Re: [RFC] [PATCH 3/3] drivers: spi: spi.c: Don't use the message queue if possible in spi_sync

[David Jander <david@xxxxxxxxxxx>]

On Wed, 25 May 2022 16:29:28 +0200
David Jander <david@xxxxxxxxxxx> wrote:

> The interaction with the controller message queue and its corresponding
> auxiliary flags and variables requires the use of the queue_lock which is
> costly. Since spi_sync will transfer the complete message anyway, and not
> return until it is finished, there is no need to put the message into the
> queue if the queue is empty. This can save a lot of overhead.
> 
> As an example of how significant this is, when using the MCP2518FD SPI CAN
> controller on a i.MX8MM SoC, the time during which the interrupt line
> stays active (during 3 relatively short spi_sync messages), is reduced
> from 98us to 72us by this patch.
> 
> Signed-off-by: David Jander <david@xxxxxxxxxxx>
> ---
>  drivers/spi/spi.c       | 244 ++++++++++++++++++++++++----------------
>  include/linux/spi/spi.h |  11 +-
>  2 files changed, 157 insertions(+), 98 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 1d50051f3d57..401ac2f4758e 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1558,6 +1558,80 @@ static void spi_idle_runtime_pm(struct spi_controller *ctlr)
>  	}
>  }
>  
> +static int __spi_pump_transfer_message(struct spi_controller *ctlr,
> +		struct spi_message *msg, bool was_busy)
> +{
> +	struct spi_transfer *xfer;
> +	int ret;
> +
> +	if (!was_busy && ctlr->auto_runtime_pm) {
> +		ret = pm_runtime_get_sync(ctlr->dev.parent);
> +		if (ret < 0) {
> +			pm_runtime_put_noidle(ctlr->dev.parent);
> +			dev_err(&ctlr->dev, "Failed to power device: %d\n",
> +				ret);
> +			return ret;
> +		}
> +	}
> +
> +	if (!was_busy)
> +		trace_spi_controller_busy(ctlr);
> +
> +	if (!was_busy && ctlr->prepare_transfer_hardware) {
> +		ret = ctlr->prepare_transfer_hardware(ctlr);
> +		if (ret) {
> +			dev_err(&ctlr->dev,
> +				"failed to prepare transfer hardware: %d\n",
> +				ret);
> +
> +			if (ctlr->auto_runtime_pm)
> +				pm_runtime_put(ctlr->dev.parent);
> +
> +			msg->status = ret;
> +			spi_finalize_message(msg);
> +
> +			return ret;
> +		}
> +	}
> +
> +	trace_spi_message_start(msg);
> +
> +	if (ctlr->prepare_message) {
> +		ret = ctlr->prepare_message(ctlr, msg);
> +		if (ret) {
> +			dev_err(&ctlr->dev, "failed to prepare message: %d\n",
> +				ret);
> +			msg->status = ret;
> +			spi_finalize_message(msg);
> +			return ret;
> +		}
> +		msg->prepared = true;
> +	}
> +
> +	ret = spi_map_msg(ctlr, msg);
> +	if (ret) {
> +		msg->status = ret;
> +		spi_finalize_message(msg);
> +		return ret;
> +	}
> +
> +	if (!ctlr->ptp_sts_supported && !ctlr->transfer_one) {
> +		list_for_each_entry(xfer, &msg->transfers, transfer_list) {
> +			xfer->ptp_sts_word_pre = 0;
> +			ptp_read_system_prets(xfer->ptp_sts);
> +		}
> +	}
> +
> +	ret = ctlr->transfer_one_message(ctlr, msg);
> +	if (ret) {
> +		dev_err(&ctlr->dev,
> +			"failed to transfer one message from queue\n");
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * __spi_pump_messages - function which processes spi message queue
>   * @ctlr: controller to process queue for
> @@ -1573,7 +1647,6 @@ static void spi_idle_runtime_pm(struct spi_controller *ctlr)
>   */
>  static void __spi_pump_messages(struct spi_controller *ctlr, bool in_kthread)
>  {
> -	struct spi_transfer *xfer;
>  	struct spi_message *msg;
>  	bool was_busy = false;
>  	unsigned long flags;
> @@ -1608,6 +1681,7 @@ static void __spi_pump_messages(struct spi_controller *ctlr, bool in_kthread)
>  			    !ctlr->unprepare_transfer_hardware) {
>  				spi_idle_runtime_pm(ctlr);
>  				ctlr->busy = false;
> +				ctlr->queue_empty = true;
>  				trace_spi_controller_idle(ctlr);
>  			} else {
>  				kthread_queue_work(ctlr->kworker,
> @@ -1634,6 +1708,7 @@ static void __spi_pump_messages(struct spi_controller *ctlr, bool in_kthread)
>  
>  		spin_lock_irqsave(&ctlr->queue_lock, flags);
>  		ctlr->idling = false;
> +		ctlr->queue_empty = true;
>  		spin_unlock_irqrestore(&ctlr->queue_lock, flags);
>  		return;
>  	}
> @@ -1650,75 +1725,7 @@ static void __spi_pump_messages(struct spi_controller *ctlr, bool in_kthread)
>  	spin_unlock_irqrestore(&ctlr->queue_lock, flags);
>  
>  	mutex_lock(&ctlr->io_mutex);
> -
> -	if (!was_busy && ctlr->auto_runtime_pm) {
> -		ret = pm_runtime_get_sync(ctlr->dev.parent);
> -		if (ret < 0) {
> -			pm_runtime_put_noidle(ctlr->dev.parent);
> -			dev_err(&ctlr->dev, "Failed to power device: %d\n",
> -				ret);
> -			mutex_unlock(&ctlr->io_mutex);
> -			return;
> -		}
> -	}
> -
> -	if (!was_busy)
> -		trace_spi_controller_busy(ctlr);
> -
> -	if (!was_busy && ctlr->prepare_transfer_hardware) {
> -		ret = ctlr->prepare_transfer_hardware(ctlr);
> -		if (ret) {
> -			dev_err(&ctlr->dev,
> -				"failed to prepare transfer hardware: %d\n",
> -				ret);
> -
> -			if (ctlr->auto_runtime_pm)
> -				pm_runtime_put(ctlr->dev.parent);
> -
> -			msg->status = ret;
> -			spi_finalize_message(msg);
> -
> -			mutex_unlock(&ctlr->io_mutex);
> -			return;
> -		}
> -	}
> -
> -	trace_spi_message_start(msg);
> -
> -	if (ctlr->prepare_message) {
> -		ret = ctlr->prepare_message(ctlr, msg);
> -		if (ret) {
> -			dev_err(&ctlr->dev, "failed to prepare message: %d\n",
> -				ret);
> -			msg->status = ret;
> -			spi_finalize_message(msg);
> -			goto out;
> -		}
> -		msg->prepared = true;
> -	}
> -
> -	ret = spi_map_msg(ctlr, msg);
> -	if (ret) {
> -		msg->status = ret;
> -		spi_finalize_message(msg);
> -		goto out;
> -	}
> -
> -	if (!ctlr->ptp_sts_supported && !ctlr->transfer_one) {
> -		list_for_each_entry(xfer, &msg->transfers, transfer_list) {
> -			xfer->ptp_sts_word_pre = 0;
> -			ptp_read_system_prets(xfer->ptp_sts);
> -		}
> -	}
> -
> -	ret = ctlr->transfer_one_message(ctlr, msg);
> -	if (ret) {
> -		dev_err(&ctlr->dev,
> -			"failed to transfer one message from queue\n");
> -		goto out;
> -	}
> -
> -out:
> +	ret = __spi_pump_transfer_message(ctlr, msg, was_busy);
>  	mutex_unlock(&ctlr->io_mutex);
>  
>  	/* Prod the scheduler in case transfer_one() was busy waiting */
> @@ -1848,6 +1855,7 @@ static int spi_init_queue(struct spi_controller *ctlr)
>  {
>  	ctlr->running = false;
>  	ctlr->busy = false;
> +	ctlr->queue_empty = true;
>  
>  	ctlr->kworker = kthread_create_worker(0, dev_name(&ctlr->dev));
>  	if (IS_ERR(ctlr->kworker)) {
> @@ -1941,11 +1949,15 @@ void spi_finalize_message(struct spi_message *mesg)
>  
>  	mesg->prepared = false;
>  
> -	spin_lock_irqsave(&ctlr->queue_lock, flags);
> -	ctlr->cur_msg = NULL;
> -	ctlr->fallback = false;
> -	kthread_queue_work(ctlr->kworker, &ctlr->pump_messages);
> -	spin_unlock_irqrestore(&ctlr->queue_lock, flags);
> +	if (!mesg->sync) {
> +		spin_lock_irqsave(&ctlr->queue_lock, flags);
> +		WARN(ctlr->cur_msg != mesg,
> +			"Finalizing queued message that is not the current head of queue!");
> +		ctlr->cur_msg = NULL;
> +		ctlr->fallback = false;
> +		kthread_queue_work(ctlr->kworker, &ctlr->pump_messages);
> +		spin_unlock_irqrestore(&ctlr->queue_lock, flags);
> +	}
>  
>  	trace_spi_message_done(mesg);
>  
> @@ -2048,6 +2060,7 @@ static int __spi_queued_transfer(struct spi_device *spi,
>  	msg->status = -EINPROGRESS;
>  
>  	list_add_tail(&msg->queue, &ctlr->queue);
> +	ctlr->queue_empty = false;
>  	if (!ctlr->busy && need_pump)
>  		kthread_queue_work(ctlr->kworker, &ctlr->pump_messages);
>  
> @@ -3937,6 +3950,42 @@ static int spi_async_locked(struct spi_device *spi, struct spi_message *message)
>  
>  }
>  
> +static void __spi_transfer_message_noqueue(struct spi_controller *ctlr, struct spi_message *msg)
> +{
> +	bool was_busy;
> +	int ret;
> +
> +	mutex_lock(&ctlr->io_mutex);
> +
> +	/* If another context is idling the device then wait */
> +	while (ctlr->idling) {
> +		printk(KERN_INFO "spi sync message processing: controller is idling!\n");
> +		usleep_range(10000, 11000);
> +	}

This is dead ugly of course, and it needs to be removed. Not yet sure how,
hence the RFC. Maybe the idle -> not busy transition can be included inside
the io_mutex? That way this while will never be hit and can be removed...

Best regards,

> +	was_busy = READ_ONCE(ctlr->busy);
> +
> +	ret = __spi_pump_transfer_message(ctlr, msg, was_busy);
> +	if (ret)
> +		goto out;
> +
> +	if (!was_busy) {
> +		kfree(ctlr->dummy_rx);
> +		ctlr->dummy_rx = NULL;
> +		kfree(ctlr->dummy_tx);
> +		ctlr->dummy_tx = NULL;
> +		if (ctlr->unprepare_transfer_hardware &&
> +		    ctlr->unprepare_transfer_hardware(ctlr))
> +			dev_err(&ctlr->dev,
> +				"failed to unprepare transfer hardware\n");
> +		spi_idle_runtime_pm(ctlr);
> +	}
> +
> +out:
> +	mutex_unlock(&ctlr->io_mutex);
> +	return;
> +}
> +
>  /*-------------------------------------------------------------------------*/
>  
>  /*
> @@ -3955,51 +4004,52 @@ static int __spi_sync(struct spi_device *spi, struct spi_message *message)
>  	DECLARE_COMPLETION_ONSTACK(done);
>  	int status;
>  	struct spi_controller *ctlr = spi->controller;
> -	unsigned long flags;
>  
>  	status = __spi_validate(spi, message);
>  	if (status != 0)
>  		return status;
>  
> -	message->complete = spi_complete;
> -	message->context = &done;
>  	message->spi = spi;
>  
>  	SPI_STATISTICS_INCREMENT_FIELD(ctlr->pcpu_statistics, spi_sync);
>  	SPI_STATISTICS_INCREMENT_FIELD(spi->pcpu_statistics, spi_sync);
>  
>  	/*
> -	 * If we're not using the legacy transfer method then we will
> -	 * try to transfer in the calling context so special case.
> -	 * This code would be less tricky if we could remove the
> -	 * support for driver implemented message queues.
> +	 * Checking queue_empty here only guarantees async/sync message
> +	 * ordering when coming from the same context. It does not need to
> +	 * guard against reentrancy from a different context. The io_mutex
> +	 * will catch those cases.
>  	 */
> -	if (ctlr->transfer == spi_queued_transfer) {
> -		spin_lock_irqsave(&ctlr->bus_lock_spinlock, flags);
> +	if (READ_ONCE(ctlr->queue_empty)) {
> +		message->sync = true;
> +		message->actual_length = 0;
> +		message->status = -EINPROGRESS;
>  
>  		trace_spi_message_submit(message);
>  
> -		status = __spi_queued_transfer(spi, message, false);
> +		SPI_STATISTICS_INCREMENT_FIELD(ctlr->pcpu_statistics, spi_sync_immediate);
> +		SPI_STATISTICS_INCREMENT_FIELD(spi->pcpu_statistics, spi_sync_immediate);
>  
> -		spin_unlock_irqrestore(&ctlr->bus_lock_spinlock, flags);
> -	} else {
> -		status = spi_async_locked(spi, message);
> +		__spi_transfer_message_noqueue(ctlr, message);
> +
> +		return message->status;
>  	}
>  
> +	/*
> +	 * There are messages in the async queue that could have originated
> +	 * from the same context, so we need to preserve ordering.
> +	 * Therefor we send the message to the async queue and wait until they
> +	 * are completed.
> +	 */
> +	message->complete = spi_complete;
> +	message->context = &done;
> +	status = spi_async_locked(spi, message);
>  	if (status == 0) {
> -		/* Push out the messages in the calling context if we can */
> -		if (ctlr->transfer == spi_queued_transfer) {
> -			SPI_STATISTICS_INCREMENT_FIELD(ctlr->pcpu_statistics,
> -						       spi_sync_immediate);
> -			SPI_STATISTICS_INCREMENT_FIELD(spi->pcpu_statistics,
> -						       spi_sync_immediate);
> -			__spi_pump_messages(ctlr, false);
> -		}
> -
>  		wait_for_completion(&done);
>  		status = message->status;
>  	}
>  	message->context = NULL;
> +
>  	return status;
>  }
>  
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index 43ec1e262913..9caed8537755 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -449,6 +449,8 @@ extern struct spi_device *spi_new_ancillary_device(struct spi_device *spi, u8 ch
>   * @irq_flags: Interrupt enable state during PTP system timestamping
>   * @fallback: fallback to pio if dma transfer return failure with
>   *	SPI_TRANS_FAIL_NO_START.
> + * @queue_empty: signal green light for opportunistically skipping the queue
> + * 	for spi_sync transfers.
>   *
>   * Each SPI controller can communicate with one or more @spi_device
>   * children.  These make a small bus, sharing MOSI, MISO and SCK signals
> @@ -665,6 +667,9 @@ struct spi_controller {
>  
>  	/* Interrupt enable state during PTP system timestamping */
>  	unsigned long		irq_flags;
> +
> +	/* Flag for enabling opportunistic skipping of the queue in spi_sync */
> +	bool			queue_empty;
>  };
>  
>  static inline void *spi_controller_get_devdata(struct spi_controller *ctlr)
> @@ -974,6 +979,7 @@ struct spi_transfer {
>   * @state: for use by whichever driver currently owns the message
>   * @resources: for resource management when the spi message is processed
>   * @prepared: spi_prepare_message was called for the this message
> + * @sync: this message took the direct sync path skipping the async queue
>   *
>   * A @spi_message is used to execute an atomic sequence of data transfers,
>   * each represented by a struct spi_transfer.  The sequence is "atomic"
> @@ -1025,7 +1031,10 @@ struct spi_message {
>  	struct list_head        resources;
>  
>  	/* spi_prepare_message was called for this message */
> -	bool                    prepared;
> +	bool			prepared;
> +
> +	/* this message is skipping the async queue */
> +	bool			sync;
>  };
>  
>  static inline void spi_message_init_no_memset(struct spi_message *m)


-- 
David Jander