re: HID: add documentation and Coccinelle script for FF registration race

Hi Dimitry

Looks like media-ci is not particularly happy about this patch:

https://gitlab.freedesktop.org/linux-media/users/ribalda/-/jobs/108127901

/usr/bin/spatch -D report --no-show-diff --very-quiet --cocci-file /builds/linux-media/users/ribalda/scripts/coccinelle/hid/ff_race.cocci --patch /builds/linux-media/users/ribalda --dir . -I /builds/linux-media/users/ribalda/arch/x86/include -I /builds/linux-media/users/ribalda/arch/x86/include/generated -I /builds/linux-media/users/ribalda/include -I /builds/linux-media/users/ribalda/include -I /builds/linux-media/users/ribalda/arch/x86/include/uapi -I /builds/linux-media/users/ribalda/arch/x86/include/generated/uapi -I /builds/linux-media/users/ribalda/include/uapi -I /builds/linux-media/users/ribalda/include/generated/uapi --include /builds/linux-media/users/ribalda/include/linux/compiler-version.h --include /builds/linux-media/users/ribalda/include/linux/kconfig.h --jobs 16 --chunksize 1
Position variable r.p2 on line 20 must be attached to some other token using @.
coccicheck failed


The following change fixes the issue for me... but you probably want to double
check it, I am not proficient at cocci.

diff --git a/scripts/coccinelle/hid/ff_race.cocci b/scripts/coccinelle/hid/ff_race.cocci
index 479f5d1e3184..7cf84936db13 100644
--- a/scripts/coccinelle/hid/ff_race.cocci
+++ b/scripts/coccinelle/hid/ff_race.cocci
@@ -9,7 +9,8 @@ virtual report

 @r@
 identifier probe_fn;
-expression hdev, flags;
+identifier hdev;
+expression flags;
 position p1, p2;
 @@

@@ -17,7 +18,7 @@ probe_fn(struct hid_device *hdev, ...) {
   <...
   hid_hw_start@p1(hdev, flags)
   ...
-  \(input_ff_create\|input_ff_create_memless\)@p2(...)
+  \(input_ff_create@p2\|input_ff_create_memless@p2\)(...)
   ...>
 }


Regards!