[U-Boot] [PATCH 3/4] config_distro_bootcmd: Scan all partitions for boot files

[Sjoerd Simons]

On Wed, 2015-01-07 at 11:17 +0000, Ian Campbell wrote:
> On Wed, 2015-01-07 at 12:01 +0100, Sjoerd Simons wrote:
> > On Wed, 2015-01-07 at 10:22 +0000, Ian Campbell wrote:
> > > On Wed, 2015-01-07 at 11:10 +0100, Sjoerd Simons wrote:
> > > > On Tue, 2015-01-06 at 17:43 -0700, Stephen Warren 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:
> > > > 
> > > > > > 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.
> > > > 
> > > > 
> > > > I don't remember exactly how many partitions with fat/ext* filesystems a
> > > > ChromiumOS installation has (order of 3-5 iirc?), but indeed it means
> > > > your boot will be a bit slower due to it probing more partitions.
> > > > Wouldn't expect it to significantly slow down the total boot time
> > > > though.
> > > 
> > > I thought u-boot would scan that partitions which were marked bootable
> > > first, in which case you just need to set the bit correctly in the
> > > partition table. I might be wrong about that though. (and re-reading
> > > $subject, it seems like changing this is the subject of the thread...)
> > 
> > Nope current u-boot just always uses the first partition. My changes
> > make it scan all the partition on the device instead to find a viable
> > set of bootfiles. As you can see in the previous mails, i do have some
> > dislike for using the legacy bootable flag for this purpose..
> 
> I think when using a legacy partition table it is fine to pay attention
> to that bit, it's not legacy in that context.
> 
> WRT GPT, I think "legacy" in that context can reasonably be inferred to
> mean "non-EFI", and u-boot isn't EFI so I don't think it is so very
> wrong for u-boot to pay some attention to it, or at least search those
> first.
> 
> GPT defines bit 1, as "No Block IO Protocol", so if you disagree with
> treating u-boot as "not EFI", perhaps you'd instead prefer to be "EFI
> compatible" to the extent of honouring that bit.

I prefer to treat GPT as a partition table layout seperate from EFI. In
the GPT context, i tend to read "legacy" as "Intel BIOS". But that's
really opinions. Respecting bit 1 would be a nice improvement though. 

Wrt. the boot flag. I can be convinced that partitions with that flag
should be prioritized in their scanning above others, although i'm
really not sure how valuable that actually is (i don't think it is).
What i'm opposed to would be *only* searching partitions with that flag
as that just tends to be unnecessarily error-prone.


> > > > I didn't think of this one my WIP is on an Odroid X2 which has a boot
> > > > selector jumper, so I have it always starting from mmc0 (which is either
> > > > SD or EMMC depending on the jumper setting). 
> > > > 
> > > > However, it raises an interesting question. The current convention for
> > > > Exynos is to first scans external storage (SD, mmc 1) and then internal
> > > > storage (eMMC, mmc 0), which opens up a whole different can of worms. As
> > > > that means that e.g. my chromebook will try to boot from whatever random
> > > > SD i've put into it first rather the OS installed on eMMC.  It would be
> > > > nice to have some general guidelines in this area so the behaviour of
> > > > various boards can be somewhat consistent in the default behaviour.
> > > 
> > > My understanding was that the various ${boot_*} envvars, including
> > > boot_targets, are considered to be user serviceable parts. IOW if you
> > > want to boot from mmc0 only then:
> > >         setenv boot_targets mmc0
> > >         saveenv
> > > 
> > > Maybe it makes sense for the default boot order to differ depending on
> > > the specific exynos platform though?
> > 
> > Sure, which is something that really should get some documentation (I've
> > had that on my mental today, together with documenting what boot scripts
> > can expect wrt. variables set).. 
> 
> http://patchwork.ozlabs.org/patch/425412/ covers all this, I think.

Oh awesome, didn't see that patch yet. Thanks for the pointer.

> > This is about the default setup though, it would be really nice to get
> > consistent behaviour. I would be inclined to say that the defaults
> > should conservatively try the internal/main storage first (assuming
> > there will be an OS is installed there) and only fallback to other
> > options later. 
> 
> I'm inclined the other way, which is to boot of a removable media first
> if someone has gone to the effort to plug one in. People building kiosks
> etc who want to lock it down to internal only can still do so.

This is the general problem of the user doing an action but the system
really being unable to devine the intention behind that action.. An
external storage device can have been plugged in for a lot of reasons,
not just to boot from (you might have wnated to copy/inspect/change some
data on the card). I would argue that the main reason for folks to plug
in external storage devices into computers is not because they'd like to
boot from it and furthermore that a lot of people would be confused if
leaving in a storage card/stick makes the system unbootable.

I suspect this argument comes down to how you expect users to normally
use the system.. Iotw as a general computing device, just like your
laptop or desktop machine or as a development toy where you regularly
boot from external media (I would expect the former, but that's just
me).


-- 
Sjoerd Simons <sjoerd.simons at collabora.co.uk>
Collabora Ltd.
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