linux-kernel - Re: [PATCH net] net: microchip: lan743x : bidirectional throughuput improvement

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Date:   Wed, 4 Oct 2023 13:02:17 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Vishvambar Panth S <vishvambarpanth.s@...rochip.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bryan.whitehead@...rochip.com, UNGLinuxDriver@...rochip.com,
        davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        andrew@...n.ch
Subject: Re: [PATCH net] net: microchip: lan743x : bidirectional throughuput
 improvement

On 10/4/23 12:20, Jakub Kicinski wrote:
> On Wed, 27 Sep 2023 16:46:23 +0530 Vishvambar Panth S wrote:
>> The LAN743x/PCI11xxx DMA descriptors are always 4 dwords long, but the
>> device supports placing the descriptors in memory back to back or
>> reserving space in between them using its DMA_DESCRIPTOR_SPACE (DSPACE)
>> configurable hardware setting. Currently DSPACE is unnecessarily set to
>> match the host's L1 cache line size, resulting in space reserved in
>> between descriptors in most platforms and causing a suboptimal behavior
>> (single PCIe Mem transaction per descriptor). By changing the setting
>> to DSPACE=16 many descriptors can be packed in a single PCIe Mem
>> transaction resulting in a massive performance improvement in
>> bidirectional tests without any negative effects.
>> Tested and verified improvements on x64 PC and several ARM platforms
>> (typical data below)
> 
> Nobody complained for 5 years, and it's not a regression.
> Let's not treat this as a fix, please repost without the Fixes tag for
> net-next.

As a driver maintainer, you may want to provide some guarantees to your 
end users/customers that from stable version X.Y.Z the performance 
issues have been fixed. Performance improvements are definitively border 
line in terms of being considered as bug fixes though.
-- 
Florian