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@xxxxxxxxxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Vivien Didelot <vivien.didelot@xxxxxxxxxxxxxxxxxxxx>
> >
> > 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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