netdev - Re: [PATCH net-next 05/14] net: ethernet: mtk_eth

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Date:   Thu, 22 Apr 2021 21:16:19 -0700
From:   Ilya Lipnitskiy <ilya.lipnitskiy@...il.com>
To:     Felix Fietkau <nbd@....name>
Cc:     John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next 05/14] net: ethernet: mtk_eth_soc: reduce MDIO
 bus access latency

On Thu, Apr 22, 2021 at 5:33 AM Felix Fietkau <nbd@....name> wrote:
>
>
> On 2021-04-22 06:09, Ilya Lipnitskiy wrote:
> > From: Felix Fietkau <nbd@....name>
> >
> > usleep_range often ends up sleeping much longer than the 10-20us provided
> > as a range here. This causes significant latency in mdio bus acceses,
> > which easily adds multiple seconds to the boot time on MT7621 when polling
> > DSA slave ports.
> >
> > Use udelay via readx_poll_timeout_atomic, since the MDIO access does not
> > take much time
> >
> > Signed-off-by: Felix Fietkau <nbd@....name>
> > [Ilya: use readx_poll_timeout_atomic instead of cond_resched]
> I still prefer the cond_resched() variant.
No problem, I will respin with your original change. Looks like we
can't take advantage of iopoll.h routine in this case, but that's not
the end of the world!

> On a fully loaded system, I'd
> prefer to let the MDIO access take longer instead of wasting cycles on
> udelay.
>
> - Felix

Ilya