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

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Date:   Thu, 22 Apr 2021 14:33:31 +0200
From:   Felix Fietkau <nbd@....name>
To:     Ilya Lipnitskiy <ilya.lipnitskiy@...il.com>,
        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@...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 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. On a fully loaded system, I'd
prefer to let the MDIO access take longer instead of wasting cycles on
udelay.

- Felix