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

From: Andy Shevchenko
Date: Thu Sep 17 2020 - 10:50:14 EST

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.

> 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

With Best Regards,
Andy Shevchenko