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

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Message-ID: <20200420144403.eoo47sq7pwp6yc7d@wunner.de>
Date:   Mon, 20 Apr 2020 16:44:03 +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 04:24:05PM +0200, Marek Vasut wrote:
> On 4/20/20 4:20 PM, Lukas Wunner wrote:
> > 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.
> 
> Why am I not able to see the same on the RPi3 then ?
> How can I replicate this observation ?

Compile this branch with CONFIG_PREEMPT_RT_FULL=y:

https://github.com/l1k/linux/commits/revpi-4.19-marek-v4

Alternatively, download this file:

http://wunner.de/ks8851-marekv4.tar

Install the "raspberrypi-kernel" deb-package included in the tarball on a
stock Raspbian image and copy one of the included ks8851.ko to:
/lib/modules/4.19.95-rt38-v7+/kernel/drivers/net/ethernet/micrel

Thanks,

Lukas