netdev - Re: [PATCH net-next 4/4] s390/qeth: use all configured RX buffers

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Message-ID: <dcc95391-0dea-e7d4-1901-25c00c7a3c60@linux.ibm.com>
Date:   Fri, 31 Jul 2020 09:50:43 +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 <hca@...ux.ibm.com>,
        Ursula Braun <ubraun@...ux.ibm.com>,
        Karsten Graul <kgraul@...ux.ibm.com>
Subject: Re: [PATCH net-next 4/4] s390/qeth: use all configured RX buffers

On 31.07.20 01:37, Jakub Kicinski wrote:
> On Thu, 30 Jul 2020 17:01:21 +0200 Julian Wiedmann wrote:
>> The (misplaced) comment doesn't make any sense, enforcing an
>> uninitialized RX buffer won't help with IRQ reduction.
>>
>> So make the best use of all available RX buffers.
> 
> Often one entry in the ring is left free to make it easy to
> differentiate between empty and full conditions. 
> 
> Is this not the reason here?
> 

Hmm no, the HW architecture works slightly different.

There's no index register that we could query for HW progress,
each ring entry has an associated state byte that needs to be
inspected and indicates HW progress (among other things).

So this was more likely just a mis-interpretation of how the
(quirky) IRQ reduction mechanism works in HW, or maybe part of
a code path that got removed during the NAPI conversion.