linux-kernel - Re: [PATCH net-next] net: ethernet: mlx4: Avoid assigning a value to ring_cons but not used it anymore in mlx4_en

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Date:   Sun, 13 Sep 2020 13:12:05 +0300
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     David Miller <davem@...emloft.net>, luojiaxing@...wei.com
Cc:     kuba@...nel.org, idos@...lanox.com, ogerlitz@...lanox.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH net-next] net: ethernet: mlx4: Avoid assigning a value to
 ring_cons but not used it anymore in mlx4_en_xmit()



On 9/13/2020 4:22 AM, David Miller wrote:
> From: Luo Jiaxing <luojiaxing@...wei.com>
> Date: Sat, 12 Sep 2020 16:08:15 +0800
> 
>> We found a set but not used variable 'ring_cons' in mlx4_en_xmit(), it will
>> cause a warning when build the kernel. And after checking the commit record
>> of this function, we found that it was introduced by a previous patch.
>>
>> So, We delete this redundant assignment code.
>>
>> Fixes: 488a9b48e398 ("net/mlx4_en: Wake TX queues only when there's enough room")
>>
>> Signed-off-by: Luo Jiaxing <luojiaxing@...wei.com>
> 
> Looks good, applied, thanks.
> 

Hi Luo,

I didn't get a chance to review it during the weekend.

The ring_cons local variable is used in line 903:
https://elixir.bootlin.com/linux/v5.9-rc4/source/drivers/net/ethernet/mellanox/mlx4/en_tx.c#L903

AVG_PERF_COUNTER depends on the compile-time definition of 
MLX4_EN_PERF_STAT. Otherwise it is a nop.

1. Your patch causes a degradation to the case when MLX4_EN_PERF_STAT is 
defined.
2. When MLX4_EN_PERF_STAT is not defined, we should totally remove the 
local variable declaration, not only its usage.

Please let me know if you're planning to fix this. Otherwise I'll do.

Regards,
Tariq