linux-kernel - [PATCH 2/2] spi: atmel: Reduce spin lock usage

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Date:   Wed, 2 Jun 2021 09:08:15 -0700
From:   Dan Sneddon <dan.sneddon@...rochip.com>
To:     <linux-spi@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
CC:     Dan Sneddon <dan.sneddon@...rochip.com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Mark Brown <broonie@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>
Subject: [PATCH 2/2] spi: atmel: Reduce spin lock usage

The current implementation of the driver holds a spin lock for the
duration of the transfer, releasing it only to enable interrupts for
short periods of time.  As this would prevent any interrupt from
happening, this could cause system performance issues every time a SPI
message is sent.  Since the spi core now handles message syncronization
we can reduce the amount of time the spin-lock is held to the regions
where both the calling thread and the interrupt might interract.

Signed-off-by: Dan Sneddon <dan.sneddon@...rochip.com>
---
 drivers/spi/spi-atmel.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/drivers/spi/spi-atmel.c b/drivers/spi/spi-atmel.c
index 0e693e72c922..2ef74885ffa2 100644
--- a/drivers/spi/spi-atmel.c
+++ b/drivers/spi/spi-atmel.c
@@ -700,7 +700,6 @@ static void atmel_spi_next_xfer_pio(struct spi_master *master,
 static int atmel_spi_next_xfer_dma_submit(struct spi_master *master,
 				struct spi_transfer *xfer,
 				u32 *plen)
-	__must_hold(&as->lock)
 {
 	struct atmel_spi	*as = spi_master_get_devdata(master);
 	struct dma_chan		*rxchan = master->dma_rx;
@@ -716,8 +715,6 @@ static int atmel_spi_next_xfer_dma_submit(struct spi_master *master,
 	if (!rxchan || !txchan)
 		return -ENODEV;
 
-	/* release lock for DMA operations */
-	atmel_spi_unlock(as);
 
 	*plen = xfer->len;
 
@@ -786,15 +783,12 @@ static int atmel_spi_next_xfer_dma_submit(struct spi_master *master,
 	rxchan->device->device_issue_pending(rxchan);
 	txchan->device->device_issue_pending(txchan);
 
-	/* take back lock */
-	atmel_spi_lock(as);
 	return 0;
 
 err_dma:
 	spi_writel(as, IDR, SPI_BIT(OVRES));
 	atmel_spi_stop_dma(master);
 err_exit:
-	atmel_spi_lock(as);
 	return -ENOMEM;
 }
 
@@ -1053,8 +1047,6 @@ atmel_spi_pump_pio_data(struct atmel_spi *as, struct spi_transfer *xfer)
 
 /* Interrupt
  *
- * No need for locking in this Interrupt handler: done_status is the
- * only information modified.
  */
 static irqreturn_t
 atmel_spi_pio_interrupt(int irq, void *dev_id)
@@ -1302,8 +1294,6 @@ static int atmel_spi_one_transfer(struct spi_master *master,
 	unsigned long		dma_timeout;
 
 	as = spi_master_get_devdata(master);
-	/* This lock was orignally taken in atmel_spi_trasfer_one_message */
-	atmel_spi_lock(as);
 
 	asd = spi->controller_state;
 	bits = (asd->csr >> 4) & 0xf;
@@ -1332,7 +1322,9 @@ static int atmel_spi_one_transfer(struct spi_master *master,
 		reinit_completion(&as->xfer_completion);
 
 		if (as->use_pdc) {
+			atmel_spi_lock(as);
 			atmel_spi_pdc_next_xfer(master, xfer);
+			atmel_spi_unlock(as);
 		} else if (atmel_spi_use_dma(as, xfer)) {
 			len = as->current_remaining_bytes;
 			ret = atmel_spi_next_xfer_dma_submit(master,
@@ -1348,14 +1340,13 @@ static int atmel_spi_one_transfer(struct spi_master *master,
 					as->current_remaining_bytes = 0;
 			}
 		} else {
+			atmel_spi_lock(as);
 			atmel_spi_next_xfer_pio(master, xfer);
+			atmel_spi_unlock(as);
 		}
 
-		/* interrupts are disabled, so free the lock for schedule */
-		atmel_spi_unlock(as);
 		dma_timeout = wait_for_completion_timeout(&as->xfer_completion,
 							  SPI_DMA_TIMEOUT);
-		atmel_spi_lock(as);
 		if (WARN_ON(dma_timeout == 0)) {
 			dev_err(&spi->dev, "spi transfer timeout\n");
 			as->done_status = -EIO;
@@ -1403,8 +1394,6 @@ static int atmel_spi_one_transfer(struct spi_master *master,
 	if (as->use_pdc)
 		atmel_spi_disable_pdc_transfer(as);
 
-	atmel_spi_unlock(as);
-
 	return as->done_status;
 }
 
-- 
2.17.1