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

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Message-ID: <20200616164101.GY2428291@smile.fi.intel.com>
Date:   Tue, 16 Jun 2020 19:41:01 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sultan Alsawaf <sultan@...neltoast.com>,
        Andi Shyti <andi.shyti@...el.com>
Cc:     jarkko.nikula@...ux.intel.com, aaron.ma@...onical.com,
        admin@...ma.net, benjamin.tissoires@...hat.com,
        hdegoede@...hat.com, hn.chen@...dahitech.com, jikos@...nel.org,
        kai.heng.feng@...onical.com, linux-i2c@...r.kernel.org,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        mika.westerberg@...ux.intel.com, vicamo.yang@...onical.com,
        wsa@...nel.org
Subject: Re: [PATCH v2] HID: i2c-hid: Use block reads when possible to save
 power

On Tue, Jun 16, 2020 at 08:49:51AM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@...neltoast.com>
> 
> 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