Re: [PATCH v2] HID: i2c-hid: Use block reads when possible to save power

[Andy Shevchenko]

On Tue, Jun 16, 2020 at 08:49:51AM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> 
> We have no way of knowing how large an incoming payload is going to be,
> so the only strategy available up until now has been to always retrieve
> the maximum possible report length over i2c, which can be quite
> inefficient. For devices that send reports in block read format, the i2c
> controller driver can read the payload length on the fly and terminate
> the i2c transaction early, resulting in considerable power savings.
> 
> On a Dell Precision 15 5540 with an i9-9880H, resting my finger on the
> touchpad causes psys power readings to go up by about 4W and hover there
> until I remove my finger. With this patch, my psys readings go from 4.7W
> down to 3.1W, yielding about 1.6W in savings. This is because my
> touchpad's max report length is 60 bytes, but all of the regular reports
> it sends for touch events are only 32 bytes, so the i2c transfer is
> roughly halved for the common case.

> +	/* Try to do a block read if the size fits in one byte */
> +	flags = size > 255 ? I2C_M_RD : I2C_M_RD | I2C_M_RECV_LEN;

AFAIR SMBus specification tells about 256. Why 255?

Andi, am I correct?

-- 
With Best Regards,
Andy Shevchenko