linux-kernel - Re: [PATCHv2] HID: i2c-hid: Add sleep between POWER ON and RESET

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Date:   Fri, 6 Jan 2017 09:11:44 +0100
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Brendan McGrath <redmcg@...mandi.dyndns.org>
Cc:     Jiri Kosina <jikos@...nel.org>, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Victor Vlasenko <victor.vlasenko@...gears.com>,
        Frederik Wenigwieser <frederik.wenigwieser@...il.com>
Subject: Re: [PATCHv2] HID: i2c-hid: Add sleep between POWER ON and RESET

On Jan 06 2017 or thereabouts, Brendan McGrath wrote:
> Support for the Asus Touchpad was recently added. It turns out this
> device can fail initialisation (and become unusable) when the RESET
> command is sent too soon after the POWER ON command.
> 
> Unfortunately the i2c-hid specification does not specify the need for
> a delay between these two commands. But it was discovered the Windows
> driver has a 1ms delay.
> 
> As a result, this patch modifies the i2c-hid module to add a sleep
> inbetween the POWER ON and RESET commands which lasts between 1ms and 5ms.
> 
> See https://github.com/vlasenko/hid-asus-dkms/issues/24 for further
> details.
> 
> Signed-off-by: Brendan McGrath <redmcg@...mandi.dyndns.org>
> ---
> Patch has been changed to use option a)
> 
> I upped the minimum sleep to 1ms to match Microsoft. I figure this is a
> safer value.
> 
>  drivers/hid/i2c-hid/i2c-hid.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/hid/i2c-hid/i2c-hid.c b/drivers/hid/i2c-hid/i2c-hid.c
> index 844662c..61ff6a6 100644
> --- a/drivers/hid/i2c-hid/i2c-hid.c
> +++ b/drivers/hid/i2c-hid/i2c-hid.c
> @@ -426,6 +426,7 @@ static int i2c_hid_hwreset(struct i2c_client *client)
>  	if (ret)
>  		goto out_unlock;
>  

Sorry to request a new version, but I think we should have a comment
here explaining the sleep. It should be noted here that it's not in the
spec but it mimics the behavior of the Windows driver, thus hardware
makers might rely on this small timeout instead of following the spec.

Cheers,
Benjamin

> +	usleep_range(1000, 5000);
>  	i2c_hid_dbg(ihid, "resetting...\n");
>  
>  	ret = i2c_hid_command(client, &hid_reset_cmd, NULL, 0);
> -- 
> 2.7.4
>