CHROMIUM: ARM: exynos: snow: Turn down drive strength for wifi pins

We are seeing EMI from the SDIO clock on Snow.  Turn down the drive
strength of these pins to reduce hard edges on the lines and improve
EMI.  We're not adjusting Daisy settings right now since WiFi is hooked
up differently there and we need extra testing to tell that the drive
strength is safe.

BUG=chrome-os-partner:12151
TEST=Boot on Snow and see WiFi still work.
TEST=Measure EMI and see it reduced.  After applying that CL, EMI is
lower than before(5dB lower at 450MHz/11dB lower at 750MHz).
TEST=Ran wifi test suites (MatFunc, Manager, SecMat) and didn't see
any instability which could be introduced by this CL

Change-Id: Ib979a57710117edc60893c0d32f5b64eeb6cc6db
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/29084
Reviewed-by: Tan Gao <tgao@chromium.org>

diff --git a/arch/arm/boot/dts/cros5250-common.dtsi b/arch/arm/boot/dts/cros5250-common.dtsi
index 7c31eef..6c9c710 100644 (file)
--- a/arch/arm/boot/dts/cros5250-common.dtsi
+++ b/arch/arm/boot/dts/cros5250-common.dtsi
@@ -373,9 +373,7 @@
 
                slot0 {
                        bus-width = <4>;
-                       gpios = <&gpc4 1 2 3 3>, <&gpc4 0 2 0 3>,
-                               <&gpc4 3 2 3 3>, <&gpc4 4 2 3 3>,
-                               <&gpc4 5 2 3 3>, <&gpc4 6 2 3 3>;
+                       /* See board-specific dts files for GPIOs */
                };
        };
 

diff --git a/arch/arm/boot/dts/exynos5250-daisy.dts b/arch/arm/boot/dts/exynos5250-daisy.dts
index e3c179d..df641ac 100644 (file)
--- a/arch/arm/boot/dts/exynos5250-daisy.dts
+++ b/arch/arm/boot/dts/exynos5250-daisy.dts
@@ -32,4 +32,18 @@
        display-port-controller {
                status = "disabled";
        };
+
+       /*
+        * On Daisy WiFi hooks up through a connector so keep a high drive
+        * strength for safety.
+        *
+        * TODO: Test various drive strengths and see what's really needed
+        */
+       dwmmc3@12230000 {
+               slot0 {
+                       gpios = <&gpc4 1 2 3 3>, <&gpc4 0 2 0 3>,
+                               <&gpc4 3 2 3 3>, <&gpc4 4 2 3 3>,
+                               <&gpc4 5 2 3 3>, <&gpc4 6 2 3 3>;
+               };
+       };
 };

diff --git a/arch/arm/boot/dts/exynos5250-snow.dts b/arch/arm/boot/dts/exynos5250-snow.dts
index 2cc8554..aa1566c 100644 (file)
--- a/arch/arm/boot/dts/exynos5250-snow.dts
+++ b/arch/arm/boot/dts/exynos5250-snow.dts
@@ -47,4 +47,16 @@
                        reset-gpio = <&gpx1 5 1 0 0>;
                };
        };
+
+       /*
+        * On Snow we've got SIP WiFi and so can keep drive strengths low to
+        * reduce EMI.
+        */
+       dwmmc3@12230000 {
+               slot0 {
+                       gpios = <&gpc4 1 2 3 0>, <&gpc4 0 2 0 0>,
+                               <&gpc4 3 2 3 0>, <&gpc4 4 2 3 0>,
+                               <&gpc4 5 2 3 0>, <&gpc4 6 2 3 0>;
+               };
+       };
 };