Message-ID: <20240125234732.3530278-2-dlechner@baylibre.com>
Date: Thu, 25 Jan 2024 17:47:31 -0600
From: David Lechner <dlechner@...libre.com>
To: Mark Brown <broonie@...nel.org>
Cc: David Lechner <dlechner@...libre.com>,
	linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] spi: avoid double validation in __spi_sync()

The __spi_sync() function calls __spi_validate() early in the function.
Later, it can call spi_async_locked() which calls __spi_validate()
again. __spi_validate() is an expensive function, so we can improve
performance measurably by avoiding calling it twice.

Instead of calling spi_async_locked(), we can call __spi_async() with
the spin lock held.

spi_async_locked() is removed since there are no more callers.

Signed-off-by: David Lechner <dlechner@...libre.com>
---

I tested this using an ADC driver that does a lot of spi_sync() calls to read
samples. I disabled the no-queue fast path by setting ctlr->must_async so that
the modified code always runs.

With this change I was able to increase the sample rate of the ADC about 6%
from 14.5 kHz to 15.4 kHz.

 drivers/spi/spi.c | 58 +++++------------------------------------------
 1 file changed, 6 insertions(+), 52 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 7a70ef47cdf6..6610aeced765 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -4278,57 +4278,6 @@ int spi_async(struct spi_device *spi, struct spi_message *message)
 }
 EXPORT_SYMBOL_GPL(spi_async);
 
-/**
- * spi_async_locked - version of spi_async with exclusive bus usage
- * @spi: device with which data will be exchanged
- * @message: describes the data transfers, including completion callback
- * Context: any (IRQs may be blocked, etc)
- *
- * This call may be used in_irq and other contexts which can't sleep,
- * as well as from task contexts which can sleep.
- *
- * The completion callback is invoked in a context which can't sleep.
- * Before that invocation, the value of message->status is undefined.
- * When the callback is issued, message->status holds either zero (to
- * indicate complete success) or a negative error code.  After that
- * callback returns, the driver which issued the transfer request may
- * deallocate the associated memory; it's no longer in use by any SPI
- * core or controller driver code.
- *
- * Note that although all messages to a spi_device are handled in
- * FIFO order, messages may go to different devices in other orders.
- * Some device might be higher priority, or have various "hard" access
- * time requirements, for example.
- *
- * On detection of any fault during the transfer, processing of
- * the entire message is aborted, and the device is deselected.
- * Until returning from the associated message completion callback,
- * no other spi_message queued to that device will be processed.
- * (This rule applies equally to all the synchronous transfer calls,
- * which are wrappers around this core asynchronous primitive.)
- *
- * Return: zero on success, else a negative error code.
- */
-static int spi_async_locked(struct spi_device *spi, struct spi_message *message)
-{
-	struct spi_controller *ctlr = spi->controller;
-	int ret;
-	unsigned long flags;
-
-	ret = __spi_validate(spi, message);
-	if (ret != 0)
-		return ret;
-
-	spin_lock_irqsave(&ctlr->bus_lock_spinlock, flags);
-
-	ret = __spi_async(spi, message);
-
-	spin_unlock_irqrestore(&ctlr->bus_lock_spinlock, flags);
-
-	return ret;
-
-}
-
 static void __spi_transfer_message_noqueue(struct spi_controller *ctlr, struct spi_message *msg)
 {
 	bool was_busy;
@@ -4376,6 +4325,7 @@ static void spi_complete(void *arg)
 static int __spi_sync(struct spi_device *spi, struct spi_message *message)
 {
 	DECLARE_COMPLETION_ONSTACK(done);
+	unsigned long flags;
 	int status;
 	struct spi_controller *ctlr = spi->controller;
 
@@ -4419,7 +4369,11 @@ static int __spi_sync(struct spi_device *spi, struct spi_message *message)
 	 */
 	message->complete = spi_complete;
 	message->context = &done;
-	status = spi_async_locked(spi, message);
+
+	spin_lock_irqsave(&ctlr->bus_lock_spinlock, flags);
+	status = __spi_async(spi, message);
+	spin_unlock_irqrestore(&ctlr->bus_lock_spinlock, flags);
+
 	if (status == 0) {
 		wait_for_completion(&done);
 		status = message->status;
-- 
2.43.0

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