spi: bcm2835: Fix for shared interrupts

BCM2711 shares an interrupt betweem 5 SPI interfaces (0, 3, 4, 5 & 6).
Another interrupt is shared between SPI1, SPI2 and UART1, which also
affects BCM2835/6/7. Acting on an interrupt intended for another
interface ought to be harmless (although potentially inefficient),
but it can cause this driver to crash - presumably because some
critical state is not ready.

Add a test to the spi-bcm2835 interrupt service routine that interrupts
are enabled on this interface to avoid the crash and improve efficiency.

Suggested by GitHub user boe-pi.

See: #5048

Signed-off-by: Phil Elwell <[email protected]>

Author: pelwell

drivers/spi/spi-bcm2835.c

@@ -372,6 +372,10 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
 	struct bcm2835_spi *bs = dev_id;
 	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
 
+	/* Bail out early if interrupts are not enabled */
+	if (!(cs & BCM2835_SPI_CS_INTR))
+		return IRQ_NONE;
+
 	/*
 	 * An interrupt is signaled either if DONE is set (TX FIFO empty)
 	 * or if RXR is set (RX FIFO >= ¾ full).
@@ -384,10 +388,6 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
 	if (bs->tx_len && cs & BCM2835_SPI_CS_DONE)
 		bcm2835_wr_fifo_blind(bs, BCM2835_SPI_FIFO_SIZE);
 
-	/* check if we got interrupt enabled */
-	if (!(bcm2835_rd(bs, BCM2835_SPI_CS) & BCM2835_SPI_CS_INTR))
-		return IRQ_NONE;
-
 	/* Read as many bytes as possible from FIFO */
 	bcm2835_rd_fifo(bs);
 	/* Write as many bytes as possible to FIFO */