netdev - Re: [PATCH V4 07/19] net: ks8851: Remove ks8851

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Date:   Mon, 20 Apr 2020 16:20:02 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Marek Vasut <marex@...x.de>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        Petr Stetiar <ynezz@...e.cz>,
        YueHaibing <yuehaibing@...wei.com>
Subject: Re: [PATCH V4 07/19] net: ks8851: Remove ks8851_rdreg32()

On Mon, Apr 20, 2020 at 04:12:59PM +0200, Marek Vasut wrote:
> On 4/20/20 4:07 PM, Lukas Wunner wrote:
> > On Tue, Apr 14, 2020 at 08:20:17PM +0200, 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.
> >>
> >> If this has noticeable performance impact on the SPI variant of KS8851,
> >> then we should consider using regmap to abstract the SPI and parallel
> >> bus options and in case of SPI, permit regmap to merge register reads
> >> of neighboring registers into single, longer, read.
> > 
> > Bisection has shown this patch to be the biggest cause of the performance
> > regression introduced by this series:  Latency increases by about 9 usec.
> 
> Just for completeness, did you perform this bisect on current linux-next
> without any patches except this series OR your patched rpi downstream
> vendor tree Linux 4.19 with preempt-rt patch applied ?

The latter because latency without CONFIG_PREEMPT_RT_FULL=y is too imprecise
to really see the difference and that's the configuration we care about.

Thanks,

Lukas