netdev - Re: [PATCH 06/14] net: ks8851: Remove ks8851

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Date:   Tue, 24 Mar 2020 13:37:17 +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>
Subject: Re: [PATCH 06/14] net: ks8851: Remove ks8851_rdreg32()

On 3/24/20 8:22 AM, Lukas Wunner wrote:
> On Tue, Mar 24, 2020 at 12:42:55AM +0100, Marek Vasut wrote:
>> The ks8851_rdreg32() is used only in one place, to read two registers
>> using a single read. To make it easier to support 16-bit accesses via
>> parallel bus later on, replace this single read with two 16-bit reads
>> from each of the registers and drop the ks8851_rdreg32() altogether.
> 
> This doubles the SPI transactions necessary to read the RX queue status,
> which happens for each received packet, so I expect the performance
> impact to be noticeable.  Can you keep the 32-bit variant for SPI
> and instead just introduce a 32-bit read for the MLL chip which performs
> two 16-bit reads internally?

Please test it before I'll be forced to rework the harder half of this
patchset. I don't have the SPI variant of the chip to collect those
statistics.

But the real fix here would be to convert the driver to regmap in the
end and permit regmap to merge neighboring register accesses over SPI.