On Mon, 12 Jan 2015 10:42:27 -0700 Stephen Warren swarren@wwwdotorg.org wrote:
On 01/10/2015 11:27 AM, Dennis Gilmore wrote:
On Tue, 06 Jan 2015 17:43:19 -0700 Stephen Warren swarren@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.
or perhaps use the ESP flag. though that might be totally confusing for all.
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.
Right, bios uses the bootable flag, UEFI uses the ESP partition which is why I guess GPT has the bootable flag as a legacy option. I'm in agreement with you on honouring the bootable flag. I was just trying to point out that if you put say a usb stick in a machine that had a live image installed on it that's what the x86 system would boot.
Dennis