Re: adm9240 error handling (was Re: [PATCHv1] hwmon: adm9240: handle temperature readings below 0)

From: Guenter Roeck
Date: Wed Oct 05 2016 - 09:23:44 EST

On 10/04/2016 10:00 PM, Chris Packham wrote:
Hi Guenter,

On 10/05/2016 11:10 AM, Guenter Roeck wrote:
On Tue, Oct 04, 2016 at 09:09:10PM +0000, Chris Packham wrote:

Of course, all that doesn't solve the real problem in this driver, which is
that it ignores error codes from the smbus functions, but that is a different

Yeah I'd noted that too. This patch is actually a port of changes we've
made to a custom LM81 driver based on the original adm9240 driver which
included some error handling. I'll look at bringing more of that code in
for a future patch.

Sounds good...

So on this. The as-yet unpublished changes we have basically consist of
changing adm9240_update_device() to do something like this

|| !data->valid) {

for (i = 0; i < 6; i++) { /* read voltages */
- data->in[i] = i2c_smbus_read_byte_data(client,
+ ret = i2c_smbus_read_byte_data(client,
+ if (ret >= 0)
+ data->in[i] = ret;

The user is still none the wiser that there was an error but at least we
haven't just attempted to write a error code to the data. Is this the
direction you want to head? Are there other hwmon drivers that do
something saner?

Another attempt we've made is to try to read something innocuous like
the CONFIG register to check that the i2c bus is in a good state before
reading the rest of the values.

As you can guess these "fixes" have piled up over time and because
they're hard to test we haven't been brave enough to remove them.

Any thoughts?

The usual approach would be to return an error from adm9240_update_device(),
and report it to the user. Hiding the error and returning stale data doesn't
sound like a good idea.

Another option would be to drop value caching from the driver entirely,
and convert it to use regmap (so only the actually changing registers
would be read again).