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Date:   Wed, 6 May 2020 09:56:41 +0200
From:   Julian Wiedmann <jwi@...ux.ibm.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ursula Braun <ubraun@...ux.ibm.com>
Subject: Re: [PATCH net-next 10/11] s390/qeth: allow reset via ethtool

On 05.05.20 23:28, Jakub Kicinski wrote:
> On Tue, 5 May 2020 21:57:43 +0200 Julian Wiedmann wrote:
>>> This is the comment from the uAPI header:
>>>
>>> /* The reset() operation must clear the flags for the components which
>>>  * were actually reset.  On successful return, the flags indicate the
>>>  * components which were not reset, either because they do not exist
>>>  * in the hardware or because they cannot be reset independently.  The
>>>  * driver must never reset any components that were not requested.
>>>  */
>>>
>>> Now let's take ETH_RESET_PHY as an example. Surely you're not resetting
>>> any PHY here, so that bit should not be cleared. Please look at the
>>> bits and select the ones which make sense, add whatever is missing.
>>>   
>>
>> It's a virtual device, _none_ of them make much sense?! We better not be
>> resetting any actual HW components, the other interfaces on the same
>> adapter would be quite unhappy about that.
> 
> Well, then, you can't use the API in its current form. You can't say
> none of the sub-options are applicable, but the sum of them does.
> 

Agreed, that's my take as well. So we'll basically need a ETH_RESET_FULL bit,
for devices that don't fit into the fine-grained component model.

>> Sorry for being dense, and I appreciate that the API leaves a lot of room
>> for sophisticated partial resets where the driver/HW allows it.
>> But it sounds like what you're suggesting is
>> (1) we select a rather arbitrary set of components that _might_ represent a
>>     full "virtual" reset, and then
>> (2) expect the user to guess a super-set of these features. And not worry
>>     when they selected too much, and this obscure PHY thing failed to reset.
> 
> No, please see the code I provided below, and read how the interface 
> is supposed to work. I posted the code comment in my previous reply. 
> I don't know what else I can do for you.
> 
> User can still pass "all" but you can't _clear_ all bits, 'cause you
> didn't reset any PHY, MAC, etc.
> 
>> So I looked at gve's implementation and thought "yep, looks simple enough".
> 
> Ugh, yeah, gve is not a good example.
> 
>> But if we start asking users to interpret HW bits that hardly make any
>> sense to them, we're worse off than with the existing custom sysfs trigger...
> 
> Actually - operationally, how do you expect people to use this reset?
> Some user space system detects the NIC is in a bad state? Does the
> interface communicate that via some log messages or such?
> 
> The commit message doesn't really explain the "why".
> 

Usually the driver will detect a hung condition itself, and trigger an
automatic reset internally (eg. from the TX watchdog).
But if that doesn't work, you'll hopefully get enough noisy log warnings
to investigate & reset the interface manually.
Besides that, it's just an easy way to exercise/test the reset code.

Integration with a daemon / management layer definitely sounds like an
option, and I'd much rather point those people towards ethtool instead
of sysfs.

>>> Then my suggestion would be something like:
>>>
>>>   #define QETH_RESET_FLAGS (flag | flag | flag)
>>>
>>>   if ((*flags & QETH_RESET_FLAGS) != QETH_RESET_FLAGS))
>>> 	return -EINVAL;
>>>   ...
>>>   *flags &= ~QETH_RESET_FLAGS;

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