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Subject: Re: [PATCH net-next 05/14] net: ethernet: mtk_eth_soc: reduce MDIO
 bus access latency
To:     Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>,
        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@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
References: <20210422040914.47788-1-ilya.lipnitskiy@gmail.com>
 <20210422040914.47788-6-ilya.lipnitskiy@gmail.com>
From:   Felix Fietkau <nbd@nbd.name>
Message-ID: <d96206db-96e2-1eb7-6b19-47c9596ccfea@nbd.name>
Date:   Thu, 22 Apr 2021 14:33:31 +0200
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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. On a fully loaded system, I'd
prefer to let the MDIO access take longer instead of wasting cycles on
udelay.

- Felix