firmware: variant: Update README with customization description
Test: Manual
Bug: 37716086
Change-Id: If227d205b804d7f94085baed5a988fd6dfae845f
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
diff --git a/firmware/variant/README b/firmware/variant/README
index 3cece9d..aa7dde4 100644
--- a/firmware/variant/README
+++ b/firmware/variant/README
@@ -1,2 +1,23 @@
-for ease of source browsing this directory may contain soft links to variant directories located elsewhere;
-build scripts fully support such layout, however this is not necessary in order to build such variants.
+0. Directory structure
+
+For ease of source browsing this directory may contain soft links to variant
+directories located elsewhere;
+build scripts fully support such layout, however this is not necessary in order
+to build such variants.
+
+1. HW Customization
+
+1.1 General description
+- wakeup-gpio has direction from Application Processor (AP) to ContextHub
+- irq1 has direction from ContextHub to AP (wakeup interrupt)
+- irq2 has direction from ContextHub to AP (non-wakeup interrupt) (optional)
+
+1.2 Kernel side
+- wakeup-gpio = SH_INT_WAKEUP
+- irq1-gpio = AP_INT_WAKEUP
+- irq2-gpio = AP_INT_NONWAKEUP
+
+1.3 ContextHub side
+- #define SH_INT_WAKEUP PX(N0)
+- #define AP_INT_WAKEUP PY(N1)
+- #define AP_INT_NONWAKEUP PZ(N2)
