linux-kernel - Re: [PATCH V2 2/7] mfd: PCF50633 adc driver

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Date:	Fri, 19 Dec 2008 12:15:47 +0000
From:	Andy Green <andy@...nmoko.com>
To:	Jonathan Cameron <Jonathan.Cameron@...il.com>
CC:	Balaji Rao <balajirrao@...nmoko.org>, linux-kernel@...r.kernel.org,
	Samuel Ortiz <sameo@...nedhand.com>
Subject: Re: [PATCH V2 2/7] mfd: PCF50633 adc driver

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Somebody in the thread at some point said:

Hi Johnathan -

| This needs a bit more explanation. Particularly as the data
| sheet describes that accsw as 'for rationmetric measurement'.

What's going on here is that ACCSW can be driven from the PMU to bias a
resistor divider to scale what's being measured... we use it to detect
resistor to 0V on ID pin of USB on our charger.

You can power accsw / that divider "by hand" which is what we're doing
here, or you can have the ratiometric state machine change the mux /
bias automatically between the two measurements it takes, which we're
not doing.

| Also, seeing as I assume this is the only driver that can touch
| these registers and you don't change them else where, why can't
| they be in initial setup code rather than here? (probably a good
| reason, but be nice to have it document here!)

ADCC2 setting concerned with ratiometric disable could indeed be moved
to init, it was done like this before I did the queuing stuff and we
took ADC measurement in two different places in the code.  But in fact
for the best we should only enable accsw until the measurement completes
and save a tiny bit of power.

|> +    /* kill ratiometric, but enable ACCSW biasing */
|> +    pcf50633_reg_write(pcf, PCF50633_REG_ADCC2, 0x00);
|> +    pcf50633_reg_write(pcf, PCF50633_REG_ADCC3, 0x01);
|> +
|> +    /* start ADC conversion on selected channel */
|> +    pcf50633_reg_write(pcf, PCF50633_REG_ADCC1, channel | avg |
| ...
|> +
|> +static void pcf50633_adc_irq(int irq, void *data)
|> +{
|> +    struct pcf50633_adc *adc = data;
|> +    struct pcf50633 *pcf = adc->pcf;
|> +    struct pcf50633_adc_request *req;
|> +    int head;
|> +    mutex_lock(&adc->queue_mutex);
|> +    head = adc->queue_head;
|> +
|> +    req = adc->queue[head];
|> +    if (WARN_ON(!req)) {
|> +        dev_err(pcf->dev, "pcf50633-adc irq: ADC queue empty!\n");
|> +        mutex_unlock(&adc->queue_mutex);
|> +        return;
|> +    }
|> +    adc->queue[head] = NULL;
|
| Weird formatting?
|
|> +    adc->queue_head = (head + 1) &
|> +                      (PCF50633_MAX_ADC_FIFO_DEPTH - 1);
|> +
|> +    mutex_unlock(&adc->queue_mutex);

This would save the power

~    pcf50633_reg_write(pcf, PCF50633_REG_ADCC3, 0x0);

|> +    req->callback(pcf, req->callback_param, adc_result(pcf));
|> +    kfree(req);
|> +
|> +    trigger_next_adc_job_if_any(pcf);
|> +}
|> +
| Rest looks good to me.
|
| Acked-by: Jonathan Cameron
|

- -Andy
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