linux-kernel - Re: [PATCH v2 1/3] HID: i2c-hid: Rely on HID descriptor fetch to probe

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Date: Wed, 24 Apr 2024 10:00:17 -0700
From: Doug Anderson <dianders@...omium.org>
To: Kenny Levinsen <kl@...wtf>
Cc: Jiri Kosina <jikos@...nel.org>, Dmitry Torokhov <dtor@...omium.org>, 
	Benjamin Tissoires <benjamin.tissoires@...hat.com>, Hans de Goede <hdegoede@...hat.com>, 
	Maxime Ripard <mripard@...nel.org>, Kai-Heng Feng <kai.heng.feng@...onical.com>, 
	Johan Hovold <johan+linaro@...nel.org>, linux-input@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Radoslaw Biernacki <rad@...omium.org>, 
	Lukasz Majczak <lma@...omium.org>
Subject: Re: [PATCH v2 1/3] HID: i2c-hid: Rely on HID descriptor fetch to probe

Hi,

On Tue, Apr 23, 2024 at 5:26 AM Kenny Levinsen <kl@...wtf> wrote:
>
> To avoid error messages when a device is not present, b3a81b6c4fc6 added
> an initial bus probe using a dummy i2c_smbus_read_byte() call.
>
> Without this probe, i2c_hid_fetch_hid_descriptor() will fail with
> EREMOTEIO. Propagate the error up so the caller can handle EREMOTEIO
> gracefully, and remove the probe as it is no longer necessary.
>
> Signed-off-by: Kenny Levinsen <kl@...wtf>
> ---
>  drivers/hid/i2c-hid/i2c-hid-core.c | 20 ++++++--------------
>  1 file changed, 6 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/hid/i2c-hid/i2c-hid-core.c b/drivers/hid/i2c-hid/i2c-hid-core.c
> index 2df1ab3c31cc..515a80dbf6c7 100644
> --- a/drivers/hid/i2c-hid/i2c-hid-core.c
> +++ b/drivers/hid/i2c-hid/i2c-hid-core.c
> @@ -894,12 +894,8 @@ static int i2c_hid_fetch_hid_descriptor(struct i2c_hid *ihid)
>                                               ihid->wHIDDescRegister,
>                                               &ihid->hdesc,
>                                               sizeof(ihid->hdesc));
> -               if (error) {
> -                       dev_err(&ihid->client->dev,
> -                               "failed to fetch HID descriptor: %d\n",
> -                               error);
> -                       return -ENODEV;
> -               }
> +               if (error)
> +                       return error;
>         }
>
>         /* Validate the length of HID descriptor, the 4 first bytes:
> @@ -1014,17 +1010,13 @@ static int __i2c_hid_core_probe(struct i2c_hid *ihid)
>         struct hid_device *hid = ihid->hid;
>         int ret;
>
> -       /* Make sure there is something at this address */
> -       ret = i2c_smbus_read_byte(client);
> -       if (ret < 0) {
> +       ret = i2c_hid_fetch_hid_descriptor(ihid);
> +       if (ret == -EREMOTEIO) {

I worry a little bit about keying just off of -EREMOTEIO. If I'm
skimming the code properly it's up to the different i2c bus controller
to decide which error code to return here. Looking at, for instance,
"i2c-qcom-geni.c", I see:

[NACK] = {-ENXIO, "NACK: slv unresponsive, check its power/reset-ln"},

Maybe we should just use dev_dbg() in all cases here when we fail to
fetch the descriptor? ...otherwise I think some boards will start
getting a noisy error message.

..and confirmed that my skim seemed to be accurate. i put your
patches on my system and then changed the system to think my
hid-over-i2c device was at 0x11 instead of the normal 0x10. Now, I
get:

[    5.973417] i2c_hid_of 4-0011: failed to fetch HID descriptor: -6
[    5.979701] i2c_hid_of 4-0011: Power on failed: -6


-Doug