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Message-ID: <391c60a4-b86f-48e4-ba64-abdcb79d71ef@redhat.com>
Date: Tue, 2 Apr 2024 13:06:11 +0200
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Subject: Re: [PATCH v2] HID: i2c-hid: Revert to await reset ACK before reading
 report descriptor
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To: Kenny Levinsen <kl@kl.wtf>, Jiri Kosina <jikos@kernel.org>,
 Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: Douglas Anderson <dianders@chromium.org>, linux-input@vger.kernel.org,
 linux-kernel@vger.kernel.org
References: <20240331182440.14477-1-kl@kl.wtf>
From: Hans de Goede <hdegoede@redhat.com>
In-Reply-To: <20240331182440.14477-1-kl@kl.wtf>
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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@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;
>