drm: bridge/dw_hdmi: introduce interfaces to enable and disable audio

iMX6 devices suffer from an errata (ERR005174) where the audio FIFO can
be emptied while it is partially full, resulting in misalignment of the
audio samples.

To prevent this, the errata workaround recommends writing N as zero
until the audio FIFO has been loaded by DMA.  Writing N=0 prevents the
HDMI bridge from reading from the audio FIFO, effectively disabling
audio.

This means we need to provide the audio driver with a pair of functions
to enable/disable audio.  These are dw_hdmi_audio_enable() and
dw_hdmi_audio_disable().

A spinlock is introduced to ensure that setting the CTS/N values can't
race, ensuring that the audio driver calling the enable/disable
functions (which are called in an atomic context) can't race with a
modeset.
Tested-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/gpu/drm/bridge/dw_hdmi.c

@@ -18,6 +18,7 @@
 #include <linux/hdmi.h>
 #include <linux/mutex.h>
 #include <linux/of_device.h>
+#include <linux/spinlock.h>
 
 #include <drm/drm_of.h>
 #include <drm/drmP.h>
@@ -123,8 +124,12 @@ struct dw_hdmi {
 	struct i2c_adapter *ddc;
 	void __iomem *regs;
 
+	spinlock_t audio_lock;
 	struct mutex audio_mutex;
 	unsigned int sample_rate;
+	unsigned int audio_cts;
+	unsigned int audio_n;
+	bool audio_enable;
 	int ratio;
 
 	void (*write)(struct dw_hdmi *hdmi, u8 val, int offset);
@@ -346,7 +351,11 @@ static void hdmi_set_clk_regenerator(struct dw_hdmi *hdmi,
 	dev_dbg(hdmi->dev, "%s: samplerate=%ukHz ratio=%d pixelclk=%luMHz N=%d cts=%d\n",
 		__func__, sample_rate, ratio, pixel_clk, n, cts);
 
-	hdmi_set_cts_n(hdmi, cts, n);
+	spin_lock_irq(&hdmi->audio_lock);
+	hdmi->audio_n = n;
+	hdmi->audio_cts = cts;
+	hdmi_set_cts_n(hdmi, cts, hdmi->audio_enable ? n : 0);
+	spin_unlock_irq(&hdmi->audio_lock);
 }
 
 static void hdmi_init_clk_regenerator(struct dw_hdmi *hdmi)
@@ -375,6 +384,28 @@ void dw_hdmi_set_sample_rate(struct dw_hdmi *hdmi, unsigned int rate)
 }
 EXPORT_SYMBOL_GPL(dw_hdmi_set_sample_rate);
 
+void dw_hdmi_audio_enable(struct dw_hdmi *hdmi)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&hdmi->audio_lock, flags);
+	hdmi->audio_enable = true;
+	hdmi_set_cts_n(hdmi, hdmi->audio_cts, hdmi->audio_n);
+	spin_unlock_irqrestore(&hdmi->audio_lock, flags);
+}
+EXPORT_SYMBOL_GPL(dw_hdmi_audio_enable);
+
+void dw_hdmi_audio_disable(struct dw_hdmi *hdmi)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&hdmi->audio_lock, flags);
+	hdmi->audio_enable = false;
+	hdmi_set_cts_n(hdmi, hdmi->audio_cts, 0);
+	spin_unlock_irqrestore(&hdmi->audio_lock, flags);
+}
+EXPORT_SYMBOL_GPL(dw_hdmi_audio_disable);
+
 /*
  * this submodule is responsible for the video data synchronization.
  * for example, for RGB 4:4:4 input, the data map is defined as
@@ -1577,6 +1608,7 @@ int dw_hdmi_bind(struct device *dev, struct device *master,
 	hdmi->encoder = encoder;
 
 	mutex_init(&hdmi->audio_mutex);
+	spin_lock_init(&hdmi->audio_lock);
 
 	of_property_read_u32(np, "reg-io-width", &val);
 

include/drm/bridge/dw_hdmi.h

@@ -63,5 +63,7 @@ int dw_hdmi_bind(struct device *dev, struct device *master,
 		 const struct dw_hdmi_plat_data *plat_data);
 
 void dw_hdmi_set_sample_rate(struct dw_hdmi *hdmi, unsigned int rate);
+void dw_hdmi_audio_enable(struct dw_hdmi *hdmi);
+void dw_hdmi_audio_disable(struct dw_hdmi *hdmi);
 
 #endif /* __IMX_HDMI_H__ */