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

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Date:   Thu, 8 Feb 2018 15:57:21 -0600
From:   Dean Nelson <dnelson@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     rric@...nel.org, sgoutham@...ium.com, netdev@...r.kernel.org,
        Vadim.Lomovtsev@...ium.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: net: thunder: change q_len's type to handle max ring size

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,
Dean