linux-kernel - Re: [PATCH net v1 2/2] lan743x: boost performance: limit PCIe bandwidth requirement

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Message-ID: <CAGngYiWFKeXL6SFMEvVyj1rMU3WuC4jDJKGSY4FB_vd=cCZsiQ@mail.gmail.com>
Date:   Wed, 16 Dec 2020 22:18:14 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, Jakub Kicinski <kuba@...nel.org>,
        Bryan Whitehead <bryan.whitehead@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        David S Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v1 2/2] lan743x: boost performance: limit PCIe
 bandwidth requirement

On Wed, Dec 16, 2020 at 8:01 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>
> x86 is a fully cache and device coherent memory architecture and there
> are smarts like DDIO to bring freshly DMA'd data into the L3 cache
> directly. For ARMv7, it depends on the hardware you have, most ARMv7
> SoCs do not have hardware maintained coherency at all, this means that
> doing the cache maintenance operations is costly. This is even true on
> platforms that use an external cache controller (PL310).

Thank you, that's quite fascinating. The functions my armv7 spends most
time in during ethernet receive (eg. v7_dma_inv_range) do appear to be
just nops on x86.