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

[Mikko Perttunen <mperttunen@nvidia.com>]

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>