[bootlin/training-materials updates] master: Add explanations about including defines for symbols defined in asm/<file>.h (aa7e3999)

[Michael Opdenacker]

Repository : https://github.com/bootlin/training-materials
On branch  : master
Link       : https://github.com/bootlin/training-materials/commit/aa7e39990b798a380d1c46e713c7e17ca551919f

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commit aa7e39990b798a380d1c46e713c7e17ca551919f
Author: Michael Opdenacker <michael.opdenacker at bootlin.com>
Date:   Tue Oct 27 11:46:14 2020 +0100

    Add explanations about including defines for symbols defined in asm/<file>.h
    
    Signed-off-by: Michael Opdenacker <michael.opdenacker at bootlin.com>


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aa7e39990b798a380d1c46e713c7e17ca551919f
 .../kernel-i2c-communication.tex                       | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/labs/kernel-i2c-communication/kernel-i2c-communication.tex b/labs/kernel-i2c-communication/kernel-i2c-communication.tex
index ecddba24..9f9b99a7 100644
--- a/labs/kernel-i2c-communication/kernel-i2c-communication.tex
+++ b/labs/kernel-i2c-communication/kernel-i2c-communication.tex
@@ -235,8 +235,24 @@ In the probe routine (run every time a matching device is found):
       using. This could reveal communication issues.  Using Elixir, find
       examples of how to handle failures properly using the same
       function.
+
 \item Let the CPU wait for 1 ms by using the \kfunc{udelay} routine.
-      You may need to use Elixir again to find the right C headers to include.
+      Let's use Elixir again to find the right C headers to include...
+
+      The Elixir results are a bit confusing here, because
+      \kfunc{udelay} is defined in \code{arch/<arch>/include/asm/delay.h} files,
+      but not in an \code{include/linux/<file>.h>} that is normally used
+      in kernel code.
+
+      However, look at \kfile{include/linux/delay.h} and you will see
+      that it includes \code{asm/delay.h} which corresponds to the
+      specific headers for the current architecture. So you need to include
+      \code{linux/delay.h}.
+
+      {\bf General rule}: whenever the symbol you're looking
+      for is defined in \code{arch/<arch>/include/asm/<file>.h}, you
+      can include \code{linux/<file>.h} in your kernel code.
+
 \item In the same way, send the \code{0xfb} and \code{0x00} bytes now.
       This completes the nunchuk initialization.
 \end{enumerate}