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Subject: Re: [PATCH v2 0/4] i2c-hid: Save power by reducing i2c xfers with
 block reads
To:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     linux-i2c <linux-i2c@vger.kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        Aaron Ma <aaron.ma@canonical.com>,
        Pavel Balan <admin@kryma.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        Hans de Goede <hdegoede@redhat.com>,
        HungNien Chen <hn.chen@weidahitech.com>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>,
        linux-input <linux-input@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        You-Sheng Yang <vicamo.yang@canonical.com>,
        Wolfram Sang <wsa@kernel.org>
References: <20200917052256.5770-1-sultan@kerneltoast.com>
 <CAHp75Vea8=WchmWVOGndgarjehv+sYQh7CNUrQYqmG8hGJCzHA@mail.gmail.com>
From:   Jarkko Nikula <jarkko.nikula@linux.intel.com>
Message-ID: <86cbe3f2-30c9-615e-6d29-c94c98d6be53@linux.intel.com>
Date:   Wed, 23 Sep 2020 16:59:24 +0300
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On 9/17/20 5:02 PM, Andy Shevchenko wrote:
> On Thu, Sep 17, 2020 at 8:26 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote:
>>
>> From: Sultan Alsawaf <sultan@kerneltoast.com>
>>
>> This is a fixed resubmission of "[PATCH 0/2] i2c-hid: Save power by reducing i2c
>> xfers with block reads". That original patchset did not have enough fixes for
>> the designware i2c adapter's I2C_M_RECV_LEN feature, which is documented
>> extensively in the original email thread.
>>
>> Here is the original cover letter, which still applies:
>> "I noticed on my Dell Precision 15 5540 with an i9-9880H that simply putting my
>> finger on the touchpad would increase my system's power consumption by 4W, which
>> is quite considerable. Resting my finger on the touchpad would generate roughly
>> 4000 i2c irqs per second, or roughly 20 i2c irqs per touchpad irq.
>>
>> Upon closer inspection, I noticed that the i2c-hid driver would always transfer
>> the maximum report size over i2c (which is 60 bytes for my touchpad), but all of
>> my touchpad's normal touch events are only 32 bytes long according to the length
>> byte contained in the buffer sequence.
>>
>> Therefore, I was able to save about 2W of power by passing the I2C_M_RECV_LEN
>> flag in i2c-hid, which says to look for the payload length in the first byte of
>> the transfer buffer and adjust the i2c transaction accordingly. The only problem
>> though is that my i2c controller's driver allows bytes other than the first one
>> to be used to retrieve the payload length, which is incorrect according to the
>> SMBus spec, and would break my i2c-hid change since not *all* of the reports
>> from my touchpad are conforming SMBus block reads.
>>
>> This patchset fixes the I2C_M_RECV_LEN behavior in the designware i2c driver and
>> modifies i2c-hid to use I2C_M_RECV_LEN to save quite a bit of power. Even if the
>> peripheral controlled by i2c-hid doesn't support block reads, the i2c controller
>> drivers should cope with this and proceed with the i2c transfer using the
>> original requested length."
> 
> Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
> for I²C DesignWare patches.
> 
Also for i2c-designware

Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>