Unverified Commit f2d4f106 authored by Mark Brown's avatar Mark Brown
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Merge series "spi: bcm2835: Interrupt-handling optimisations" from Robin... 

Merge series "spi: bcm2835: Interrupt-handling optimisations" from Robin Murphy <robin.murphy@arm.com>:

Hi all,

Although Florian was concerned about a trivial inline check to deal with
shared IRQs adding overhead, the reality is that it would be so small as
to not be worth even thinking about unless the driver was already tuned
to squeeze out every last cycle. And a brief look over the code shows
that that clearly isn't the case.

This is an example of some of the easy low-hanging fruit that jumps out
just from code inspection. Based on disassembly and ARM1176 cycle
timings, patch #2 should save the equivalent of 2-3 shared interrupt
checks off the critical path in all cases, and patch #3 possibly up to
about 100x more. I don't have any means to test these patches, let alone
measure performance, so they're only backed by the principle that less
code - and in particular fewer memory accesses - is almost always
better.

There is almost certainly a *lot* more to be had from careful use of
relaxed I/O accessors, not doing a read-modify-write of CS at every
reset, tweaking the loops further to avoid unnecessary writebacks to
variables, and so on. However since I'm not invested in this personally
I'm not going to pursue it any further; I'm throwing these patches out
as more of a demonstration to back up my original drive-by review
comments, so if anyone want to pick them up and run with them then
please do so.

Robin.

Robin Murphy (3):
  spi: bcm3835: Tidy up bcm2835_spi_reset_hw()
  spi: bcm2835: Micro-optimise IRQ handler
  spi: bcm2835: Micro-optimise FIFO loops

 drivers/spi/spi-bcm2835.c | 45 +++++++++++++++++++--------------------
 1 file changed, 22 insertions(+), 23 deletions(-)

--
2.23.0.dirty

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parents 95f2fd2e 26751de2

drivers/spi/spi-bcm2835.c

+22 −23
Original line number Diff line number Diff line
@@ -86,6 +86,7 @@ MODULE_PARM_DESC(polling_limit_us, 
 * @clk: core clock, divided to calculate serial clock

 * @irq: interrupt, signals TX FIFO empty or RX FIFO ¾ full

 * @tfr: SPI transfer currently processed

 * @ctlr: SPI controller reverse lookup

 * @tx_buf: pointer whence next transmitted byte is read

 * @rx_buf: pointer where next received byte is written

 * @tx_len: remaining bytes to transmit
@@ -125,6 +126,7 @@ struct bcm2835_spi { 
	struct clk *clk;

	int irq;

	struct spi_transfer *tfr;

	struct spi_controller *ctlr;

	const u8 *tx_buf;

	u8 *rx_buf;

	int tx_len;
@@ -243,13 +245,13 @@ static inline void bcm2835_rd_fifo_count(struct bcm2835_spi *bs, int count) 


	bs->rx_len -= count;



	while (count > 0) {

	do {

		val = bcm2835_rd(bs, BCM2835_SPI_FIFO);

		len = min(count, 4);

		memcpy(bs->rx_buf, &val, len);

		bs->rx_buf += len;

		count -= 4;

	}

	} while (count > 0);

}



/**
@@ -269,7 +271,7 @@ static inline void bcm2835_wr_fifo_count(struct bcm2835_spi *bs, int count) 


	bs->tx_len -= count;



	while (count > 0) {

	do {

		if (bs->tx_buf) {

			len = min(count, 4);

			memcpy(&val, bs->tx_buf, len);
@@ -279,7 +281,7 @@ static inline void bcm2835_wr_fifo_count(struct bcm2835_spi *bs, int count) 
		}

		bcm2835_wr(bs, BCM2835_SPI_FIFO, val);

		count -= 4;

	}

	} while (count > 0);

}



/**
@@ -308,12 +310,11 @@ static inline void bcm2835_rd_fifo_blind(struct bcm2835_spi *bs, int count) 
	count = min(count, bs->rx_len);

	bs->rx_len -= count;



	while (count) {

	do {

		val = bcm2835_rd(bs, BCM2835_SPI_FIFO);

		if (bs->rx_buf)

			*bs->rx_buf++ = val;

		count--;

	}

	} while (--count);

}



/**
@@ -328,16 +329,14 @@ static inline void bcm2835_wr_fifo_blind(struct bcm2835_spi *bs, int count) 
	count = min(count, bs->tx_len);

	bs->tx_len -= count;



	while (count) {

	do {

		val = bs->tx_buf ? *bs->tx_buf++ : 0;

		bcm2835_wr(bs, BCM2835_SPI_FIFO, val);

		count--;

	}

	} while (--count);

}



static void bcm2835_spi_reset_hw(struct spi_controller *ctlr)

static void bcm2835_spi_reset_hw(struct bcm2835_spi *bs)

{

	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);

	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);



	/* Disable SPI interrupts and transfer */
@@ -363,8 +362,7 @@ static void bcm2835_spi_reset_hw(struct spi_controller *ctlr) 


static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)

{

	struct spi_controller *ctlr = dev_id;

	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);

	struct bcm2835_spi *bs = dev_id;

	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);



	/*
@@ -386,9 +384,9 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id) 


	if (!bs->rx_len) {

		/* Transfer complete - reset SPI HW */

		bcm2835_spi_reset_hw(ctlr);

		bcm2835_spi_reset_hw(bs);

		/* wake up the framework */

		complete(&ctlr->xfer_completion);

		complete(&bs->ctlr->xfer_completion);

	}



	return IRQ_HANDLED;
@@ -607,7 +605,7 @@ static void bcm2835_spi_dma_rx_done(void *data) 
	bcm2835_spi_undo_prologue(bs);



	/* reset fifo and HW */

	bcm2835_spi_reset_hw(ctlr);

	bcm2835_spi_reset_hw(bs);



	/* and mark as completed */;

	complete(&ctlr->xfer_completion);
@@ -641,7 +639,7 @@ static void bcm2835_spi_dma_tx_done(void *data) 
		dmaengine_terminate_async(ctlr->dma_rx);



	bcm2835_spi_undo_prologue(bs);

	bcm2835_spi_reset_hw(ctlr);

	bcm2835_spi_reset_hw(bs);

	complete(&ctlr->xfer_completion);

}
@@ -825,14 +823,14 @@ static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr, 
	if (!bs->rx_buf && !bs->tx_dma_active &&

	    cmpxchg(&bs->rx_dma_active, true, false)) {

		dmaengine_terminate_async(ctlr->dma_rx);

		bcm2835_spi_reset_hw(ctlr);

		bcm2835_spi_reset_hw(bs);

	}



	/* wait for wakeup in framework */

	return 1;



err_reset_hw:

	bcm2835_spi_reset_hw(ctlr);

	bcm2835_spi_reset_hw(bs);

	bcm2835_spi_undo_prologue(bs);

	return ret;

}
@@ -1074,7 +1072,7 @@ static int bcm2835_spi_transfer_one_poll(struct spi_controller *ctlr, 
	}



	/* Transfer complete - reset SPI HW */

	bcm2835_spi_reset_hw(ctlr);

	bcm2835_spi_reset_hw(bs);

	/* and return without waiting for completion */

	return 0;

}
@@ -1182,7 +1180,7 @@ static void bcm2835_spi_handle_err(struct spi_controller *ctlr, 
	bcm2835_spi_undo_prologue(bs);



	/* and reset */

	bcm2835_spi_reset_hw(ctlr);

	bcm2835_spi_reset_hw(bs);

}



static int chip_match_name(struct gpio_chip *chip, void *data)
@@ -1311,6 +1309,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev) 
	ctlr->dev.of_node = pdev->dev.of_node;



	bs = spi_controller_get_devdata(ctlr);

	bs->ctlr = ctlr;



	bs->regs = devm_platform_ioremap_resource(pdev, 0);

	if (IS_ERR(bs->regs)) {
@@ -1345,7 +1344,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev) 
		   BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX);



	err = devm_request_irq(&pdev->dev, bs->irq, bcm2835_spi_interrupt, 0,

			       dev_name(&pdev->dev), ctlr);

			       dev_name(&pdev->dev), bs);

	if (err) {

		dev_err(&pdev->dev, "could not request IRQ: %d\n", err);

		goto out_dma_release;

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