linux-kernel - Re: [PATCH] Input: elan_i2c - check if device is there before really probing

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Message-ID: <20170508211031.GA31381@dtor-ws>
Date:   Mon, 8 May 2017 14:10:31 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-input@...r.kernel.org, KT Liao <ktalex.liao@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: elan_i2c - check if device is there before really
 probing

On Wed, May 03, 2017 at 11:42:54PM -0700, Guenter Roeck wrote:
> On 05/03/2017 11:29 PM, Dmitry Torokhov wrote:
> >Before trying to properly initialize the touchpad and generate bunch of
> >errors, let's first see it there is anything at the given address. If we
> >get error, fail silently with -ENXIO.
> >
> >Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> >---
> > drivers/input/mouse/elan_i2c_core.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> >diff --git a/drivers/input/mouse/elan_i2c_core.c b/drivers/input/mouse/elan_i2c_core.c
> >index 5d3b53dd2fa2..0479730e7c5c 100644
> >--- a/drivers/input/mouse/elan_i2c_core.c
> >+++ b/drivers/input/mouse/elan_i2c_core.c
> >@@ -1040,6 +1040,7 @@ static int elan_probe(struct i2c_client *client,
> > 	const struct elan_transport_ops *transport_ops;
> > 	struct device *dev = &client->dev;
> > 	struct elan_tp_data *data;
> >+	union i2c_smbus_data dummy;
> > 	unsigned long irqflags;
> > 	int error;
> >
> >@@ -1091,6 +1092,14 @@ static int elan_probe(struct i2c_client *client,
> > 		return error;
> > 	}
> >
> >+	/* Make sure there is something at this address */
> >+	error = i2c_smbus_xfer(client->adapter, client->addr, 0,
> >+			       I2C_SMBUS_READ, 0, I2C_SMBUS_BYTE, &dummy);
> 
> Can't you just call i2c_smbus_read_byte_data(client, 0); and check if it returns
> an error ?

Are we guaranteed to get a result from command "0"? I am not sure, and
looking at least at i2c_smbus_xfer_emulated() I2C_SMBUS_READ with
I2C_SMBUS_BYTE modifier works differently form
I2C_SMBUS_READ/I2C_SMBUS_BYTE_DATA (the former uses a single i2c message
vs 2 for the latter).

Also the form above is what i2c core uses to do detection when adapter
does not implement I2C_SMBUS_QUICK, so I'd rather leave it as it was.

Thanks.

-- 
Dmitry