Re: [PATCH v2 0/4] i2c-hid: Save power by reducing i2c xfers with block reads

[Jarkko Nikula]

On 9/17/20 5:02 PM, Andy Shevchenko wrote:
> On Thu, Sep 17, 2020 at 8:26 AM Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
> > From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> for I²C DesignWare patches.
Also for i2c-designware

Acked-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>