netdev - Re: [PATCH net 1/5] ibmvnic: harden interrupt handler

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Message-Id: <0cea7654-6722-5e68-cd05-cc51bbf7a5ae@linux.vnet.ibm.com>
Date:   Thu, 26 Jan 2017 10:44:22 -0600
From:   Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, wvoigt@...ibm.com,
        jallen@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org,
        nfont@...ux.vnet.ibm.com
Subject: Re: [PATCH net 1/5] ibmvnic: harden interrupt handler

On 01/25/2017 10:04 PM, David Miller wrote:
> From: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
> Date: Wed, 25 Jan 2017 15:02:19 -0600
>
>> Move most interrupt handler processing into a tasklet, and
>> introduce a delay after re-enabling interrupts to fix timing
>> issues encountered in hardware testing.
>>
>> Signed-off-by: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
> I don't think you have any idea what the real problem is.  This looks
> like a hack, at best.  Next patch you'll increase the delay to "20",
> right?  And if that doesn't work you'll try "40".
>
> Or if you do know the reason, you need to explain it in detail in this
> commit message so that we can properly evaluate this patch.

You're right, I should have given more explanation in the commit message about the bug encountered and our reasons for this sort of fix.  The issue is that there are some scenarios during the device init where multiple messages are sent by firmware in one interrupt request. 

We have observed behavior where messages are delayed, resulting in the interrupt handler completing before the delayed messages can be processed, fouling up the device bring-up in the device probing and elsewhere.  The goal of the delay is to buy some time for the hypervisor to forward all the CRQ messages from the firmware.
>
> Furthermore, if you're going to move all of your packet processing
> into software interrupt context, you might as well use NAPI polling
> which is a purposefully built piece of infrastructure for doing
> exactly this.
>
This interrupt handler doesn't handle packet processing, but communications between the driver and firmware for device settings and resource allocation.  Packet processing is done with different interrupts that do use NAPI polling.