ichsmb - correct locking strategy?

[Svatopluk Kraus]

On Tue, Feb 22, 2011 at 3:37 PM, John Baldwin <jhb at freebsd.org> wrote:
> On Friday, February 18, 2011 9:10:47 am Svatopluk Kraus wrote:
>> Hi,
>>
>>   I try to figure out locking strategy in FreeBSD and found 'ichsmb'
>> device. There is a mutex which protects smb bus (ichsmb device). For
>> example in ichsmb_readw() in sys/dev/ichsmb/ichsmb.c, the mutex is
>> locked and a command is written to bus, then unbounded (but with
>> timeout) sleep is done (mutex is unlocked during sleep). After sleep a
>> word is read from bus and the mutex is unlocked.
>>
>>   1. If an use of the device IS NOT serialized by layers around then
>> more calls to this function (or others) can be started or even done
>> before the first one is finished. The mutex don't protect sms bus.
>>
>>   2. If an use of the device IS serialized by layers around then the
>> mutex is useless.
>>
>>   Moreover, I don't mension interrupt routine which uses the mutex and
>> smb bus too.
>>
>>   Am I right? Or did I miss something?
>
> Hmm, the mutex could be useful if you have an smb controller with an interrupt
> handler (I think ichsmb or maybe intpm can support an interrupt handler) to
> prevent concurrent access to device registers.  That is the purpose of the
> mutex at least.  There is a separate locking layer in smbus itself in (see
> smbus_request_bus(), etc.).
>
> --
> John Baldwin
>

I see. So, multiple accesses to bus are protected by upper smbus layer
itself. And the mutex encloses each single access in respect of
interrupt. I.e., an interrupt can be assigned to a command (bus is
either command processing or idle) and a wait to command result can be
done atomically (no wakeup is missed). Am I right?

BTW, a mutex priority propagation isn't too much exploited here.
Maybe, it will be better for me to not take this feature into account
when thinking about locking strategy and just take a mutex in most
cases as a low level locking primitive which is indeed. Well, it seems
that things become more clear.