comedi: dmm32at: serialize use of paged registers

Some of the hardware registers of the DMM-32-AT board are multiplexed,
using the least significant two bits of the Miscellaneous Control
register to select the function of registers at offsets 12 to 15:

 00 => 8254 timer/counter registers are accessible
 01 => 8255 digital I/O registers are accessible
 10 => Reserved
 11 => Calibration registers are accessible

The interrupt service routine (`dmm32at_isr()`) clobbers the bottom two
bits of the register with value 00, which would interfere with access to
the 8255 registers by the `dm32at_8255_io()` function (used for Comedi
instruction handling on the digital I/O subdevice).

Make use of the generic Comedi device spin-lock `dev->spinlock` (which
is otherwise unused by this driver) to serialize access to the
miscellaneous control register and paged registers.

Fixes: 3c50188 ("Staging: comedi: add dmm32at driver")
Cc: stable@vger.kernel.org
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20260112162835.91688-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ian-abbott

drivers/comedi/drivers/dmm32at.c

@@ -330,6 +330,7 @@ static int dmm32at_ai_cmdtest(struct comedi_device *dev,
 
 static void dmm32at_setaitimer(struct comedi_device *dev, unsigned int nansec)
 {
+	unsigned long irq_flags;
 	unsigned char lo1, lo2, hi2;
 	unsigned short both2;
 
@@ -342,6 +343,9 @@ static void dmm32at_setaitimer(struct comedi_device *dev, unsigned int nansec)
 	/* set counter clocks to 10MHz, disable all aux dio */
 	outb(0, dev->iobase + DMM32AT_CTRDIO_CFG_REG);
 
+	/* serialize access to control register and paged registers */
+	spin_lock_irqsave(&dev->spinlock, irq_flags);
+
 	/* get access to the clock regs */
 	outb(DMM32AT_CTRL_PAGE_8254, dev->iobase + DMM32AT_CTRL_REG);
 
@@ -354,6 +358,8 @@ static void dmm32at_setaitimer(struct comedi_device *dev, unsigned int nansec)
 	outb(lo2, dev->iobase + DMM32AT_CLK2);
 	outb(hi2, dev->iobase + DMM32AT_CLK2);
 
+	spin_unlock_irqrestore(&dev->spinlock, irq_flags);
+
 	/* enable the ai conversion interrupt and the clock to start scans */
 	outb(DMM32AT_INTCLK_ADINT |
 	     DMM32AT_INTCLK_CLKEN | DMM32AT_INTCLK_CLKSEL,
@@ -363,13 +369,19 @@ static void dmm32at_setaitimer(struct comedi_device *dev, unsigned int nansec)
 static int dmm32at_ai_cmd(struct comedi_device *dev, struct comedi_subdevice *s)
 {
 	struct comedi_cmd *cmd = &s->async->cmd;
+	unsigned long irq_flags;
 	int ret;
 
 	dmm32at_ai_set_chanspec(dev, s, cmd->chanlist[0], cmd->chanlist_len);
 
+	/* serialize access to control register and paged registers */
+	spin_lock_irqsave(&dev->spinlock, irq_flags);
+
 	/* reset the interrupt just in case */
 	outb(DMM32AT_CTRL_INTRST, dev->iobase + DMM32AT_CTRL_REG);
 
+	spin_unlock_irqrestore(&dev->spinlock, irq_flags);
+
 	/*
 	 * wait for circuit to settle
 	 * we don't have the 'insn' here but it's not needed
@@ -429,8 +441,13 @@ static irqreturn_t dmm32at_isr(int irq, void *d)
 		comedi_handle_events(dev, s);
 	}
 
+	/* serialize access to control register and paged registers */
+	spin_lock(&dev->spinlock);
+
 	/* reset the interrupt */
 	outb(DMM32AT_CTRL_INTRST, dev->iobase + DMM32AT_CTRL_REG);
+
+	spin_unlock(&dev->spinlock);
 	return IRQ_HANDLED;
 }
 
@@ -481,14 +498,25 @@ static int dmm32at_ao_insn_write(struct comedi_device *dev,
 static int dmm32at_8255_io(struct comedi_device *dev,
 			   int dir, int port, int data, unsigned long regbase)
 {
+	unsigned long irq_flags;
+	int ret;
+
+	/* serialize access to control register and paged registers */
+	spin_lock_irqsave(&dev->spinlock, irq_flags);
+
 	/* get access to the DIO regs */
 	outb(DMM32AT_CTRL_PAGE_8255, dev->iobase + DMM32AT_CTRL_REG);
 
 	if (dir) {
 		outb(data, dev->iobase + regbase + port);
-		return 0;
+		ret = 0;
+	} else {
+		ret = inb(dev->iobase + regbase + port);
 	}
-	return inb(dev->iobase + regbase + port);
+
+	spin_unlock_irqrestore(&dev->spinlock, irq_flags);
+
+	return ret;
 }
 
 /* Make sure the board is there and put it to a known state */