linux-kernel - Re: [PATCH v6 2/5] efi: Add embedded peripheral firmware support

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Date:   Fri, 1 Jun 2018 16:40:26 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Hans de Goede <hdegoede@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     Peter Jones <pjones@...hat.com>, Dave Olsthoorn <dave@...aar.me>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        David Howells <dhowells@...hat.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        dmitry.torokhov@...il.com, mfuzzey@...keon.com,
        Kalle Valo <kvalo@...eaurora.org>,
        Arend Van Spriel <arend.vanspriel@...adcom.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        nbroeking@...com, bjorn.andersson@...aro.org,
        Torsten Duwe <duwe@...e.de>, Kees Cook <keescook@...omium.org>,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/5] efi: Add embedded peripheral firmware support

On 06/01/2018 05:53 AM, Hans de Goede wrote:
> 
> Reported-by: Dave Olsthoorn <dave@...aar.me>
> Suggested-by: Peter Jones <pjones@...hat.com>
> Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Hans de Goede <hdegoede@...hat.com>
> ---
> ---
>  .../driver-api/firmware/request_firmware.rst  |  76 +++++++++
>  drivers/base/firmware_loader/Makefile         |   1 +
>  drivers/base/firmware_loader/fallback.h       |  12 ++
>  drivers/base/firmware_loader/fallback_efi.c   |  56 +++++++
>  drivers/base/firmware_loader/main.c           |   2 +
>  drivers/firmware/efi/Kconfig                  |   3 +
>  drivers/firmware/efi/Makefile                 |   1 +
>  drivers/firmware/efi/embedded-firmware.c      | 148 ++++++++++++++++++
>  include/linux/efi.h                           |   6 +
>  include/linux/efi_embedded_fw.h               |  25 +++
>  include/linux/fs.h                            |   1 +
>  init/main.c                                   |   3 +
>  12 files changed, 334 insertions(+)
>  create mode 100644 drivers/base/firmware_loader/fallback_efi.c
>  create mode 100644 drivers/firmware/efi/embedded-firmware.c
>  create mode 100644 include/linux/efi_embedded_fw.h
> 
> diff --git a/Documentation/driver-api/firmware/request_firmware.rst b/Documentation/driver-api/firmware/request_firmware.rst
> index f62bdcbfed5b..66ab91f3357d 100644
> --- a/Documentation/driver-api/firmware/request_firmware.rst
> +++ b/Documentation/driver-api/firmware/request_firmware.rst
> @@ -73,3 +73,79 @@ If something went wrong request_firmware() returns non-zero and fw_entry
>  is set to NULL. Once your driver is done with processing the firmware it
>  can call call release_firmware(fw_entry) to release the firmware image
>  and any related resource.
> +
> +EFI embedded firmware support
> +=============================
> +
> +On some devices the system's EFI code / ROM may contain an embedded copy
> +of firmware for some of the system's integrated peripheral devices and
> +the peripheral's Linux device-driver needs to access this firmware.
> +
> +A device driver which needs this can describe the firmware it needs
> +using an efi_embedded_fw_desc struct:
> +
> +.. kernel-doc:: include/linux/efi_embedded_fw.h
> +   :functions: efi_embedded_fw_desc
> +
> +The EFI embedded-fw code works by scanning all EFI_BOOT_SERVICES_CODE memory
> +segments for an eight byte sequence matching prefix, if the prefix is found it

                                                prefix; if

> +then does a crc32 over length bytes and if that matches makes a copy of length
> +bytes and adds that to its list with found firmwares.
> +
> +To avoid doing this somewhat expensive scan on all systems, dmi matching is
> +used. Drivers are expected to export a dmi_system_id array, with each entries'
> +driver_data pointing to an efi_embedded_fw_desc.
> +
> +To register this array with the efi-embedded-fw code, a driver needs to:
> +
> +1. Always be builtin to the kernel or store the dmi_system_id array in a
> +   separate object file which always gets builtin.
> +
> +2. Add an extern declaration for the dmi_system_id array to
> +   include/linux/efi_embedded_fw.h.
> +
> +3. Add the dmi_system_id array to the embedded_fw_table in
> +   drivers/firmware/efi/embedded-firmware.c wrapped in a #ifdef testing that
> +   the driver is being builtin.
> +
> +4. Add "select EFI_EMBEDDED_FIRMWARE if EFI_STUB" to its Kconfig entry.
> +
> +The request_firmware() function will always first try to load firmware with
> +the specified name directly from the disk, so the EFI embedded-fw can always
> +be overridden by placing a file under /lib/firmare.

                                         /lib/firmware.

> +
> +To make request_firmware() fallback to trying EFI embedded firmwares after this,
> +the driver must set a boolean "efi-embedded-firmware" device-property on the
> +device before passing it to request_firmware(). Note that this disables the
> +usual usermodehelper fallback, so you may want to only set this on systems
> +which match your dmi_system_id array.
> +
> +Once the device-property is set, the driver can use the regular
> +request_firmware() function to get the firmware, using the name filled in
> +in the efi_embedded_fw_desc.
> +
> +Note that:
> +
> +1. The code scanning for EFI embbedded-firmware runs near the end
> +   of start_kernel(), just before calling rest_init(). For normal drivers and
> +   subsystems using subsys_initcall() to register themselves this does not
> +   matter. This means that code running earlier cannot use EFI
> +   embbedded-firmware.
> +
> +2. ATM the EFI embedded-fw code assumes that firmwares always start at an offset

s/ATM/At the moment/

> +   which is a multiple of 8 bytes, if this is not true for your case send in

                               bytes; if

> +   a patch to fix this.
> +
> +3. ATM the EFI embedded-fw code only works on x86 because other archs free

At the moment

> +   EFI_BOOT_SERVICES_CODE before the EFI embedded-fw code gets a chance to
> +   scan it.
> +
> +4. The current brute-force scanning of EFI_BOOT_SERVICES_CODE is an ad-hoc
> +   brute-force solution. There has been discussion to use the PI spec's
> +   Firmware Volume protocol. This has been rejected because the FV Protocol
> +   relies on *internal* interfaces of PI spec, and:
> +   1. The The PI spec does not define firmware at all
> +   2. The internal interfaces of PI Spec does not guarantee any backward
> +      compatibility. Any implementation details in FV may be subject to change,
> +      and may vary system to system. Supporting the FV Protocol would be
> +      difficult as it is purposely ambiguous.

What/where is this PI spec?

thanks,
-- 
~Randy