On 01/10/2015 11:27 AM, Dennis Gilmore wrote:
On Tue, 06 Jan 2015 17:43:19 -0700
Stephen Warren <swar...@wwwdotorg.org> wrote:
(CCing Dennis so he can comment from a distro perspective re:
partition table bootable flags v.s. scanning all partitions)
On 01/06/2015 10:07 AM, Sjoerd Simons wrote:
On Mon, 2015-01-05 at 13:24 -0700, Stephen Warren wrote:
On 01/05/2015 10:13 AM, Sjoerd Simons wrote:
Not all devices use the convention that the boot scripts are on
the first partition. For example on chromebooks it seems common
for the first two partitions to be ChromeOS kernel partitions.
ChromeOS seems to have adopted its own unique setup. it is not a
typical configuration.
So instead of just the first partition scan all partitions on a
device with a filesystem u-boot can recognize.
I had planned (but obviously never got around to...) enhancing the
scripts to look up the (set of?) bootable partition(s) on the disk
and to attempt to load the boot files from there. Bootable would
be defined as the MBR bootable flag, or GPT legacy bootable
attribute.
That would allow the code to zero in on the one specific partition
that it was supposed to look at, rather than searching all
partitions.
Do you have any thoughts re: which option is better?
I did wonder about this as well. I do personally consider the
bootable flag as a rather obsolete/legacy thing (GPT even specifies
it as a legacy flag), so i was wary about using it.. Also i've been
bitten a few times on systems that did rely on the bootable flag
(what, what, why does it not boot, oooooohhhh), which was another
reason for heading this route.
I really like the idea of using the bootable flag and looking at it but
if its legacy in GPT will it go away in some future partition table
layout? UEFI Requires that a ESP exist. I think requiring that the
bootable flag exist is acceptable.
One other alternative for GPT is to invent a new partition type UUID for
bootable partitions. This likely has more implications though, since any
tool that looks at the partition type UUID would have to be updated. I
have no idea how many such tools exist though.
This way does no extra work if the first partition is the partition
with the boot partition when compared to only checking partitions
with the bootable flag as both would need to list existing
partitions.
If the first few partitions have no filesystems, the extra work
compared to the bootable-flag approach would just be probing the
filesystem type, which tends to be relatively simple, so i don't
see a big issue there (it's more work to scan for a missing boot
file).
If your first few partitions are ones without the bootfiles, some
more effort is wasted as it will be probing those for viable boot
files.. However, in my experience, partition layouts with the
bootfiles not on the first filesystem partitions is rather
uncommmon. So again, i didn't feel that that was problematic. If
you have an odd parition layout, your boot time will be ever so
slightly longer :)
The only "issue" in my mind is when multiple partitions, for
whatever reason, have bootfiles. In which case the first one will
get picked with this approach, while with the partition-boot-flag
approach you'd have a way to specify, no really just look at that
one.. However, i suspect the likelihood of forgetting to set the
boot flag is higher (been there, done that) then accidentally
leaving boot files on partitions before the intended boot partition
(which also requires on uncommon layout), so even then i suspect
this approach is more friendly/less error-prone.
This patch looks fine assuming this option (rather than bootable
flag) is selected.
Well my thoughts on the matter are above, If folks feel strongly
about this approach being the wrong way I'd love to hear their
arguments :).
One issue with this approach is that there's no way for the user to
short-circuit the scanning. If I put a ChromiumOS install on an SD
card and leave it plugged into a system that's going to end up
booting from eMMC since that's where the boot files are, there are
lots of partitions to scan on that SD card, which will be a bit
annoying.
That is what happens on x86 today though. if you had a bootable
cdrom/dvdrom or usb stick it will boot from that before the local
install.
x86 doesn't search all the partitions though, only those marked with the
bootable flag. That's why I'm trying to drive the standard distro boot
process (as implemented by U-Boot) to honor the bootable flag and ignore
other partitions.
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