linux-kernel - Re: net: thunder: change q_len's type to handle max ring size

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Date:   Fri, 9 Feb 2018 09:59:52 +0530
From:   Sunil Kovvuri <sunil.kovvuri@...il.com>
To:     Dean Nelson <dnelson@...hat.com>
Cc:     David Miller <davem@...emloft.net>, Vadim.Lomovtsev@...ium.com,
        Robert Richter <rric@...nel.org>,
        Linux Netdev List <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sunil Goutham <sgoutham@...ium.com>,
        LAKML <linux-arm-kernel@...ts.infradead.org>
Subject: Re: net: thunder: change q_len's type to handle max ring size

On Fri, Feb 9, 2018 at 3:27 AM, Dean Nelson <dnelson@...hat.com> wrote:
> On 02/08/2018 02:34 PM, David Miller wrote:
>>
>> From: Dean Nelson <dnelson@...hat.com>
>> Date:
>>
>>> The Cavium thunder nicvf driver supports rx/tx rings of up to 65536
>>> entries per.
>>> The number of entires are stored in the q_len member of struct
>>> q_desc_mem. The
>>> problem is that q_len being a u16, results in 65536 becoming 0.
>>>
>>> In getting pointers to descriptors in the rings, the driver uses q_len
>>> minus 1
>>> as a mask after incrementing the pointer, in order to go back to the
>>> beginning
>>> and not go past the end of the ring.
>>>
>>> With the q_len set to 0 the mask is no longer correct and the driver does
>>> go
>>> beyond the end of the ring, causing various ills. Usually the first thing
>>> that
>>> shows up is a "NETDEV WATCHDOG: enP2p1s0f1 (nicvf): transmit queue 7
>>> timed out"
>>> warning.
>>>
>>> This patch remedies the problem by changing q_len to a u32.
>>>
>>> Signed-off-by: Dean Nelson <dnelson@...hat.com>
>>
>>
>> Applied, thanks.
>
>
> Thank you!
>
>>
>> Another way to solve this could have been to encode that length
>> as "length - 1"
>
>
> True. I had pondered that, but felt that since changing q_len's type
> didn't add any length to the structure and that it was less impactful
> from a number-of-lines of code changed perspective, I'd opt for this
> route.
>
> Cavium, if you'd prefer this goes the route that Dave just mentioned,
> please let me know and I can make a new patch against what's been
> applied?

Thanks for fixing this and i think the current patch is fine.

Thanks,
Sunil.

>
> Thanks,
> Dean
>
>
>
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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