[PATCH 0/2] media: tegra_cec: a fix and an enhancement

[PATCH 0/2] media: tegra_cec: a fix and an enhancement

[Hans Verkuil]

Two tegra_cec patches: the first works around a HW bug where, if
a transmit was NACKed, the retransmit would contain corrupt data.

The second logs when a Low Drive is received (i.e. generated by
another CEC device). This helps debug issues with (typically) bad
cables or CEC voltages.

Regards,

	Hans

Hans Verkuil (2):
  media: cec: tegra_cec: don't break off msg on NACK
  media: cec: tegra_cec: keep track of number of Rx Low Drives

 drivers/media/cec/platform/tegra/tegra_cec.c | 32 +++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

-- 
2.53.0

[PATCH 1/2] media: cec: tegra_cec: don't break off msg on NACK

[Hans Verkuil]

The Tegra CEC hardware has a bug where, if the first attempt
to transmit a message is NACKed so the transmit is aborted,
then the second attempt can contain corrupt data.

Ensure that the full message is always transmitted to avoid hitting
this bug.

I suspect some internal state is not reset in the case of aborting
a message due to a NACK.

Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
---
 drivers/media/cec/platform/tegra/tegra_cec.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/media/cec/platform/tegra/tegra_cec.c b/drivers/media/cec/platform/tegra/tegra_cec.c
index fe66336e734f..f8ffaeb78118 100644
--- a/drivers/media/cec/platform/tegra/tegra_cec.c
+++ b/drivers/media/cec/platform/tegra/tegra_cec.c
@@ -243,7 +243,18 @@ static int tegra_cec_adap_enable(struct cec_adapter *adap, bool enable)
 		  TEGRA_CEC_INT_MASK_RX_REGISTER_FULL |
 		  TEGRA_CEC_INT_MASK_RX_START_BIT_DETECTED);
 
-	cec_write(cec, TEGRA_CEC_HW_CONTROL, TEGRA_CEC_HWCTRL_TX_RX_MODE);
+	/*
+	 * TX_NAK_MODE ensures that the whole message is transmitted even
+	 * if each byte is NACKed. Without this flag the retransmit of the
+	 * messages after a NACK can be corrupt. This is a bug in the hardware.
+	 *
+	 * While less efficient, in practice you rarely transmit messages
+	 * that can be NACKed, with the exception of POLL messages which
+	 * are just one byte anyway.
+	 */
+	cec_write(cec, TEGRA_CEC_HW_CONTROL,
+		  TEGRA_CEC_HWCTRL_TX_RX_MODE |
+		  TEGRA_CEC_HWCTRL_TX_NAK_MODE);
 	return 0;
 }
 
-- 
2.53.0

Re: [PATCH 1/2] media: cec: tegra_cec: don't break off msg on NACK

[Mikko Perttunen]

On Friday, July 10, 2026 6:53 PM Hans Verkuil wrote:
> The Tegra CEC hardware has a bug where, if the first attempt
> to transmit a message is NACKed so the transmit is aborted,
> then the second attempt can contain corrupt data.
> 
> Ensure that the full message is always transmitted to avoid hitting
> this bug.
> 
> I suspect some internal state is not reset in the case of aborting
> a message due to a NACK.
> 
> Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
> ---
>  drivers/media/cec/platform/tegra/tegra_cec.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/cec/platform/tegra/tegra_cec.c b/drivers/media/cec/platform/tegra/tegra_cec.c
> index fe66336e734f..f8ffaeb78118 100644
> --- a/drivers/media/cec/platform/tegra/tegra_cec.c
> +++ b/drivers/media/cec/platform/tegra/tegra_cec.c
> @@ -243,7 +243,18 @@ static int tegra_cec_adap_enable(struct cec_adapter *adap, bool enable)
>  		  TEGRA_CEC_INT_MASK_RX_REGISTER_FULL |
>  		  TEGRA_CEC_INT_MASK_RX_START_BIT_DETECTED);
>  
> -	cec_write(cec, TEGRA_CEC_HW_CONTROL, TEGRA_CEC_HWCTRL_TX_RX_MODE);
> +	/*
> +	 * TX_NAK_MODE ensures that the whole message is transmitted even
> +	 * if each byte is NACKed. Without this flag the retransmit of the
> +	 * messages after a NACK can be corrupt. This is a bug in the hardware.
> +	 *
> +	 * While less efficient, in practice you rarely transmit messages
> +	 * that can be NACKed, with the exception of POLL messages which
> +	 * are just one byte anyway.
> +	 */
> +	cec_write(cec, TEGRA_CEC_HW_CONTROL,
> +		  TEGRA_CEC_HWCTRL_TX_RX_MODE |
> +		  TEGRA_CEC_HWCTRL_TX_NAK_MODE);
>  	return 0;
>  }
>  
> -- 
> 2.53.0
> 
> 

Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>

[PATCH 2/2] media: cec: tegra_cec: keep track of number of Rx Low Drives

[Hans Verkuil]

Keep track of Rx Low Drive conditions. Useful for detecting
potential hardware/cable problems as it suggests unstable or
incorrect voltage levels.

Note that the Tx Low Drive conditions are already logged in
the CEC core.

Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
---
 drivers/media/cec/platform/tegra/tegra_cec.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/media/cec/platform/tegra/tegra_cec.c b/drivers/media/cec/platform/tegra/tegra_cec.c
index f8ffaeb78118..e22a4625f192 100644
--- a/drivers/media/cec/platform/tegra/tegra_cec.c
+++ b/drivers/media/cec/platform/tegra/tegra_cec.c
@@ -24,6 +24,7 @@
 #include <linux/of.h>
 #include <linux/of_platform.h>
 #include <linux/platform_device.h>
+#include <linux/seq_file.h>
 #include <linux/clk/tegra.h>
 
 #include <media/cec-notifier.h>
@@ -47,6 +48,7 @@ struct tegra_cec {
 	u32			tx_buf[CEC_MAX_MSG_SIZE];
 	u8			tx_buf_cur;
 	u8			tx_buf_cnt;
+	u32			rx_total_low_drives;
 };
 
 static inline u32 cec_read(struct tegra_cec *cec, u32 reg)
@@ -116,6 +118,13 @@ static irqreturn_t tegra_cec_irq_handler(int irq, void *data)
 		return IRQ_WAKE_THREAD;
 	}
 
+	if (status & TEGRA_CEC_INT_STAT_RX_BUS_ERROR_DETECTED) {
+		dev_warn_ratelimited(dev, "RX bus error detected, generated low drive\n");
+		cec->rx_total_low_drives++;
+		cec_write(cec, TEGRA_CEC_INT_STAT,
+			  TEGRA_CEC_INT_STAT_RX_BUS_ERROR_DETECTED);
+	}
+
 	if ((status & TEGRA_CEC_INT_STAT_TX_ARBITRATION_FAILED) ||
 		   (status & TEGRA_CEC_INT_STAT_TX_BUS_ANOMALY_DETECTED)) {
 		tegra_cec_error_recovery(cec);
@@ -241,6 +250,7 @@ static int tegra_cec_adap_enable(struct cec_adapter *adap, bool enable)
 		  TEGRA_CEC_INT_MASK_TX_BUS_ANOMALY_DETECTED |
 		  TEGRA_CEC_INT_MASK_TX_FRAME_TRANSMITTED |
 		  TEGRA_CEC_INT_MASK_RX_REGISTER_FULL |
+		  TEGRA_CEC_INT_MASK_RX_BUS_ERROR_DETECTED |
 		  TEGRA_CEC_INT_MASK_RX_START_BIT_DETECTED);
 
 	/*
@@ -318,11 +328,20 @@ static int tegra_cec_adap_transmit(struct cec_adapter *adap, u8 attempts,
 	return 0;
 }
 
+static void tegra_cec_adap_status(struct cec_adapter *adap, struct seq_file *file)
+{
+	struct tegra_cec *cec = adap->priv;
+
+	seq_printf(file, "receive low drive count: %u\n",
+		   cec->rx_total_low_drives);
+}
+
 static const struct cec_adap_ops tegra_cec_ops = {
 	.adap_enable = tegra_cec_adap_enable,
 	.adap_log_addr = tegra_cec_adap_log_addr,
 	.adap_transmit = tegra_cec_adap_transmit,
 	.adap_monitor_all_enable = tegra_cec_adap_monitor_all_enable,
+	.adap_status = tegra_cec_adap_status,
 };
 
 static int tegra_cec_probe(struct platform_device *pdev)
-- 
2.53.0

Re: [PATCH 2/2] media: cec: tegra_cec: keep track of number of Rx Low Drives

[Mikko Perttunen]

On Friday, July 10, 2026 6:53 PM Hans Verkuil wrote:
> Keep track of Rx Low Drive conditions. Useful for detecting
> potential hardware/cable problems as it suggests unstable or
> incorrect voltage levels.
> 
> Note that the Tx Low Drive conditions are already logged in
> the CEC core.
> 
> Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
> ---
>  drivers/media/cec/platform/tegra/tegra_cec.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/media/cec/platform/tegra/tegra_cec.c b/drivers/media/cec/platform/tegra/tegra_cec.c
> index f8ffaeb78118..e22a4625f192 100644
> --- a/drivers/media/cec/platform/tegra/tegra_cec.c
> +++ b/drivers/media/cec/platform/tegra/tegra_cec.c
> @@ -24,6 +24,7 @@
>  #include <linux/of.h>
>  #include <linux/of_platform.h>
>  #include <linux/platform_device.h>
> +#include <linux/seq_file.h>
>  #include <linux/clk/tegra.h>
>  
>  #include <media/cec-notifier.h>
> @@ -47,6 +48,7 @@ struct tegra_cec {
>  	u32			tx_buf[CEC_MAX_MSG_SIZE];
>  	u8			tx_buf_cur;
>  	u8			tx_buf_cnt;
> +	u32			rx_total_low_drives;
>  };
>  
>  static inline u32 cec_read(struct tegra_cec *cec, u32 reg)
> @@ -116,6 +118,13 @@ static irqreturn_t tegra_cec_irq_handler(int irq, void *data)
>  		return IRQ_WAKE_THREAD;
>  	}
>  
> +	if (status & TEGRA_CEC_INT_STAT_RX_BUS_ERROR_DETECTED) {
> +		dev_warn_ratelimited(dev, "RX bus error detected, generated low drive\n");
> +		cec->rx_total_low_drives++;
> +		cec_write(cec, TEGRA_CEC_INT_STAT,
> +			  TEGRA_CEC_INT_STAT_RX_BUS_ERROR_DETECTED);
> +	}
> +
>  	if ((status & TEGRA_CEC_INT_STAT_TX_ARBITRATION_FAILED) ||
>  		   (status & TEGRA_CEC_INT_STAT_TX_BUS_ANOMALY_DETECTED)) {
>  		tegra_cec_error_recovery(cec);
> @@ -241,6 +250,7 @@ static int tegra_cec_adap_enable(struct cec_adapter *adap, bool enable)
>  		  TEGRA_CEC_INT_MASK_TX_BUS_ANOMALY_DETECTED |
>  		  TEGRA_CEC_INT_MASK_TX_FRAME_TRANSMITTED |
>  		  TEGRA_CEC_INT_MASK_RX_REGISTER_FULL |
> +		  TEGRA_CEC_INT_MASK_RX_BUS_ERROR_DETECTED |
>  		  TEGRA_CEC_INT_MASK_RX_START_BIT_DETECTED);
>  
>  	/*
> @@ -318,11 +328,20 @@ static int tegra_cec_adap_transmit(struct cec_adapter *adap, u8 attempts,
>  	return 0;
>  }
>  
> +static void tegra_cec_adap_status(struct cec_adapter *adap, struct seq_file *file)
> +{
> +	struct tegra_cec *cec = adap->priv;
> +
> +	seq_printf(file, "receive low drive count: %u\n",
> +		   cec->rx_total_low_drives);
> +}
> +
>  static const struct cec_adap_ops tegra_cec_ops = {
>  	.adap_enable = tegra_cec_adap_enable,
>  	.adap_log_addr = tegra_cec_adap_log_addr,
>  	.adap_transmit = tegra_cec_adap_transmit,
>  	.adap_monitor_all_enable = tegra_cec_adap_monitor_all_enable,
> +	.adap_status = tegra_cec_adap_status,
>  };
>  
>  static int tegra_cec_probe(struct platform_device *pdev)
> -- 
> 2.53.0
> 
> 

Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>