Re: [RFC PATCH 0/1] Add driver for bootstage stash

[Federico Giovanardi <fede.dev@giovanardi.dev>]

Hello,

The note about the data format also was my initial thought, by just 
copying a C structure we might have issues as soon one party changes it, 
and they might not be perfectly aligned.

To avoid inventing yet-another-data-format I've used msgpack in the past 
for that (the format 
https://github.com/msgpack/msgpack/blob/master/spec.md, not the library  
); because the specs are so simple they can be implemented in a few 
lines, and it's something with a reference. But I don't have a lot of 
experience in upstreaming stuff on the kernel, so I don't know if it 
might cause someone to don't be happy. Anyway, I can contribute the 
implementation if needed.

Something as simple as an array of fixarray will give extensibility with 
only a few bytes of overhead.

Which gets encoded as:

0xdc # lenght 16 bit << array header
      # 0xB << 4 | ( array_size & 0xF) << fixarray header ( 3 elements, 
simplest case)
            # 0xce # time_us
            # 0xce # start_us
            # 0xc << 4 | strlen(name) # name
            /*no flags, no id*/
      # 0xB << 4 | ( array_size & 0xF) << fixarray header ( 5 elements 
bigger case)
            # 0xce # time_us
            # 0xce # start_us
            # 0xc << 4 | strlen(name) # name
            # 0xcc # flags
            # 0xcc # id
      .. repeat ...


Since the goal is to use that in many different contexts, defining the 
fields that we need early is important.

Bye
Federico

> -------------------------
> 
> Da: Francesco Valla <francesco@valla.it>
> Inviato: venerdì 23 maggio 2025 00:42
> A: linux-embedded@vger.kernel.org <linux-embedded@vger.kernel.org>
> Oggetto: [RFC PATCH 0/1] Add driver for bootstage stash
> 
> Questo messaggio proviene da un mittente esterno: fai attenzione ad
> allegati e link.
> 
> Hello,
> 
> after the discussion on the "Unified Boot Log" topic during the latest
> Boot Time SIG special meeting [1], I tried to mock up a driver that
> reads a bootstage stash saved by the U-Boot bootloader in a given
> memory
> area and exposes the data in a user- and machine- friendly through
> both
> sysfs and debugfs attributes. Details on the interfaces, as well as
> example output for the debugfs interfaces, can be found on the
> documentation that is part of the patchset.
> 
> To use this driver, a memory area shall be reserved inside the Linux
> kernel devicetree as follows (possibly changing the address and the
> size
> of the memory area):
> 
>     bootstage@a4300000 {
>         compatible = "bootstage";
>         reg = <0 0xa4300000 0 0x1000>;
>         no-map;
>     };
> 
> At U-Boot side, following configuration shall then be set:
> 
>     CONFIG_BOOTSTAGE=y
>     CONFIG_BOOTSTAGE_STASH_ADDR=0xa4300000
>     CONFIG_BOOTSTAGE_STASH_SIZE=0x1000
> 
> Once booted, the bootstage data can will be found at:
> 
>     - /sys/devices/platform/a4300000.bootstage/
>     - /sys/kernel/debug/bootstage/a4300000.bootstage/
> 
> The device name is purposely part of the sysfs and debugfs paths to
> support multiple bootstage areas, as this _might_ then be used for
> multiple bootstage sources, e.g. bootloaders running on different
> cores inside a SoC with different architectures.
> 
> Note that this is not really meant to be integrated as-is, not only
> because it's a single patch including code, documentation and
> devicetree
> bindings, but also because the bootstage stash format itself may need
> to
> be touched up a bit. In particular, fixed data type should probably be
> evaluated for the bootstage record, in order to increase compatibility
> with different data sources.
> 
> Comments are of course welcome.
> 
> Regards,
> 
> Francesco
> 
> [1]
> https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-embedded%2FMW5PR13MB5632B8FA3279D77F2F9217BBFD9CA%40MW5PR13MB5632.namprd13.prod.outlook.com%2F&data=05%7C02%7Cfederico.giovanardi%40cnh.com%7C68aacb29d80340fc5d3208dd99904415%7C79310fb0d39b486bb77b25f3e0c82a0e%7C0%7C0%7C638835567100175385%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Hrd3rjtJ7sLciUzkFymL7y2agCAMZSAKKF7evt20LQU%3D&reserved=0
> [1]
> 
> Francesco Valla (1):
>   drivers: misc: add driver for bootstage stash
> 
>  .../bindings/reserved-memory/bootstage.yaml   |  44 +++
>  Documentation/misc-devices/bootstage.rst      |  53 ++++
>  Documentation/misc-devices/index.rst          |   1 +
>  MAINTAINERS                                   |   7 +
>  drivers/misc/Kconfig                          |  10 +
>  drivers/misc/Makefile                         |   1 +
>  drivers/misc/bootstage.c                      | 292
> ++++++++++++++++++
>  drivers/of/platform.c                         |   1 +
>  8 files changed, 409 insertions(+)
>  create mode 100644
> Documentation/devicetree/bindings/reserved-memory/bootstage.yaml
>  create mode 100644 Documentation/misc-devices/bootstage.rst
>  create mode 100644 drivers/misc/bootstage.c
> 
> --
> 2.49.0
> 
> 
> 
> Links:
> ------
> [1] 
> https://lore.kernel.org/linux-embedded/MW5PR13MB5632B8FA3279D77F2F9217BBFD9CA@MW5PR13MB5632.namprd13.prod.outlook.com/