I'm writing a SPI Slave kernel module for the at91sam9g20 that will respond to NSSR and RDRF interrupts. The problem I'm having is after I have configured the SPI port, assigned an Interrupt Service Routine and enable the port, my Service Routine gets called constantly. When checking the Interrupt Mask and Status Register I get: IMR = 0x0 and SR = 0x103FB. As soon as I leave the ISR it is called again. I have tried reading the SR during the open function but it always is set to 0x103FB even when no SPI traffic occurs.

static irqreturn_t spi_slave_isr (int irq, void *dev_id)
{
struct spi_slave_dev *dev = dev_id;
u16 val;
u32 status, pending, imr;
int ret = IRQ_NONE;

spi_writel(dev, IDR, SPI_BIT(RDRF));
spi_writel(dev, PTCR, SPI_BIT(RXTDIS));

imr = spi_readl(dev, IMR);
status = spi_readl(dev, SR);
pending = status & imr;

val = spi_readl(dev, RDR) & 0xFFFF;

CDBG("imr (0x%X) sr(0x%X) val(0x%X)\n", imr, status, val);

if (pending & SPI_BIT(RDRF))
ret = IRQ_HANDLED;

sr = spi_readl(dev, SR);

spi_writel(dev, IER, SPI_BIT(RDRF) | SPI_BIT(NSSR));
spi_writel(dev, PTCR, SPI_BIT(RXTEN));

return ret;
}

My setup of the Port / IRQ:
static int spi_slave_open (struct inode *inode, struct file *filp)
{
u32 mr, csr;
int ret;
struct spi_slave_dev *dev;

/* Find Device */
dev = container_of(inode->i_cdev, struct spi_slave_dev, cdev);

dev->regs = ioremap_wc(AT91C_BASE_SPI1, 0x4000);
if (!dev->regs)
{
printk(KERN_INFO "Failed ioremap_wc\n");
return -ENOMEM;
}
/* Disable SPI */
spi_writel(dev, CR, SPI_BIT(SPIDIS));

/* Setup ISR */
ret = request_irq(AT91C_ID_SPI1), // IRQ Number
spi_slave_isr, // ISR
0, // Flags
"spi_slave", // /proc/interrupts
dev);

if (ret)
{
printk(KERN_INFO "SPI Slave Unable to register IRQ\n");
return ret;
}

/* Execute software reset twice per Atmel's AT91Lib*/
spi_writel(dev, CR, SPI_BIT(SWRST));
spi_writel(dev, CR, SPI_BIT(SWRST));

/* Configure MR */
spi_writel(dev, MR, (SPI_BIT(MODFDIS)));

/* Configure PTCR */
spi_writel(dev, PTCR, SPI_BIT(RXTDIS) | SPI_BIT(TXTDIS));

/* Configure CS */
spi_writel(dev, CSR0, (SPI_BIT(CPOL) | SPI_BF(BITS, 0)));

/* Clearing everything else */
spi_writel(dev, TDR, 0x00BC);
spi_readl(dev, RDR);

/* Enable SPI */
spi_writel(dev, CR, SPI_BIT(SPIEN));

/* Enable IER & PTCR */
spi_writel(dev, IER, SPI_BIT(RDRF) | SPI_BIT(NSSR));
spi_writel(dev, PTCR, SPI_BIT(RXTEN));

filp->private_data = dev;

return 0;
}

I figured out that the PMC was not maintaining the configuration assigned in U-Boot after Linux booted. I assume this was the issue as I made the following change and the code now works.

......
at91_set_A_periph(AT91_PIN_PB0, 0); /* MISO */
at91_set_A_periph(AT91_PIN_PB1, 0); /* MOSI */
at91_set_A_periph(AT91_PIN_PB2, 0); /* SPCK */
at91_set_A_periph(AT91_PIN_PB3, 0); /* NPCS0 */

/* Enable SPI in PMC */
at91spi1_clk = clk_get(NULL, "spi1_clk");
clk_enable(at91spi1_clk);
.......