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References: <20200917052256.5770-1-sultan@kerneltoast.com>
In-Reply-To: <20200917052256.5770-1-sultan@kerneltoast.com>
From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 17 Sep 2020 17:02:27 +0300
Message-ID: <CAHp75Vea8=WchmWVOGndgarjehv+sYQh7CNUrQYqmG8hGJCzHA@mail.gmail.com>
Subject: Re: [PATCH v2 0/4] i2c-hid: Save power by reducing i2c xfers with
 block reads
To:     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>,
        Jarkko Nikula <jarkko.nikula@linux.intel.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>
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On Thu, Sep 17, 2020 at 8:26 AM Sultan Alsawaf <sultan@kerneltoast.com> wro=
te:
>
> From: Sultan Alsawaf <sultan@kerneltoast.com>
>
> This is a fixed resubmission of "[PATCH 0/2] i2c-hid: Save power by reduc=
ing 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 putt=
ing 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 r=
oughly
> 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 tr=
ansfer
> 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 b=
yte 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 fir=
st one
> to be used to retrieve the payload length, which is incorrect according t=
o the
> SMBus spec, and would break my i2c-hid change since not *all* of the repo=
rts
> from my touchpad are conforming SMBus block reads.
>
> This patchset fixes the I2C_M_RECV_LEN behavior in the designware i2c dri=
ver 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 con=
troller
> 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 I=C2=B2C DesignWare patches.

>
> Sultan
>
> Sultan Alsawaf (4):
>   i2c: designware: Fix transfer failures for invalid SMBus block reads
>   i2c: designware: Ensure tx_buf_len is nonzero for SMBus block reads
>   i2c: designware: Allow SMBus block reads up to 255 bytes in length
>   HID: i2c-hid: Use block reads when possible to save power
>
>  drivers/hid/i2c-hid/i2c-hid-core.c         |  5 ++++-
>  drivers/i2c/busses/i2c-designware-master.c | 15 +++++++++------
>  2 files changed, 13 insertions(+), 7 deletions(-)
>
> --
> 2.28.0
>


--=20
With Best Regards,
Andy Shevchenko