[PATCH] mmc: core: Allow speed modes to be adjusted via module param

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

During board verification, there is a need to test the various supported
eMMC/SD speed modes.  However, since the framework chooses the best mode
supported by the card and the host controller's caps, this currently
necessitates changing the devicetree for every iteration.

To make changing the modes easier, allow the various host controller
capabilities to be cleared via a module parameter.  (A per-controller
debugfs wouldn't work since the controller needs to be re-probed to
trigger re-init of cards.  A module parameter is used instead of a
global debugfs to allow this to be also set via the kernel command
line.)

The values to be written are the raw MMC_CAP* values from
include/linux/mmc/host.h.  This is rather low-level, and these defines
are not guaranteed to be stable, but it is perhaps good enough for the
indented use case.  A warning is emitted when the caps clearing is in
effect.

Example of use:

 # grep timing /sys/kernel/debug/mmc0/ios
 timing spec:	9 (mmc HS200)

 // MMC_CAP2_HS200_1_8V_SDR
 # echo $((1 << 5)) > /sys/module/mmc_core/parameters/caps2_clear

 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/unbind
 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/bind
 # grep timing /sys/kernel/debug/mmc0/ios
 timing spec:	8 (mmc DDR52)

 // MMC_CAP_1_8V_DDR
 # echo $((1 << 12)) > /sys/module/mmc_core/parameters/caps_clear

 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/unbind
 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/bind
 # grep timing /sys/kernel/debug/mmc0/ios
 timing spec:	1 (mmc high-speed)

 # echo 0 > /sys/module/mmc_core/parameters/caps2_clear

 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/unbind
 # echo 16d40000.mmc > /sys/bus/platform/drivers/dwmmc_exynos/bind
 # grep timing /sys/kernel/debug/mmc0/ios
 timing spec:	9 (mmc HS200)

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
 drivers/mmc/core/host.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
index 2ed2b4d5e5a5..37971b7c7f62 100644
--- a/drivers/mmc/core/host.c
+++ b/drivers/mmc/core/host.c
@@ -34,6 +34,10 @@
 #define cls_dev_to_mmc_host(d)	container_of(d, struct mmc_host, class_dev)
 
 static DEFINE_IDA(mmc_host_ida);
+static unsigned int caps_clear, caps2_clear;
+
+module_param(caps_clear, uint, 0644);
+module_param(caps2_clear, uint, 0644);
 
 #ifdef CONFIG_PM_SLEEP
 static int mmc_host_class_prepare(struct device *dev)
@@ -411,6 +415,14 @@ int mmc_of_parse(struct mmc_host *host)
 		host->caps2 &= ~(MMC_CAP2_HS400_1_8V | MMC_CAP2_HS400_1_2V |
 				 MMC_CAP2_HS400_ES);
 
+	if (caps_clear || caps2_clear)
+		dev_warn(host->parent,
+			 "clearing host controller caps %#x caps2 %#x\n",
+			 caps_clear, caps2_clear);
+
+	host->caps &= ~caps_clear;
+	host->caps2 &= ~caps2_clear;
+
 	/* Must be after "non-removable" check */
 	if (device_property_read_u32(dev, "fixed-emmc-driver-type", &drv_type) == 0) {
 		if (host->caps & MMC_CAP_NONREMOVABLE)
-- 
2.34.1