mfd: revise locking for pcf50633 ADC

Current implementation is prone to races, this patch attempts to remove all
but one (in pcf50633_adc_sync_read).

The idea is that we need to guard the queue access only on inserting and
removing items. If we insert and there're no more items in the queue it
means that the last irq already happened and we need to trigger ADC
manually. If not, then the next conversion will be triggered by the irq
handler upon completion of the previous.

Signed-off-by: Paul Fertser <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

drivers/mfd/pcf50633-adc.c

···
 	struct pcf50633_adc *adc = __to_adc(pcf);
 	int head;
 
-	mutex_lock(&adc->queue_mutex);
-
 	head = adc->queue_head;
 
-	if (!adc->queue[head]) {
-		mutex_unlock(&adc->queue_mutex);
+	if (!adc->queue[head])
 		return;
-	}
-	mutex_unlock(&adc->queue_mutex);
 
 	adc_setup(pcf, adc->queue[head]->mux, adc->queue[head]->avg);
 }
···
 
 	if (adc->queue[tail]) {
 		mutex_unlock(&adc->queue_mutex);
+		dev_err(pcf->dev, "ADC queue is full, dropping request\n");
 		return -EBUSY;
 	}
 
 	adc->queue[tail] = req;
+	if (head == tail)
+		trigger_next_adc_job_if_any(pcf);
 	adc->queue_tail = (tail + 1) & (PCF50633_MAX_ADC_FIFO_DEPTH - 1);
 
 	mutex_unlock(&adc->queue_mutex);
-
-	trigger_next_adc_job_if_any(pcf);
 
 	return 0;
 }
···
 int pcf50633_adc_sync_read(struct pcf50633 *pcf, int mux, int avg)
 {
 	struct pcf50633_adc_request *req;
+	int err;
 
 	/* req is freed when the result is ready, in interrupt handler */
 	req = kzalloc(sizeof(*req), GFP_KERNEL);
···
 	req->callback_param = req;
 
 	init_completion(&req->completion);
-	adc_enqueue_request(pcf, req);
+	err = adc_enqueue_request(pcf, req);
+	if (err)
+		return err;
+
 	wait_for_completion(&req->completion);
 
+	/* FIXME by this time req might be already freed */
 	return req->result;
 }
 EXPORT_SYMBOL_GPL(pcf50633_adc_sync_read);
···
 	req->callback = callback;
 	req->callback_param = callback_param;
 
-	adc_enqueue_request(pcf, req);
-
-	return 0;
+	return adc_enqueue_request(pcf, req);
 }
 EXPORT_SYMBOL_GPL(pcf50633_adc_async_read);
 
···
 	struct pcf50633_adc *adc = data;
 	struct pcf50633 *pcf = adc->pcf;
 	struct pcf50633_adc_request *req;
-	int head;
+	int head, res;
 
 	mutex_lock(&adc->queue_mutex);
 	head = adc->queue_head;
···
 	adc->queue_head = (head + 1) &
 				      (PCF50633_MAX_ADC_FIFO_DEPTH - 1);
 
+	res = adc_result(pcf);
+	trigger_next_adc_job_if_any(pcf);
+
 	mutex_unlock(&adc->queue_mutex);
 
-	req->callback(pcf, req->callback_param, adc_result(pcf));
+	req->callback(pcf, req->callback_param, res);
 	kfree(req);
-
-	trigger_next_adc_job_if_any(pcf);
 }
 
 static int __devinit pcf50633_adc_probe(struct platform_device *pdev)