Yocto and patches. How are they applied? Can I patch a patch?

qtSucks · wrote on 21 May 2025, 20:49

I have a boot2qt recipe that I have inherited based on 5.15.5 that has been customised. One of the customisations is to alter the kernel splash screen but I can't figure out how it is applied and I need to as I need to further update this file. The updated file is logo_linux_clut224.ppm.
I initially thought it was done by a .bbapend file that mentions logo_linux_clut224.ppm but that file (linux-raspberrypi_4.9.bbappend) has not corresponding .bb file. I checked the linux-raspberrypi package and it is using linux-raspberrypi_4.19.bb.

I greped for logo_linux_clut224.ppmlogo_linux_clut224.ppm and it also appears in linux.inc but greping for linux.inc and that is not used in any .bb or .bbappend files.

The last place (outside the build directory) is in a patch file, dynamic-layers/raspberrypi/recipes-kernel/linux/linux-raspberrypi/raspberrypi-cm3/0004-Change-boot-logo.patch this file just show the git hash the file deletion and insertion and then the content of the new PPM file

From ec56a56d223adb7507fe8263567164de4d9dc1e6 Mon Sep 17 00:00:00 2001
From: aperson@aemail.com
Date: Fri, 01 Jan 2021 18:23:11 +6500
Subject: [PATCH 4/4] Change boot logo 

---                                                                                                                                                                drivers/video/logo/logo_linux_clut224.ppm | 103282 ++++++++++++++++++++++++++-                                                                                    1 file changed, 102401 insertions(+), 881 deletions(-)                                                                                                                                                                                                                                                                             diff --git a/drivers/video/logo/logo_linux_clut224.ppm b/drivers/video/logo/logo_linux_clut224.ppm                                                                index 7626beb..441bdba 100644                                                                                                                                     --- a/drivers/video/logo/logo_linux_clut224.ppm                                                                                                                   +++ b/drivers/video/logo/logo_linux_clut224.ppm                                                                                                                   @@ -1,883 +1,102403 @@                                                                                                                                             P3          bla bla bla                                                                                                                                                     -63 80

This file is referred to in linux-raspberrypi_4.19.bbappend, the append file for the recipe used for linux-raspberrypi package. So it must be somehow applied via this .bbappend. That makes sense. However what I don't understand is how this is applied to the linux-raspberrypi recipe. There is just the patch file (amongst other patch files) listed. How does it know how to apply the file? Is it just based on the .patch extension?

If I alter the ppm file do I need to create a new patch file? Would that be a patch on top of the patch or would I replace the patch? I have read about the PR variable but I don't see that as being updated in the .bbapend file or .bb file and if I apply a new patch I think I have to update this to trigger the package to be rebuilt (and hence all of the other packages that have linux-raspberrypi as a dependency.

Thanks for any pointers