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Subject: Re: [PATCH 1/1] docs: admin-guide: fdt and initrd load in EFI stub
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        linux-efi <linux-efi@vger.kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        =?UTF-8?Q?Fran=c3=a7ois_Ozog?= <francois.ozog@linaro.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>
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From:   Heinrich Schuchardt <xypron.glpk@gmx.de>
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Date:   Fri, 2 Oct 2020 19:50:33 +0200
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On 02.10.20 19:21, Ard Biesheuvel wrote:
> Hi Heinrich,
>
> Thanks for documenting this.
>
>
> On Fri, 2 Oct 2020 at 19:11, Heinrich Schuchardt <xypron.glpk@gmx.de> wr=
ote:
>>
>> 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@gmx.de>
>> ---
>>  Documentation/admin-guide/efi-stub.rst | 35 ++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/efi-stub.rst b/Documentation/adm=
in-guide/efi-stub.rst
>> index 833edb0d0bc4..86f50a33884c 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 provi=
de 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=
=3D" command
>> +  line parameter.
>> +* The prior boot stage may pass the location of the initial RAM disk v=
ia the
>> +  "linux,initrd-start" and "linux,initrd-end" properties of the "/chos=
en" node
>> +  of the device-tree.
>> +
>
> On x86, the boot_params struct is used to pass the address and size of
> the initrd in memory. Maybe include that for completeness?

Sure we should add it. But I will just wait for more review comments.

>
>> +The first two items are inhibited by the "noinitrd" command line param=
eter.
>> +
>
> Interesting. Are you saying noinitrd is ignored by the kernel itself?
>
> Looking at the code, it might only work for preventing the load of old
> style initrd ramdisks, whereas initramfs images are handled
> separately.
>
> This is something that we should probably fix one way or the other.
>

initrd_load() seems to depend on the value and will not create /dev/ram
if "noinitrd" is set.
init/do_mounts_initrd.o is compiled for ARMv8.

But my ARMv8 Odroid C2 boots fine via U-Boot->GRUB->EFI stub->Linux with:

[  +0.000000] Kernel command line: BOOT_IMAGE=3D/vmlinuz-5.9.0-rc6-arm64+
root=3DUUID=3D.. ro earlycon=3Dmeson,0xc81004c0,115200n8 noinitrd

So I assume initrd_load() is either not called or at least not needed
for the FDT case.

Best regards

Heinrich

>
>> +Passing a device-tree to the EFI Boot Stub
>> +------------------------------------------
>> +
>> +A device-tree can be passed to the EFI Boot Stub in decreasing priorit=
y using
>> +
>> +* command line option dtb=3D
>> +* a UEFI configuration table with GUID b1b621d5-f19c-41a5-830b-d9152c6=
9aae0.
>> +
>> +The command line option is only available if CONFIG_EFI_ARMSTUB_DTB_LO=
ADER=3Dy
>> +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=3DttyS0 root=3D/dev/sda4
>>
>> +The "noinitrd" option
>> +---------------------
>> +
>> +The "noinitrd" option stops the EFI stub from loading an initial RAM d=
isk.
>>
>>  The "initrd=3D" option
>>  --------------------
>> @@ -98,3 +130,6 @@ CONFIGURATION TABLE.
>>
>>  "dtb=3D" is processed in the same manner as the "initrd=3D" option tha=
t is
>>  described above.
>> +
>> +This option is only available if CONFIG_EFI_ARMSTUB_DTB_LOADER=3Dy and=
 secure
>> +boot is disabled.
>> --
>> 2.28.0
>>