Better way to upstream a single-board computer?

From: Lucas Tanure <tanure@linux.com>
To: kernelnewbies <kernelnewbies@kernelnewbies.org>,
	linux-amlogic@lists.infradead.org
Subject: Better way to upstream a single-board computer?
Date: Tue, 13 Jun 2023 11:02:01 +0100	[thread overview]
Message-ID: <CAJX_Q+0Ei+6gueLBxnRsaqS8HeFPJC2UU1+k7rxd68Tdq1NyJg@mail.gmail.com> (raw)

Hi,

I have been trying to port Vim4 to Mainline for the past few weeks.
Downstream branch:
  https://github.com/khadas/linux/tree/khadas-vims-5.4.y

- First approach: The compatible string way
In that 5.4 branches, clean the device tree to the bare minimum to
have a serial console, sd-card mount and ethernet, and reduce the
kernel config to only build the necessary modules.
With those 2 in hand (minimum dts and config), I could port the
necessary drivers for which the dts have compatible strings. I did
manage to boot once a v6.3 kernel with that dts and config.
But it would hang a few seconds after booting the kernel. And after a
few days of trying to debug the problem without success, I decided to
move to another approach.

- Second approach: The fat patch
I compared a clean 5.4 branch and khadas 5.4 branch and generated a
big fat patch with around 2.5 million lines, where when applied to
clean 5.4, it would boot and work.
I started to chop the fat patch to remove stuff that did not need to
get vim4 working with only serial, ethernet and sd-card.
I managed to reduce to 1.3 million lines the fat patch, but after
that, things started to hang and crash, so I was unable to move on.
At this point, I realized that 5.4 khadas was actually an
Android12-5.4 branch with amlogic and khadas stuff.

- Thrid approch: The android fat patch
I am doing the same thing as the second approach but using an
android12-5.4-lts branch as the start, where the comparison shows me
only the amlogic and khadas changes made.
I am still going through this method, but it is promising. Right now,
I have the fat patch with 0.9 million lines.
My idea here is that when that fat patch becomes small enough, I can
apply it to a clean 5.4 (no Android) to work it out more. And when
that patch becomes something tinny, I could apply it on mainline,
adjusting for the kernel version jump.

- Fourth approach: The rebase way (On hold)
I could compare the android12-5.4-lts git log with the khandas-5.4 log
and start to cherry-pick patches to a clean 5.4 (no Android), but from
experience, I think that's going to take forever, and it would be very
hard to apply patches as they refer to an Android branch, not
mainline.

The intermediate goal is to have a patch on top of the mainline that
boots vim4, no matter how ugly. With that in hand, I can start
re-structuring and getting ready for upstream.

Now my question for the community:
Is there a better way to port a board to the mainline when you only
have an old kernel that boots?

Thanks
Lucas Tanure

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