Bitbake recipe not applying patch as expected - patch

Question

I have a tarball src.tar.gz whose contents are unpacked into src/ and a patch of this sources generated with this command:
$ diff -Nurp src/ src_mod/ > my.patch
The patch header starts with this three lines:
diff -Nurp src/path/to/file src_PATCHED/path/to/file
--- src/path/to/file 2012-10-22 05:52:59.000000000 +0200
+++ src_PATCHED/path/to/file 2016-03-14 12:27:52.892802283 +0100
My bitbake recipe references both path and tarball files using this SRC_URI:
SRC_URI = " \
file://my.patch \
file://src.tar.gz \
"
do_fetch and do_unpack tasks work as expected, leaving my.patch and src/ inside ${S} directory, that is:
${S}/my.path
${S}/src.tar.gz
But do_patch task is failing with this ERROR message:
ERROR: Command Error: exit status: 1 Output:
Applying patch my.patch
can't find file to patch at input line 4
Perhaps you used the wrong -p or --strip option?
I have tested different alternatives, for example setting "patchdir" attribute like showed below:
SRC_URI = " \
file://my.patch;patchdir=${S}/src \
file://src.tar.gz \
"
I expected "patchdir" being the same as using "patch -d dir". But it doesn't work as expected, it always returns the same ERROR message.
What I am doing wrong?

Answer 1

My variable ${S} was re-defined inside my recipe with this content:
S = "${WORKDIR}/${PN}-${PV}"
But the fetchers downloads my.patch and src/ inside ${WORKDIR}, not inside ${S} directory, so:
${WORKDIR}/my.path
${WORKDIR}/src.tar.gz
And tarball was also extracted inside ${WORKDIR}
${WORKDIR}/src/
The fix was setting "patchdir" attribute properly, replacing ${S} by ${WORKDIR}
SRC_URI = " \
file://my.patch;patchdir=${WORKDIR}/src \
file://src.tar.gz \
"
That is already working!

Answer 2

Though the author provided their own solution to their distinct problem, this question is the first result that comes up when searching for solutions to the "can't find file to patch" error in Yocto builds, so I want to share a solution for a different situation that produces the same error output.
In my case, I was trying to use a .bbappend file to apply an override controlled patch to a pre-existing recipe, and was receiving the "can't find file to patch" error. The thread at https://community.nxp.com/thread/474138 identified the solution: using the '_append' syntax instead of the '+=' syntax. That is, use:
SRC_URI_append_machineoverride = " file://my.patch"
Instead of
SRC_URI_machineoverride += "file://my.patch"
Note that the use of '_append' requires a leading space (contra the trailing space noted in the thread linked above). I have not yet investigated this enough to explain why this syntax is necessary, but I thought this would still be a valuable addition to the record for this question.