spi: axi-spi-engine: move msg finalization out of irq handler

As a general principal, it is best to do as little as possible in an
interrupt handler. This patch reworks the AXI SPI Engine driver to move
timer_delete_sync() and spi_finalize_current_message() out of the
interrupt handler. Instead, spi_finalize_current_message() is moved to
the transfer_one_message function (similar to nearly all other SPI
controllers). A completion is now used to wait for the sync interrupt
that indicates that the message is complete. The watchdog timer is no
longer needed since we can use the wait_for_completion_timeout()
function to wait for the message to complete with the same effect.

As a bonus, these changes also improve throughput of the SPI controller.
For example, this was tested on a ZynqMP with a 80MHz SCLK reading 4
byte samples from an ADC. The max measured throughput increased from
26k to 28k samples per second.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240207-axi-spi-engine-round-2-1-v2-2-40c0b4e85352@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-axi-spi-engine.c

@@ -6,14 +6,14 @@
  */
 
 #include <linux/clk.h>
+#include <linux/completion.h>
 #include <linux/fpga/adi-axi-common.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>
 #include <linux/of.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/spi/spi.h>
-#include <linux/timer.h>
 
 #define SPI_ENGINE_REG_RESET			0x40
 
@@ -109,9 +109,7 @@ struct spi_engine {
 	spinlock_t lock;
 
 	void __iomem *base;
-	struct timer_list watchdog_timer;
-	struct spi_controller *controller;
-
+	struct completion msg_complete;
 	unsigned int int_enable;
 };
 
@@ -483,11 +481,9 @@ static irqreturn_t spi_engine_irq(int irq, void *devid)
 
 	if (pending & SPI_ENGINE_INT_SYNC && msg) {
 		if (completed_id == AXI_SPI_ENGINE_CUR_MSG_SYNC_ID) {
-			if (timer_delete_sync(&spi_engine->watchdog_timer)) {
-				msg->status = 0;
-				msg->actual_length = msg->frame_length;
-				spi_finalize_current_message(host);
-			}
+			msg->status = 0;
+			msg->actual_length = msg->frame_length;
+			complete(&spi_engine->msg_complete);
 			disable_int |= SPI_ENGINE_INT_SYNC;
 		}
 	}
@@ -558,7 +554,7 @@ static int spi_engine_transfer_one_message(struct spi_controller *host,
 	unsigned int int_enable = 0;
 	unsigned long flags;
 
-	mod_timer(&spi_engine->watchdog_timer, jiffies + msecs_to_jiffies(5000));
+	reinit_completion(&spi_engine->msg_complete);
 
 	spin_lock_irqsave(&spi_engine->lock, flags);
 
@@ -580,21 +576,16 @@ static int spi_engine_transfer_one_message(struct spi_controller *host,
 	spi_engine->int_enable = int_enable;
 	spin_unlock_irqrestore(&spi_engine->lock, flags);
 
-	return 0;
-}
-
-static void spi_engine_timeout(struct timer_list *timer)
-{
-	struct spi_engine *spi_engine = from_timer(spi_engine, timer, watchdog_timer);
-	struct spi_controller *host = spi_engine->controller;
-
-	if (WARN_ON(!host->cur_msg))
-		return;
+	if (!wait_for_completion_timeout(&spi_engine->msg_complete,
+					 msecs_to_jiffies(5000))) {
+		dev_err(&host->dev,
+			"Timeout occurred while waiting for transfer to complete. Hardware is probably broken.\n");
+		msg->status = -ETIMEDOUT;
+	}
 
-	dev_err(&host->dev,
-		"Timeout occurred while waiting for transfer to complete. Hardware is probably broken.\n");
-	host->cur_msg->status = -ETIMEDOUT;
 	spi_finalize_current_message(host);
+
+	return msg->status;
 }
 
 static void spi_engine_release_hw(void *p)
@@ -625,8 +616,7 @@ static int spi_engine_probe(struct platform_device *pdev)
 	spi_engine = spi_controller_get_devdata(host);
 
 	spin_lock_init(&spi_engine->lock);
-	timer_setup(&spi_engine->watchdog_timer, spi_engine_timeout, TIMER_IRQSAFE);
-	spi_engine->controller = host;
+	init_completion(&spi_engine->msg_complete);
 
 	spi_engine->clk = devm_clk_get_enabled(&pdev->dev, "s_axi_aclk");
 	if (IS_ERR(spi_engine->clk))