[PATCH v3 3/4] drm/vc4: dsi: Serialize host transfers

[Henry Zhang <henryzhang62@gmail.com>]

vc4_dsi_host_transfer() fills the command FIFOs and the TXPKT registers
and waits on a single completion without any locking. Two callers
sending at the same time, for example a panel driver and a backlight
update, can overwrite each other's packets. Nothing keeps a transfer
from running while the bridge callbacks reprogram the block either,
for example while pre_enable sets up the PHY timings.

Add a mutex and hold it for the whole host transfer and in the bridge
pre_enable, enable, disable and post_disable callbacks. None of these
callbacks calls into the DSI device, so the lock is never taken
recursively.

Signed-off-by: Henry Zhang <henryzhang62@gmail.com>
---
 drivers/gpu/drm/vc4/vc4_dsi.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index face44a23..ea3a6d06b 100644
--- a/drivers/gpu/drm/vc4/vc4_dsi.c
+++ b/drivers/gpu/drm/vc4/vc4_dsi.c
@@ -25,6 +25,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/dmaengine.h>
 #include <linux/io.h>
+#include <linux/mutex.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/platform_device.h>
@@ -595,6 +596,11 @@ struct vc4_dsi {
 	 */
 	struct clk *pixel_clock;
 
+	/*
+	 * Serializes host transfers against each other and against the
+	 * bridge enable/disable paths, which reprogram the block.
+	 */
+	struct mutex transfer_lock;
 	struct completion xfer_completion;
 	int xfer_result;
 
@@ -805,6 +811,8 @@ static void vc4_dsi_bridge_disable(struct drm_bridge *bridge,
 	struct vc4_dsi *dsi = bridge_to_vc4_dsi(bridge);
 	u32 disp0_ctrl;
 
+	guard(mutex)(&dsi->transfer_lock);
+
 	disp0_ctrl = DSI_PORT_READ(DISP0_CTRL);
 	disp0_ctrl &= ~DSI_DISP0_ENABLE;
 	DSI_PORT_WRITE(DISP0_CTRL, disp0_ctrl);
@@ -816,6 +824,8 @@ static void vc4_dsi_bridge_post_disable(struct drm_bridge *bridge,
 	struct vc4_dsi *dsi = bridge_to_vc4_dsi(bridge);
 	struct device *dev = &dsi->pdev->dev;
 
+	guard(mutex)(&dsi->transfer_lock);
+
 	clk_disable_unprepare(dsi->pll_phy_clock);
 	clk_disable_unprepare(dsi->escape_clock);
 	clk_disable_unprepare(dsi->pixel_clock);
@@ -891,6 +901,8 @@ static void vc4_dsi_bridge_pre_enable(struct drm_bridge *bridge,
 	unsigned long phy_clock;
 	int ret;
 
+	guard(mutex)(&dsi->transfer_lock);
+
 	ret = pm_runtime_resume_and_get(dev);
 	if (ret) {
 		drm_err(bridge->dev, "Failed to runtime PM enable on DSI%d\n", dsi->variant->port);
@@ -1148,6 +1160,8 @@ static void vc4_dsi_bridge_enable(struct drm_bridge *bridge,
 	bool debug_dump_regs = false;
 	u32 disp0_ctrl;
 
+	guard(mutex)(&dsi->transfer_lock);
+
 	disp0_ctrl = DSI_PORT_READ(DISP0_CTRL);
 	disp0_ctrl |= DSI_DISP0_ENABLE;
 	DSI_PORT_WRITE(DISP0_CTRL, disp0_ctrl);
@@ -1181,6 +1195,8 @@ static ssize_t vc4_dsi_host_transfer(struct mipi_dsi_host *host,
 	bool is_long = mipi_dsi_packet_format_is_long(msg->type);
 	u32 cmd_fifo_len = 0, pix_fifo_len = 0;
 
+	guard(mutex)(&dsi->transfer_lock);
+
 	mipi_dsi_create_packet(&packet, msg);
 
 	pkth |= VC4_SET_FIELD(packet.header[0], DSI_TXPKT1H_BC_DT);
@@ -1807,6 +1823,7 @@ static int vc4_dsi_dev_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
 	struct vc4_dsi *dsi;
+	int ret;
 
 	dsi = devm_drm_bridge_alloc(&pdev->dev, struct vc4_dsi, bridge, &vc4_dsi_bridge_funcs);
 	if (IS_ERR(dsi))
@@ -1814,6 +1831,9 @@ static int vc4_dsi_dev_probe(struct platform_device *pdev)
 	dev_set_drvdata(dev, dsi);
 
 	dsi->pdev = pdev;
+	ret = devm_mutex_init(dev, &dsi->transfer_lock);
+	if (ret)
+		return ret;
 #ifdef CONFIG_OF
 	dsi->bridge.of_node = dev->of_node;
 #endif
-- 
2.47.3