> file to make them match. While I think we should work
>> >> >> to remove this string from the kernel, userspace
>> >> >> depending on the DT model string is a bad idea. For
>> >> >> example, if you had some platform with multiple OEMs
>> >> >> just rebranding the same base design, they may all want
>> >> >> to put their own model string into each product. The
>> >> >> true h/w id is the compatible string.
>> >> >>
>> >> >> Serial number is easy. There is already a standard
>> >> >> although not widely used property for it with
>> >> >> "/serial-number". You just need to wire it up to
>> >> >> cpuinfo.
>> >> >
>> >> > There is no "/serial-number" property in kernel:
>> >> >
>> >> > $ ls -l -a /sys/bus/soc/devices/soc0/
>> >> > drwxr-xr-x    3 0        0                0 Jan  1 00:00
>> >> > . drwxr-xr-x   18 0        0                0 Jan  1
>> >> > 00:00 .. -r--r--r--    1 0        0             4096 Jan
>> >> >  1 00:00 family -r--r--r--    1 0        0
>> >> > 4096 Jan  1 00:00 machine drwxr-xr-x    2 0        0
>> >> >            0 Jan  1 00:00 power -r--r--r--    1 0
>> >> > 0
>> >> > 4096 Jan  1 00:00 revision lrwxrwxrwx    1 0        0
>> >> >
>> >> >          0 Jan  1 00:00 subsystem -> ../../bus/soc
>> >> >
>> >> > -r--r--r--    1 0        0             4096 Jan  1 00:00
>> >> > type -rw-r--r--    1 0        0             4096 Jan  1
>> >> > 00:00 uevent
>> >>
>> >> Wrong place. If you put /serial-number in your DT, then it
>> >> will be in /proc/device-tree/serial-number.
>> >>
>> >> If you want to wire that up to /proc/cpuinfo, I would be
>> >> fine with that.
>> >>
>> >> Rob
>> >
>> > I cannot hardcode it into DT, because it is set by
>> > bootloader. And bootloader tell it to kernel via ATAGs
>> > structure. Bootloader is closed and proprietary and cannot
>> > be replaced (there is no documentation for hw init).
>>
>> Either the bootloader can put the serial number into the DTB
>> instead of ATAGs or the kernel decompressor can copy it from
>> ATAGs to DTB like is done already for other ATAGs. I presume
>> you cannot update the bootloader and will need the latter.
>>
>> Rob
>
> Rob, thanks for info! Btw, theoretically I can update bootloader
> if someone write it. So in practise I need to use second option.
>
> Can you tell me which CONFIG_ needs to be enabled for kernel
> decompressor (so it copy some ATAGs) and where to start looking
> at code (for adding new code for copying other ATAGs)?

See CONFIG_ARM_ATAG_DTB_COMPAT and arch/arm/boot/compressed/atags_to_fdt.c.

Rob
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