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References: <20210422040914.47788-1-ilya.lipnitskiy@gmail.com>
 <20210422040914.47788-6-ilya.lipnitskiy@gmail.com> <d96206db-96e2-1eb7-6b19-47c9596ccfea@nbd.name>
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From:   Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Date:   Thu, 22 Apr 2021 21:16:19 -0700
Message-ID: <CALCv0x0dRPDsrkNYOgWW-+rFj_7ysNfj+x_2AmFRa0f3No2YZg@mail.gmail.com>
Subject: Re: [PATCH net-next 05/14] net: ethernet: mtk_eth_soc: reduce MDIO
 bus access latency
To:     Felix Fietkau <nbd@nbd.name>
Cc:     John Crispin <john@phrozen.org>,
        Sean Wang <sean.wang@mediatek.com>,
        Mark Lee <Mark-MC.Lee@mediatek.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
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On Thu, Apr 22, 2021 at 5:33 AM Felix Fietkau <nbd@nbd.name> wrote:
>
>
> On 2021-04-22 06:09, Ilya Lipnitskiy wrote:
> > From: Felix Fietkau <nbd@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@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