[PATCH 1/2] DM_USB: allow building without OF_CONTROL

[Merlijn Wajer]

Hi Tom, Marek,

On 25/06/2021 23:59, Tom Rini wrote:
> On Fri, Jun 25, 2021 at 11:51:51PM +0200, Pali Rohár wrote:
>> On Friday 25 June 2021 17:37:44 Tom Rini wrote:
>>> One thing I want to say here as I think it maybe wasn't clear in Marek's
>>> suggestion.  Why not have X-Loader boot SPL which loads U-Boot from extN
>>> on the eMMC?
>>
>> Hello Tom! I have already answered this in my previous email.
> 
> I just re-read things and I don't see it.  But perhaps I'm not being
> clear enough.  Why can't you just have NOLO start SPL, not re-initialize
> things (which is a really common case now thanks to aarch64) and just
> use that to load full U-Boot from a not size restricted place?
> 

I think there are a few problems.

1. One is a practical one, from Pali's email:

> There is no easy access to eMMC until you start full U-Boot. So even if  all these problems are solved then "bootstrapping" or flashing U-Boot into such location is not possible, plus there is no recovery. Plus this loose existing and working operating system, which is no-go. So this way is basically undebugable and therefore perfectly hard to develop.

Not being able to access the eMMC to write u-boot until u-boot is
started does sound like it would make things a bit more tricky.

2. According to Pali, adding SPL support would not be a trivial task,
and might end up with a lot more "#ifdef"s in SPL than the one in
Ivaylo's patch. As I understand it, this hypothetical SPL would mostly
not do any hardware initialisation on the N900, so that might be where
instead hacks would need to be added to SPL.

Pali also wrote:

> U-Boot for N900 does not use SPL. There is no SPL code implemented.
> Nobody ever tried to implement it and neither tested. As you have
> correctly pointed instead of SPL is used vendor X-Loader binary, which
> is signed by RSA key.
And when I asked him today:

> 12:11 < Pali> in past (10 years ago?) I was investigating the way how we can boot u-boot and the only reasable way was the current one, directly load main u-boot by x-loader/nolo
> 12:12 < Pali> I have already spend some time with spl on n900 and at that time I have rejected this idea, because it did not work that
> 12:13 < Pali> spl is also doing hw initialization which cannot be called on n900 as this code basically crash / freeze n900
So perhaps it would make sense to get more clarity on that, since that
seems to be where the argument gets stuck.


Also, I'd like to point what Ivaylo wrote earlier:

>> This sounds like a workaround though. Can you instead do the full conversion of the board? I am sure the board removal patch can be postponed if there is plan to convert it.
> 
> Hard to say if migration to device-tree is even possible on N900 ATM, enabling OF_CONTROL increases the size of the produced binary with some 100k (.dtb not included), making the size of the binary way above our budget of ~256k. Sure, board config does not enable -mthumb, but omap3 in rx-51 suffers from ARM errata 430973 and noone can guarantee we're not going to see SIGILL faults if we enable it. Which it seems we are forced to do even with DM_USB migration only.
> 
> Re workaround - I took examples of #ifdef's from the current u-boot code (mmc, i2c, etc.) so workaround or not, it is no different to what the other drivers are doing.

If the other drivers have the same logic, it seems a bit weird to me
that the change made in Ivaylo's patch would be rejected because of a
preference to port the board to DT. Unless maybe this was the first
driver to be migrated to support only DT and the patch is in effect a
reversal of some prior work?

Of course, in the long run all boards should follow u-boot technological
preferences, but it feels a bit off that this patch would be treated
differently if the existing drivers already use the same logic, and as
far as I can see there is no deadline currently to convert boards to DT,
just for converting them to DM.

Regards,
Merlijn