[PATCH v2 1/2] docs: admin-guide: fdt and initrd load in EFI stub

[Heinrich Schuchardt]

On 03.10.20 09:34, Atish Patra wrote:
> On Fri, Oct 2, 2020 at 11:38 PM Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>>
>> Describe how a device tree and an initial RAM disk can be passed to the EFI
>> Boot Stub.
>>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
>> ---
>> v2:
>>         mention EFI_GENERIC_STUB_INITRD_CMDLINE_LOADER (thx Atish)
>> ---
>>  Documentation/admin-guide/efi-stub.rst | 35 ++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/admin-guide/efi-stub.rst
>> index 833edb0d0bc4..4965dec48af4 100644
>> --- a/Documentation/admin-guide/efi-stub.rst
>> +++ b/Documentation/admin-guide/efi-stub.rst
>> @@ -38,6 +38,34 @@ arch/arm/boot/zImage should be copied to the system partition, and it
>>  may not need to be renamed. Similarly for arm64, arch/arm64/boot/Image
>>  should be copied but not necessarily renamed.
>>
>> +Passing an initial RAM disk to the EFI Boot Stub
>> +------------------------------------------------
>> +
>> +The following means sorted by decreasing priority can be used to provide an
>> +initial RAM disk to the EFI Boot Stub:
>> +
>> +* The firmware may provide a UEFI Load File 2 Protocol. The stub will try to
>> +  load the RAM disk by calling the LoadFile() service of the protocol using
>> +  a vendor device path with the vendor GUID
>> +  5568e427-0x68fc-4f3d-ac74-ca555231cc68.
>> +* Next the EFI stub will try to load the file indicated by the "initrd=" command
>> +  line parameter if CONFIG_EFI_GENERIC_STUB_INITRD_CMDLINE_LOADER is enabled.
>> +* The prior boot stage may pass the location of the initial RAM disk via the
>> +  "linux,initrd-start" and "linux,initrd-end" properties of the "/chosen" node
>> +  of the device-tree.
>> +
>
> Should we also specify which method is enabled by default for which
> ARCH and recommended methods?

The user relevant configuration is not the Linux' defconfig but what the
distribution maintainer has baked. I doubt mentioning Linux' defaults is
meaningful here.

>
> For example, It's recommended to use the LoadFile method for RISC-V
> and new ARM systems.

GRUB does not implement the LoadFile2 protocol yet. In U-Boot it is only
good for testing. I am not aware of usability with unmodified EDK II.
Why should we recommend anything before building the ecosystem that
makes it useful?

What is best may depend on the use case. There is nothing insecure in
passing the initrd via "linux,initrd-start" and "linux,initrd-end" if
you control the load options.

The EBBR (https://github.com/arm-software/ebbr) might be a better place
for a recommendation.

> Existing ARM ones will continue to use the initrd argument as that's
> the method enabled by default.

Only if if the LoadFile2 protocol is not available because that has a
higher priority for ARM, x86, and RISC-V.

Should I consider my i.mx6 Wandboard Quad bought in 2013 "old" while it
is running the U-Boot v2020.10-rc5, Linux v5.9-rc7, and Debian testing?
A distinction between "old" and "new" systems seems irrelevant here. All
are treated equal by the EFI stub.

>
>> +The first two items are inhibited by the "noinitrd" command line parameter.
>> +
>> +Passing a device-tree to the EFI Boot Stub
>> +------------------------------------------
>> +
>> +A device-tree can be passed to the EFI Boot Stub in decreasing priority using
>> +
>> +* command line option dtb=
>> +* a UEFI configuration table with GUID b1b621d5-f19c-41a5-830b-d9152c69aae0.
>> +
>
> I am just curious. Is there any specific reason why efistub tries to
> load the dtb from the command line first
> and loads from the config table only if it fails from the first approach ?

As we disable dtb= in secure boot it would make sense to turn the
priorities around for non-secure boot too.

But this is beyond the scope of a documentation patch.

Best regards

Heinrich

>
>> +The command line option is only available if CONFIG_EFI_ARMSTUB_DTB_LOADER=y
>> +and secure boot is disabled.
>>
>>  Passing kernel parameters from the EFI shell
>>  --------------------------------------------
>> @@ -46,6 +74,10 @@ Arguments to the kernel can be passed after bzImage.efi, e.g.::
>>
>>         fs0:> bzImage.efi console=ttyS0 root=/dev/sda4
>>
>> +The "noinitrd" option
>> +---------------------
>> +
>> +The "noinitrd" option stops the EFI stub from loading an initial RAM disk.
>>
>>  The "initrd=" option
>>  --------------------
>> @@ -98,3 +130,6 @@ CONFIGURATION TABLE.
>>
>>  "dtb=" is processed in the same manner as the "initrd=" option that is
>>  described above.
>> +
>> +This option is only available if CONFIG_EFI_ARMSTUB_DTB_LOADER=y and secure
>> +boot is disabled.
>> --
>> 2.28.0
>>
>>
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>
>
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