proposal: require ivar accessors to succeed

[Andriy Gapon]

On 27/05/2019 21:10, Bjoern A. Zeeb wrote:
> On 27 May 2019, at 5:44, Andriy Gapon wrote:
> 
>> __BUS_ACCESSOR() macro is used to define accessors to bus IVAR variables.
>> Unfortunately, accessors defined in such a fashion completely ignore return
>> values of BUS_READ_IVAR() and BUS_WRITE_IVAR() method calls.  There is no way to
>> see if a call is successful.  Typically, this should not be a problem as a
>> device driver targets a specific bus (sometimes, buses) and it should know what
>> IVARs the bus has.  So, the driver should make only those IVAR calls that are
>> supposed to always succeed on the bus.
>> But sometimes things can go wrong as with everything else.
>>
>> So, I am proposing to add some code to __BUS_ACCESSOR to verify the success.
>> For example, we can panic when a call fails.  The checks could be under
>> INVARIANTS or under DIAGNOSTICS or under a new kernel option.
>> A less drastic option is to print a warning message on an error.
>>
>> This is mostly intended to help driver writers and maintainers.
>>
>> Opinions, suggestions, etc are welcome.
> 
> What about “fixing” the KPI (possibly adding a 2nd one), deprecating the old
> one, and (slowly over time) migrating old stuff over?

I think that the two proposals are not mutually exclusive.
And I think that both make sense.
However, it's hard for me to imagine a desire to replace code like this
  devid = pci_get_devid(dev);
with this
  err = pci_get2_devid(dev, &devid);
  if (err != 0) {
    ...
  }

Especially given that, modulo bugs, dev is going to be a device on the pci bus
and the call is going to succeed.
In other words, in my opinion, the only cases where an accessor is allowed to
fail are:
- a driver somehow attached to a device on an unexpected bus
- uncoordinated changes in between a bus driver and a device driver
So, programming errors.


-- 
Andriy Gapon