linux-kernel - Re: [PATCH v2] HID: i2c-hid: Revert to await reset ACK before reading report descriptor

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Date: Tue, 2 Apr 2024 13:06:11 +0200
From: Hans de Goede <hdegoede@...hat.com>
To: Kenny Levinsen <kl@...wtf>, Jiri Kosina <jikos@...nel.org>,
 Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc: Douglas Anderson <dianders@...omium.org>, linux-input@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] HID: i2c-hid: Revert to await reset ACK before reading
 report descriptor

Hi Kenny,

Sorry for causing this regression and thank you for your fix.

One small remark comment below. In the hope of getting this merged
soon I'll prepare a v3 addressing this myself (keeping you as the author).

On 3/31/24 8:24 PM, Kenny Levinsen wrote:
> In af93a167eda9, i2c_hid_parse was changed to continue with reading the
> report descriptor before waiting for reset to be acknowledged.
> 
> This has lead to two regressions:
> 
> 1. We fail to handle reset acknowledgement if it happens while reading
>    the report descriptor. The transfer sets I2C_HID_READ_PENDING, which
>    causes the IRQ handler to return without doing anything.
> 
>    This affects both a Wacom touchscreen and a Sensel touchpad.
> 
> 2. On a Sensel touchpad, reading the report descriptor this quickly
>    after reset results in all zeroes or partial zeroes.
> 
> The issues were observed on the Lenovo Thinkpad Z16 Gen 2.
> 
> The change in question was made based on a Microsoft article[0] stating
> that Windows 8 *may* read the report descriptor in parallel with
> awaiting reset acknowledgement, intended as a slight reset performance
> optimization. Perhaps they only do this if reset is not completing
> quickly enough for their tastes?
> 
> As the code is not currently ready to read registers in parallel with a
> pending reset acknowledgement, and as reading quickly breaks the report
> descriptor on the Sensel touchpad, revert to waiting for reset
> acknowledgement before proceeding to read the report descriptor.
> 
> [0]: https://learn.microsoft.com/en-us/windows-hardware/drivers/hid/plug-and-play-support-and-power-management
> 
> Fixes: af93a167eda9 ("HID: i2c-hid: Move i2c_hid_finish_hwreset() to after reading the report-descriptor")
> Signed-off-by: Kenny Levinsen <kl@...wtf>
> ---
>  drivers/hid/i2c-hid/i2c-hid-core.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
> index 2df1ab3c31cc..72d2bccf5621 100644
> --- a/drivers/hid/i2c-hid/i2c-hid-core.c
> +++ b/drivers/hid/i2c-hid/i2c-hid-core.c
> @@ -735,9 +735,12 @@ static int i2c_hid_parse(struct hid_device *hid)
>  	mutex_lock(&ihid->reset_lock);
>  	do {
>  		ret = i2c_hid_start_hwreset(ihid);
> -		if (ret)
> +		if (ret == 0)
> +			ret = i2c_hid_finish_hwreset(ihid);
> +		else
>  			msleep(1000);
>  	} while (tries-- > 0 && ret);
> +	mutex_unlock(&ihid->reset_lock);
>  
>  	if (ret)
>  		goto abort_reset;

The abort_reset label here and in other places now is no longer
necessary. i2c_hid_start_hwreset() (on error) and i2c_hid_finish_hwreset()
(regardless of error or not) always clear I2C_HID_RESET_PENDING.

And we only do "goto abort_reset;" here and in 2 other places
below in a "if (ret) {}" branch, and abort_reset itself is:

abort_reset:
        clear_bit(I2C_HID_RESET_PENDING, &ihid->flags);
        if (ret)
                goto out;

Since the reset loop now always exits with I2C_HID_RESET_PENDING
cleared, the clear_bit() is not necessary after your changes and
ret != 0 is always true when doing goto abort_reset so
"goto abort_reset" can be replaced with "goto out" or if there is
nothing to cleanup with a simple "return ret".

As mentioned above I'll post a v3 with this addressed myself,
so that we can hopefully get the fix upstream soonest.

Regards,

Hans









> @@ -767,16 +770,8 @@ static int i2c_hid_parse(struct hid_device *hid)
>  		}
>  	}
>  
> -	/*
> -	 * Windows directly reads the report-descriptor after sending reset
> -	 * and then waits for resets completion afterwards. Some touchpads
> -	 * actually wait for the report-descriptor to be read before signalling
> -	 * reset completion.
> -	 */
> -	ret = i2c_hid_finish_hwreset(ihid);
>  abort_reset:
>  	clear_bit(I2C_HID_RESET_PENDING, &ihid->flags);
> -	mutex_unlock(&ihid->reset_lock);
>  	if (ret)
>  		goto out;
>