On 02/19/2017 12:53 AM, Lucas Tanure wrote:
On 19/02/2017 02:10, Guenter Roeck wrote:
Hi Lucas,
On 02/17/2017 08:07 AM, Lucas Tanure wrote:
Hi,
We have here in our company an internal hardware platform for our linux
development that can measure current in a few circuits with nanoamps precision.
The measured values are exposed by a 32bit register that must read as a 32bit
float.
If we follow the hwmon sysfs interface we loss precision converting to
milliamps, where the final value exposed is zero.
Example :
Measure 291nA becomes 0mA under sysfs curr1_input
The best solution for me would be a file in sysfs that I could just expose the
float from my register (like curr1_input_float), this solution has two benefits
for me:
- I wouldn't need to convert my internal float to int
- This would give the best precision that I can have from my hardware
The value could be exposed as a 32bit hex string, like 0x349c3abc.
So, how should I proceed with this ?
A hex string would be really bad; userspace (generic) would not know what to do
with it.
You could on purpose violate the ABI and report the current in nano-Amps with
currX_input
(properly documented). This way the sensors command would still work after
proper
adjustments in /etc/sensors3.conf. If the current sense resistor is
configurable,
you could also not tell me at all and assume a current sense resistor which is
1,000,000 times larger than the one you actually use (after all, currency is
usually measured as voltage loss over a current sense resistor).
Another possibility would be to add a curr1_input_na attribute, but I would
prefer
the above mechanism.
Guenter
Hi Guenter,
You don't see any option to expose the 32bit float to user space ?
I would rather not deal with float conversion inside the kernel, as the numbers
become too large to deal and I lose precision.
I'd rather not have to deal with one of many float formats in a kernel ABI.
After all, an ABI is expected to convert from internal formats to standardized
formats.
Besides, u32 can report ~4 A in nano-Ampere, and u64 can report 18,446,744,073 A
in nano-Ampere, so I am not entirely sure I can follow your argument about
loosing precision.
Do you plan to submit your driver upstream ? Maybe it would help to see the
code.
Thanks,
Guenter
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