Syslinux - Jun 2015 - chainloading syslinux from an EFI partition to an ext4

On 6 June 2015 at 17:22, Ady via Syslinux <syslinux at zytor.com> wrote:
> FWIW, the most current (or "up-to-date") documentation about
chain.c32
> at this time is located at:
>  http://www.syslinux.org/wiki/index.php/Chain.c32
Yes, I had a look at that, but frankly the archlinux pages were
more newbie-friendly, so my research is 50% based on what I found
there and for the rest syslinux.org and the internet at large.
> Note that, in the official wiki of The Syslinux Project, the wiki pages
> containing "... /Doc/ ..." are mainly copies of documents already
> included in some official release, which are usually outdated.
>
>
> As per the intended setup, it is not possible. There are several
> reasons for such impossibility.
Could you elaborate a bit more? What are the impediments?
> *Independently of this particular case*, if someone wants to improve
> chain.c32 (or any other part of Syslinux), patches are welcome.
I have no experience with bootloaders, but I could give it a shot,
if I had some confidence that it's a task I can tackle within the
time I have been given to find a solution.

Unfortunately my set of choices is fairly limited, so syslinux
still seems a possible solution, if the effort to modify it is
compatible with my skills and time frame.

In that sense, a hint of what to do would also be very appreciated.

Ex: would it be more reasonable to improve support for chainloading
or to add support for loading a kernel from another partition?

-- 
igor

(snip) 
 
> > As per the intended setup, it is not possible. There are several
> > reasons for such impossibility.
> 
> Could you elaborate a bit more? What are the impediments?
 
 (snip) 
 
> -- 
> igor
 
Posting an explanation for every current impediment (as of v.6.03) and, 
optionally, potential future improvements, would take (me) some (long) 
time, it would be boring to read, and it would not actually bring you 
to a realistic working solution of your attempted setup.

The most basic problem with your attempted setup, IMHO, is that 
initially you want to boot with a UEFI bootloader (syslinux.efi) in 
UEFI mode - that's fine - but then you want to chain to a BIOS 
bootloader (EXTLINUX, ldlinux.sys).

As of version 6.03, Syslinux in UEFI systems lacks many 
potentially-useful features (e.g. chainloading another EFI tool). I 
would tend to think that chainloading from one (U)EFI firmware 
architecture to a BIOS-based bootloader would probably be a low 
priority for developers (if it would be ever possible).

Regarding the alternative, loading kernels located in other partitions 
(and using different file systems), there have been some attempts to 
add such feature to Syslinux (this is not as an easy task as some 
people might think). I guess that the suggested patches might be 
evaluated at some point, but I am going to assume (as an educated 
guess) that it will take more time than you are willing to wait.

Throughout this email thread, there have been some comments about the 
possibility to boot Windows and Linux, both being located in the same 
physical device, and (for some unknown reason) not willing to use the 
ESP for kernels. These conditions might add complexity; whether 
unnecessarily or not, I don't know.

Since I was asked to expand on my prior reply, I should modify my 
initial qualification from "impossible" (which I used as an
exaggerated
short description so to convey the very low probability to be 
successful in your attempted setup and conditions while using Syslinux 
v.6.03) to some other qualifier. How about: "hopefully it will be 
possible in some future version, using some alternative method"?

Regards,
Ady.
 
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On 7 June 2015 at 21:48, Ady via Syslinux <syslinux at zytor.com> wrote:
> The most basic problem with your attempted setup, IMHO, is that
> initially you want to boot with a UEFI bootloader (syslinux.efi) in
> UEFI mode - that's fine - but then you want to chain to a BIOS
> bootloader (EXTLINUX, ldlinux.sys).

I have the possibility of adjusting the number/content of the partitions.
One option I had considered less desirable, but it might be a game
changer here, is to put the desired kernel in a separate partition, as raw
binary data.

So this new scenario would require to boot the UEFI bootloader and
then to chainload the content of the partition, which is a plain kernel.

Would this work any better?

I could do the same with the initrd. Actually in this case I could even use
initramfs model, where kernel and initrd are one single blob.

-- 
thanks, igor