From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> Hi, 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. Sultan Sultan Alsawaf (2): i2c: designware: Only check the first byte for SMBus block read length HID: i2c-hid: Use block reads when possible to save power drivers/hid/i2c-hid/i2c-hid-core.c | 3 ++- drivers/i2c/busses/i2c-designware-master.c | 10 +++++----- 2 files changed, 7 insertions(+), 6 deletions(-) -- 2.27.0
