staging: comedi: das1800: document the spinlock

The comedi_device spinlock is used to protect the indirect addressing
selected by the DAS1800_SELECT register. It also prevents races between
the interrupt handler and the analog input (*poll).

Update the comments to make this clear.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/comedi/drivers/das1800.c b/drivers/staging/comedi/drivers/das1800.c
index 677f430b5c3e44ba09b4c41dd411087311dcdd3d..365a3d5231a9395bc7d170af8617e8f9e6d43511 100644 (file)
--- a/drivers/staging/comedi/drivers/das1800.c
+++ b/drivers/staging/comedi/drivers/das1800.c
@@ -490,8 +490,9 @@ static void das1800_ai_handler(struct comedi_device *dev)
        struct comedi_cmd *cmd = &async->cmd;
        unsigned int status = inb(dev->iobase + DAS1800_STATUS);
 
-       /*  select adc for base address + 0 */
+       /* select adc register (spinlock is already held) */
        outb(ADC, dev->iobase + DAS1800_SELECT);
+
        /*  dma buffer full */
        if (devpriv->irq_dma_bits & DMA_ENABLED) {
                /*  look for data from dma transfer even if dma terminal count hasn't happened yet */
@@ -535,9 +536,14 @@ static int das1800_ai_poll(struct comedi_device *dev,
 {
        unsigned long flags;
 
-       /*  prevent race with interrupt handler */
+       /*
+        * Protects the indirect addressing selected by DAS1800_SELECT
+        * in das1800_ai_handler() also prevents race with das1800_interrupt().
+        */
        spin_lock_irqsave(&dev->spinlock, flags);
+
        das1800_ai_handler(dev);
+
        spin_unlock_irqrestore(&dev->spinlock, flags);
 
        return comedi_buf_n_bytes_ready(s);
@@ -553,9 +559,12 @@ static irqreturn_t das1800_interrupt(int irq, void *d)
                return IRQ_HANDLED;
        }
 
-       /* Prevent race with das1800_ai_poll() on multi processor systems.
-        * Also protects indirect addressing in das1800_ai_handler */
+       /*
+        * Protects the indirect addressing selected by DAS1800_SELECT
+        * in das1800_ai_handler() also prevents race with das1800_ai_poll().
+        */
        spin_lock(&dev->spinlock);
+
        status = inb(dev->iobase + DAS1800_STATUS);
 
        /* if interrupt was not caused by das-1800 */
@@ -1289,6 +1298,7 @@ static int das1800_attach(struct comedi_device *dev,
 
                /* initialize all channels to 0V */
                for (i = 0; i < s->n_chan; i++) {
+                       /* spinlock is not necessary during the attach */
                        outb(DAC(i), dev->iobase + DAS1800_SELECT);
                        outw(0, dev->iobase + DAS1800_DAC);
                }