On 10/19/23 23:05, Jon Humphreys wrote:
Hi all. I need clarification on where firmware should be located when
booting from removable shared storage. Statements in the intro of 4.2
seem to contradict later statements in 4.2.2 of the EBBR [1].
Section 4.2.2 states that to make it easier to use stock OS images on
removable storage, firmware should be located on the ESP partition (so
you don't have to create a dedicated firmware partition).
On removable media, firmware should be stored in the ESP under the
/FIRMWARE directory structure as described in Firmware Partition
Filesystem.
However, prior in the 4.2 section, it states:
The sections below discuss the requirements when using both fixed and
removable storage. However, it should be noted that the recommended
behaviour of firmware should be identical regardless of storage type.
In both cases, the recommended boot sequence is to first search for
firmware in a dedicated firmware partition, and second search for
firmware in the ESP.
Why would the boot sequence for shared storage look in two places if it
is always in the ESP? Or does the statement in 4.2.2 only apply in the
case of using a stock OS image?
High Jon,
thanks for pointing out where the specification is unclear.
I guess a half-sentence is missing saying "if both a dedicated firmware
partition and the EFI system partition are both supported by the firmware".
The EBBR is still evolving. If you have a suggestion for improvement,
please, create an incident or merge request on Github. Consider joining
one of our meetings. See
https://github.com/ARM-software/ebbr/wiki/EBBR-Meetings#datetime
As a side question, I'm not sure of the practicality of recommending a
boot sequence that looks in two places, at least for the very first
firmware stages, as ROM usually is very simple and only looks in a
single location.
The documentation is not only about what a boot ROM does. E.g. I have
seen a boot ROM loading U-Boot SPL from flash and U-Boot SPL loading EDK
II from /EFI/vendor/board/firmware.itb. But U-Boot SPL in this case
would only scan the ESP.
Best regards
Heinrich
thanks
Jon
[1] https://arm-software.github.io/ebbr/
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