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Date:   Thu, 7 May 2020 16:22:54 +0200
From:   Marek Vasut <>
To:     Andrew Lunn <>
Cc:     Lukas Wunner <>,,
        "David S . Miller" <>,
        Petr Stetiar <>,
        YueHaibing <>
Subject: Re: [PATCH V4 07/19] net: ks8851: Remove ks8851_rdreg32()

On 4/20/20 5:50 PM, Andrew Lunn wrote:
> On Mon, Apr 20, 2020 at 05:38:16PM +0200, Marek Vasut wrote:
>> On 4/20/20 4:44 PM, Lukas Wunner wrote:
>>> 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:
>>> Alternatively, download this file:
>>> 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
>> Why don't you rather try to replicate this problem in linux-next?
> Hi Lukas
> 4.19 is dead in terms of development work. It is now over 18 months
> old. All mainline development work is done against either the last
> -rc1 kernel, or a subsystems specific 'for-next' branch.
> Please test Marek patches against net-next. If there are performance
> regressions with net-next, they should be addressed, but mainline does
> not care about some random long time dead tree.

Are there any news on this one ?

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