Re: [Intel-wired-lan] [PATCH net-next 2/2] ice: Remove gettime HW semaphore

[Jacob Keller <jacob.e.keller@intel.com>]

On 10/5/2022 2:10 PM, Vinicius Costa Gomes wrote:
> "Kolacinski, Karol" <karol.kolacinski@intel.com> writes:
> 
>> Hi Vinicius,
>>
>>> I think the problem is less about concurrent writes/reads than
>>> concurrent reads: the fact that the registers are latched when the
>>> "lower" register is read, makes me worried that there's a (very narrow)
>>> window during rollover in which the "losing" read (of multiple threads
>>> doing reads) can return a wrong value.
>>
>> The issue in this case is, it's either risk of reading slightly wrong
>> value or having multiple timeouts and errors.
>> We experienced a lot of simultaneous reads on multiple PFs (especially
>> on E822 HW with 8 ports) and even with increased timeout to acquire
>> the HW semaphore, it still failed.
> 
> I am wondering if using a hw semaphore is making the problem worse than
> it needs to be. Why a kernel spinlock can't be used?
> 
> 
> Cheers,

The same clock is shared across multiple ports which operate as
independent PCIe devices, hence having their own instance of the ice
driver structures. A spinlock doesn't work because they wouldn't be
using the same lock.

We could try to share the lock in software between PFs, but its actually
quite difficult to do that with the existing PCIe driver model.
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