netdev - Re: [PATCH 08/14] net: ks8851: Use 16-bit read of RXFC register

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Date:   Tue, 24 Mar 2020 13:42:45 +0100
From:   Marek Vasut <marex@...x.de>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Petr Stetiar <ynezz@...e.cz>,
        YueHaibing <yuehaibing@...wei.com>, Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH 08/14] net: ks8851: Use 16-bit read of RXFC register

On 3/24/20 11:41 AM, Lukas Wunner wrote:
> On Tue, Mar 24, 2020 at 12:42:57AM +0100, Marek Vasut wrote:
>> The RXFC register is the only one being read using 8-bit accessors.
>> To make it easier to support the 16-bit accesses used by the parallel
>> bus variant of KS8851, use 16-bit accessor to read RXFC register as
>> well as neighboring RXFCTR register.
> 
> This means that an additional 8 bits need to be transferred over the
> SPI bus whenever a set of packets is read from the RX queue.  This
> should be avoided.  I'd suggest adding a separate hook to read RXFC
> and thus keep the 8-bit read function for the SPI variant.

See my comment about the 32bit read and regmap. It is slightly less
efficient, but it also makes the conversion much easier. Can you check
on the real hardware whether the is measurable performance impact ?