Re: [PATCH 1/2] thermal: qcom: tsens: atomic temperature read with hardware-guided retries

Daniel Lezcano
2026-05-04T17:29:40Z

On 4/30/26 07:44, Priyansh Jain wrote:
> The existing TSENS temperature read logic polls the valid bit and then
> reads the temperature register. When temperature reads are triggered
> at very short intervals, this can race with hardware updates and allow
> the temperature field to be read while it is still being updated.
>
> In this case, the valid bit may already be asserted even though the
> temperature value is transitioning, resulting in an incorrect reading.
>
> Hardware programming guidelines require the temperature value and the
> valid bit to be sampled atomically in the same read transaction. A
> reading is considered valid only if the valid bit is observed set in
> that same sample.
>
> The guidelines further specify that software should attempt the
> temperature read up to three times to account for transient update
> windows. If none of the attempts observe a valid sample, a stable
> fallback value must be returned: if the first and second samples match,
> the second value is returned; otherwise, if the second and third
> samples match, the third value is returned.
>
> Update the TSENS sensor read logic to implement atomic sampling along
> with the recommended retry-and-compare fallback behavior. This removes
> the race window and ensures deterministic temperature values in
> accordance with hardware requirements.
>
> Signed-off-by: Priyansh Jain <priyansh.jain@oss.qualcomm.com>
> ---
> drivers/thermal/qcom/tsens-v1.c | 6 +-
> drivers/thermal/qcom/tsens-v2.c | 6 +-
> drivers/thermal/qcom/tsens.c | 118 +++++++++++++++++++++-----------
> drivers/thermal/qcom/tsens.h | 22 ++----
> 4 files changed, 91 insertions(+), 61 deletions(-)
>
> diff --git a/drivers/thermal/qcom/tsens-v1.c b/drivers/thermal/qcom/tsens-v1.c
> index faa5d00788ca..2e0a01348c48 100644
> --- a/drivers/thermal/qcom/tsens-v1.c
> +++ b/drivers/thermal/qcom/tsens-v1.c
> @@ -77,6 +77,9 @@ static struct tsens_features tsens_v1_feat = {
> .max_sensors = 11,
> .trip_min_temp = -40000,
> .trip_max_temp = 120000,
> + .valid_bit = BIT(14),
> + .last_temp_mask = 0x3FF,

This is GENMASK(9, 0)

> + .last_temp_resolution = 9,

Please comply with the SSOT, in the init function compute the mask with:

->last_temp_mask = GENMASK(9, 0);

and remove the initialization here

> };
>
> static struct tsens_features tsens_v1_no_rpm_feat = {
> @@ -132,8 +135,7 @@ static const struct reg_field tsens_v1_regfields[MAX_REGFIELDS] = {
> /* NO CRITICAL INTERRUPT SUPPORT on v1 */
>
> /* Sn_STATUS */
> - REG_FIELD_FOR_EACH_SENSOR11(LAST_TEMP, TM_Sn_STATUS_OFF, 0, 9),
> - REG_FIELD_FOR_EACH_SENSOR11(VALID, TM_Sn_STATUS_OFF, 14, 14),
> + REG_FIELD_FOR_EACH_SENSOR11(LAST_TEMP, TM_Sn_STATUS_OFF, 0, 14),
> /* xxx_STATUS bits: 1 == threshold violated */
> REG_FIELD_FOR_EACH_SENSOR11(MIN_STATUS, TM_Sn_STATUS_OFF, 10, 10),
> REG_FIELD_FOR_EACH_SENSOR11(LOWER_STATUS, TM_Sn_STATUS_OFF, 11, 11),
> diff --git a/drivers/thermal/qcom/tsens-v2.c b/drivers/thermal/qcom/tsens-v2.c
> index 8d9698ea3ec4..814147735ba5 100644
> --- a/drivers/thermal/qcom/tsens-v2.c
> +++ b/drivers/thermal/qcom/tsens-v2.c
> @@ -56,6 +56,9 @@ static struct tsens_features tsens_v2_feat = {
> .max_sensors = 16,
> .trip_min_temp = -40000,
> .trip_max_temp = 120000,
> + .valid_bit = BIT(21),
> + .last_temp_mask = 0xFFF,
> + .last_temp_resolution = 11,

Ditto

> };
>
> static struct tsens_features ipq8074_feat = {
> @@ -125,8 +128,7 @@ static const struct reg_field tsens_v2_regfields[MAX_REGFIELDS] = {
> [WDOG_BARK_COUNT] = REG_FIELD(TM_WDOG_LOG_OFF, 0, 7),
>
> /* Sn_STATUS */
> - REG_FIELD_FOR_EACH_SENSOR16(LAST_TEMP, TM_Sn_STATUS_OFF, 0, 11),
> - REG_FIELD_FOR_EACH_SENSOR16(VALID, TM_Sn_STATUS_OFF, 21, 21),
> + REG_FIELD_FOR_EACH_SENSOR16(LAST_TEMP, TM_Sn_STATUS_OFF, 0, 21),
> /* xxx_STATUS bits: 1 == threshold violated */
> REG_FIELD_FOR_EACH_SENSOR16(MIN_STATUS, TM_Sn_STATUS_OFF, 16, 16),
> REG_FIELD_FOR_EACH_SENSOR16(LOWER_STATUS, TM_Sn_STATUS_OFF, 17, 17),
> diff --git a/drivers/thermal/qcom/tsens.c b/drivers/thermal/qcom/tsens.c
> index a2422ebee816..15392a17ef41 100644
> --- a/drivers/thermal/qcom/tsens.c
> +++ b/drivers/thermal/qcom/tsens.c
> @@ -315,10 +315,66 @@ static inline int code_to_degc(u32 adc_code, const struct tsens_sensor *s)
> return degc;
> }
>
> +static inline enum tsens_ver tsens_version(struct tsens_priv *priv)
> +{
> + return priv->feat->ver_major;
> +}

I agree putting accessor functions is a good practice but here as it
results in duplicating the function, the benefit is discutable.

> +/**
> + * tsens_read_temp - To read temperature from hw in deciCelsius.
> + * @s: Pointer to sensor struct
> + * @field: Index into regmap_field array pointing to temperature data
> + * @temp: temperature in deciCelsius to be read from hardware
> + *
> + * This function handles temperature returned in ADC code or deciCelsius
> + * depending on IP version.
> + *
> + * Return: 0 on success, a negative errno will be returned in error cases
> + */
> +static int tsens_read_temp(const struct tsens_sensor *s, int field, int *temp)
> +{
> + struct tsens_priv *priv = s->priv;
> + int temp_val[3] = {0};
> + unsigned int status = 0;
> + int ret = 0, i;
> + int max_retry = 3;

Please avoid litterals. Add a macro for max number of retries. As the
value 3 is not an arbitrary value but a documented value, add a small
comment to tell it is a hardware requirement.

> + ret = regmap_field_read(priv->rf[field], &status);
> + if (ret)
> + return ret;
> +
> + /* VER_0 doesn't have VALID bit */
> + if (tsens_version(priv) == VER_0) {
> + *temp = status;
> + return ret;
> + }

Please use a callback for v0 and v1. Set it at probe time, so the
version does not have to be checked at very read.

> + for (i = 0; i < max_retry; i++) {
> + temp_val[i] = status & priv->feat->last_temp_mask;
> + if (() {
> + *temp = temp_val[i];
> + return ret;
> + }
> + ret = regmap_field_read(priv->rf[field], &status);
> + if (ret)
> + return ret;

It looks like more than max_retry is happening. One time before the
loop, then 3 times in loop. So 4 times in total.

> + }
> +
> + if (temp_val[0] == temp_val[1])
> + *temp = temp_val[1];
> + else if (temp_val[1] == temp_val[2])
> + *temp = temp_val[2];
> + else
> + return -EAGAIN;

We have a, b and c.

if a == b, then return b
else b == c, then return c
else return -EAGAIN

It is like we have two consecutives successful read. IMO that could be
simplified to:

int prev = INTMAX;

/*
* An explanation ...
*/

for (i = 0; i < max_retry; i++) {

int value, valid;

ret = regmap_field_read(priv->rf[field], &status);
if (ret)
return ret;

value = FIELD_GET(priv->feat->last_temp_mask, status);

valid = FIELD_GET(priv->feat->valid_bit, status)
if (valid)
return value;

if (value == prev)
return value;

prev = value;
}

return -EAGAIN;

(Not tested)