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media: cec: tegra_cec: don't break off msg on NACK

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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>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
This commit is contained in:
Hans Verkuil 2026-07-10 11:53:17 +02:00 committed by Mauro Carvalho Chehab
parent 6eac737108
commit be627f257c
1 changed files with 12 additions and 1 deletions
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13
drivers/media/cec/platform/tegra/tegra_cec.c
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@ -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;
}