linux-kernel - Re: [PATCH v4 2/2] hwmon: (ads7828) add support for ADS7830

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Date:	Wed, 3 Oct 2012 08:13:56 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:	lm-sensors@...sensors.org,
	Guillaume Roguez <guillaume.roguez@...oirfairelinux.com>,
	Jean Delvare <khali@...ux-fr.org>,
	linux-kernel@...r.kernel.org, Steve Hardy <shardy@...hat.com>
Subject: Re: [PATCH v4 2/2] hwmon: (ads7828) add support for ADS7830

On Wed, Oct 03, 2012 at 10:03:08AM -0400, Vivien Didelot wrote:
> Hi Guenter,
> 
> On Tue, 2012-10-02 at 22:07 -0700, Guenter Roeck wrote:
> > On Tue, Oct 02, 2012 at 11:33:27PM -0400, Vivien Didelot wrote:
> > > From: Guillaume Roguez <guillaume.roguez@...oirfairelinux.com>
> > > 
> > > The ADS7830 device is almost the same as the ADS7828,
> > > except that it does 8-bit sampling, instead of 12-bit.
> > > This patch extends the ads7828 driver to support this chip.
> > > 
> > > Signed-off-by: Guillaume Roguez <guillaume.roguez@...oirfairelinux.com>
> > > Signed-off-by: Vivien Didelot <vivien.didelot@...oirfairelinux.com>
> > 
> > Guillaume,
> > Vivien,
> > 
> > [ ... ]
> > 
> > > @@ -147,6 +152,7 @@ static int ads7828_detect(struct i2c_client *client,
> > >  {
> > >  	struct i2c_adapter *adapter = client->adapter;
> > >  	u8 default_cmd_byte = ADS7828_CMD_SD_SE | ADS7828_CMD_PD3;
> > > +	bool is_8bit = false;
> > >  	int ch;
> > >  
> > >  	/* Check we have a valid client */
> > > @@ -158,7 +164,9 @@ static int ads7828_detect(struct i2c_client *client,
> > >  	 * dedicated register so attempt to sanity check using knowledge of
> > >  	 * the chip
> > >  	 * - Read from the 8 channel addresses
> > > -	 * - Check the top 4 bits of each result are not set (12 data bits)
> > > +	 * - Check the top 4 bits of each result:
> > > +	 *   - They should not be set in case of 12-bit samples
> > > +	 *   - The two bytes should be equal in case of 8-bit samples
> > >  	 */
> > >  	for (ch = 0; ch < ADS7828_NCH; ch++) {
> > >  		u8 cmd = ads7828_cmd_byte(default_cmd_byte, ch);
> > > @@ -168,13 +176,20 @@ static int ads7828_detect(struct i2c_client *client,
> > >  			return -ENODEV;
> > >  
> > >  		if (in_data & 0xF000) {
> > > -			pr_debug("%s : Doesn't look like an ads7828 device\n",
> > > -				 __func__);
> > > -			return -ENODEV;
> > > +			if ((in_data >> 8) == (in_data & 0xFF)) {
> > > +				/* Seems to be an ADS7830 (8-bit sample) */
> > > +				is_8bit = true;
> > > +			} else {
> > > +				dev_dbg(&client->dev, "doesn't look like an ADS7828 compatible device\n");
> > > +				return -ENODEV;
> > > +			}
> > >  		}
> > >  	}
> > 
> > I have been thinking about this. The detection function is already quite weak,
> > and this makes it even weaker. Reason is that you conly check for ADS7830 if the
> > check for ADS7828 failed, and you repeat the pattern for each channel.
> > Unfortunately, that means that you don't check for the ADS7830 condition if the
> > value returned for a channel happens to be a valid ADS7828 value, even if it is
> > not valid for ADS7830 (and even if you already know that the chip is not a
> > ADS7828).
> > 
> > Example:
> > 	ch=0: 0x1818	--> You know it is not ADS7828
> > 	ch=1: 0x0818	--> You know it is not ADS7830, but you don't check for it
> > 
> > I don't know an optimal solution right now, but maybe something like
> > 
> >  	maybe_7828 = true;
> > 	maybe_7830 = true;
> > 	for (ch = 0; ch < ADS7828_NCH && (maybe_7828 || maybe_7830); ch++) {
> > 		...
> > 		if (in_data & 0xF000)
> > 			maybe_7828 = false;
> > 		if ((in_data >> 8) != (in_data & 0xFF))
> > 			maybe_7830 = false;
> > 	}
> > 	if (!maybe_7828 && !maybe_7830)
> > 		return -ENODEV;
> > 
> > 	if (maybe_7828)
> > 		strlcpy(info->type, "ads7828", I2C_NAME_SIZE);
> > 	else
> > 		strlcpy(info->type, "ads7830", I2C_NAME_SIZE);
> > 
> > Frankly I would prefer to get rid of the _detect function entirely, I just don't
> > know if that would negatively affect some users. To give you an example for a
> > bad result: The function will wrongly detect an ADS7830 as ADS7828 if all ADC
> > channels report a value between 0x00 and 0x0f.
> 
> We totally agree with you here. There is no clean way to detect (i.e. to
> be sure) that this *is* an ADS7828 compatible device.
> 
> > How do you use the chip ? Do you need the detect function in your application ?
> 
> In our application, this device is statically declared in the platform
> support code, so we don't need to "detect" it.
> 
> I propose to re-send a v5 with the "s/u16 in_data/int in_data/" fix and
> the ads7828_detect() removal in the first cleanup patch, then the
> ADS7830 support. Does it sound good for you?
> 
Yes.

Thanks,
Guenter
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