Re: [PATCH v2] hwmon: (sbtsi) Don't read sensor more than once if it doesn't respond
From: Konstantin Aladyshev
Date: Tue Apr 06 2021 - 17:10:01 EST
Thanks for the answer!
Sorry for the confusion, by the "CPU is off" I meant "CPU is present,
but currently it is in the powered off state".
Therefore it is not possible to put these checks only in a probe
function. And I don't know either if it is a good idea to cache
config/min/max values.
I use this driver on an OpenBMC system, which uses other software
rather than lm-sensors utility. I guess that is why my priorities are
shifted.
By the way, I've noticed that the mutex check is absent in a
SBTSI_REG_CONFIG read call both in the original driver version and in
my patch, is this an error?
Best regards,
Konstantin Aladyshev
On Tue, Apr 6, 2021 at 11:09 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On 4/6/21 12:20 PM, Konstantin Aladyshev wrote:
> > Thanks for the comment.
> > Yes, you are correct, this patch adds an extra 'i2c_smbus_read_byte_data' call for the temp_max/temp_min reads.
> > I guess I did that intentionally because I just wanted to keep the restructured code concise. After all I thought, 'temp_input' generally is read more often than 'temp_max/temp_min'.
> > As I understand now, it seems like it is not acceptable. Therefore could you point me in the right direction about what I should do?
> > Should I just stick with the original driver version and simply add two more i2c call checks for the first operations for min/max?
> >
>
> Correct, it is not acceptable. A normal use case for hwmon devices is to use the "sensors"
> command which _will_ read all attributes. i2c reads are expensive, and unnecessary read
> operations should be avoided.
>
> There are several ways to solve the problem. Checking return values after each
> read is the simple option. There are other possibilities, such as reading the limits
> and the read order only once during probe, but I don't know enough about the
> hardware to suggest a more sophisticated solution. For example, I don't know
> what "CPU is off" means. Offline ? Not present ? If it means "not present",
> or if the status is permanent, the condition should be handled in the is_visible
> function (or the driver should not be instantiated in the first place).
> Otherwise, the code should possibly return -ENODATA instead of -ETIMEDOUT
> on error. But, again, I can not really suggest a better solution since
> I don't know enough (nothing, actually) about the hardware (and the public
> part of the SBTSI specification doesn't say anything about expected behavior
> for offline CPUs or CPU cores).
>
> What I did find, though, is that the driver does not implement temperature
> offset support, and it that doesn't support reporting alerts. I'd have assumed
> this to be more important than optimizing error handling, but that is just
> my personal opinion.
>
> Thanks,
> Guenter
>
> > Best regards,
> > Konstantin Aladyshev
> >
> >
> > On Tue, Apr 6, 2021 at 9:42 PM Guenter Roeck <linux@xxxxxxxxxxxx <mailto:linux@xxxxxxxxxxxx>> wrote:
> >
> > On 4/6/21 11:16 AM, Konstantin Aladyshev wrote:
> > > SBTSI sensors don't work when the CPU is off.
> > > In this case every 'i2c_smbus_read_byte_data' function would fail
> > > by a timeout.
> > > Currently temp1_max/temp1_min file reads can cause two such timeouts
> > > for every read.
> > > Restructure code so there will be no more than one timeout for every
> > > read operation.
> > >
> > > Signed-off-by: Konstantin Aladyshev <aladyshev22@xxxxxxxxx <mailto:aladyshev22@xxxxxxxxx>>
> > > ---
> > > Changes in v2:
> > > - Fix typo in a commit message
> > > - Don't swap temp_int/temp_dec checks at the end of the 'sbtsi_read' function
> > >
> >
> > This doesn't explain the reason for the extra read operation for
> > limits. Preventing a second read in error cases is not an argument
> > for adding an extra read for non-error cases.
> >
> > Guenter
> >
> > > drivers/hwmon/sbtsi_temp.c | 55 +++++++++++++++++++-------------------
> > > 1 file changed, 27 insertions(+), 28 deletions(-)
> > >
> > > diff --git a/drivers/hwmon/sbtsi_temp.c b/drivers/hwmon/sbtsi_temp.c
> > > index e35357c48b8e..4ae48635bb31 100644
> > > --- a/drivers/hwmon/sbtsi_temp.c
> > > +++ b/drivers/hwmon/sbtsi_temp.c
> > > @@ -74,48 +74,47 @@ static int sbtsi_read(struct device *dev, enum hwmon_sensor_types type,
> > > u32 attr, int channel, long *val)
> > > {
> > > struct sbtsi_data *data = dev_get_drvdata(dev);
> > > + u8 temp_int_reg, temp_dec_reg;
> > > s32 temp_int, temp_dec;
> > > int err;
> > >
> > > switch (attr) {
> > > case hwmon_temp_input:
> > > - /*
> > > - * ReadOrder bit specifies the reading order of integer and
> > > - * decimal part of CPU temp for atomic reads. If bit == 0,
> > > - * reading integer part triggers latching of the decimal part,
> > > - * so integer part should be read first. If bit == 1, read
> > > - * order should be reversed.
> > > - */
> > > - err = i2c_smbus_read_byte_data(data->client, SBTSI_REG_CONFIG);
> > > - if (err < 0)
> > > - return err;
> > > -
> > > - mutex_lock(&data->lock);
> > > - if (err & BIT(SBTSI_CONFIG_READ_ORDER_SHIFT)) {
> > > - temp_dec = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_DEC);
> > > - temp_int = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_INT);
> > > - } else {
> > > - temp_int = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_INT);
> > > - temp_dec = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_DEC);
> > > - }
> > > - mutex_unlock(&data->lock);
> > > + temp_int_reg = SBTSI_REG_TEMP_INT;
> > > + temp_dec_reg = SBTSI_REG_TEMP_DEC;
> > > break;
> > > case hwmon_temp_max:
> > > - mutex_lock(&data->lock);
> > > - temp_int = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_HIGH_INT);
> > > - temp_dec = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_HIGH_DEC);
> > > - mutex_unlock(&data->lock);
> > > + temp_int_reg = SBTSI_REG_TEMP_HIGH_INT;
> > > + temp_dec_reg = SBTSI_REG_TEMP_HIGH_DEC;
> > > break;
> > > case hwmon_temp_min:
> > > - mutex_lock(&data->lock);
> > > - temp_int = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_LOW_INT);
> > > - temp_dec = i2c_smbus_read_byte_data(data->client, SBTSI_REG_TEMP_LOW_DEC);
> > > - mutex_unlock(&data->lock);
> > > + temp_int_reg = SBTSI_REG_TEMP_LOW_INT;
> > > + temp_dec_reg = SBTSI_REG_TEMP_LOW_DEC;
> > > break;
> > > default:
> > > return -EINVAL;
> > > }
> > >
> > > + /*
> > > + * ReadOrder bit specifies the reading order of integer and
> > > + * decimal part of CPU temp for atomic reads. If bit == 0,
> > > + * reading integer part triggers latching of the decimal part,
> > > + * so integer part should be read first. If bit == 1, read
> > > + * order should be reversed.
> > > + */
> > > + err = i2c_smbus_read_byte_data(data->client, SBTSI_REG_CONFIG);
> > > + if (err < 0)
> > > + return err;
> > > +
> > > + mutex_lock(&data->lock);
> > > + if (err & BIT(SBTSI_CONFIG_READ_ORDER_SHIFT)) {
> > > + temp_dec = i2c_smbus_read_byte_data(data->client, temp_dec_reg);
> > > + temp_int = i2c_smbus_read_byte_data(data->client, temp_int_reg);
> > > + } else {
> > > + temp_int = i2c_smbus_read_byte_data(data->client, temp_int_reg);
> > > + temp_dec = i2c_smbus_read_byte_data(data->client, temp_dec_reg);
> > > + }
> > > + mutex_unlock(&data->lock);
> > >
> > > if (temp_int < 0)
> > > return temp_int;
> > >
> >
>