From 571e31fa60b3697d5db26140e16d5c45c51c9815 Mon Sep 17 00:00:00 2001 From: Lukas Wunner Date: Wed, 11 Sep 2019 12:15:30 +0200 Subject: spi: bcm2835: Cache CS register value for ->prepare_message() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The BCM2835 SPI driver needs to set up the clock polarity in its ->prepare_message() hook before spi_transfer_one_message() asserts chip select to avoid a gratuitous clock signal edge (cf. commit acace73df2c1 ("spi: bcm2835: set up spi-mode before asserting cs-gpio")). Precalculate the CS register value (which selects the clock polarity) once in ->setup() and use that cached value in ->prepare_message() and ->transfer_one(). This avoids one MMIO read per message and one per transfer, yielding a small latency improvement. Additionally, a forthcoming commit will use the precalculated value to derive the register value for clearing the RX FIFO, which will eliminate the need for an RX dummy buffer when performing TX-only DMA transfers. Tested-by: Nuno Sá Tested-by: Noralf Trønnes Signed-off-by: Lukas Wunner Acked-by: Stefan Wahren Acked-by: Martin Sperl Link: https://lore.kernel.org/r/d17c1d7fcdc97fffa961b8737cfd80eeb14f9416.1568187525.git.lukas@wunner.de Signed-off-by: Mark Brown --- drivers/spi/spi-bcm2835.c | 46 ++++++++++++++++++++++++++-------------------- 1 file changed, 26 insertions(+), 20 deletions(-) (limited to 'drivers') diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c index 60255ac837f5..f01d18d521f2 100644 --- a/drivers/spi/spi-bcm2835.c +++ b/drivers/spi/spi-bcm2835.c @@ -68,6 +68,7 @@ #define BCM2835_SPI_FIFO_SIZE 64 #define BCM2835_SPI_FIFO_SIZE_3_4 48 #define BCM2835_SPI_DMA_MIN_LENGTH 96 +#define BCM2835_SPI_NUM_CS 3 /* raise as necessary */ #define BCM2835_SPI_MODE_BITS (SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \ | SPI_NO_CS | SPI_3WIRE) @@ -94,6 +95,8 @@ MODULE_PARM_DESC(polling_limit_us, * @rx_prologue: bytes received without DMA if first RX sglist entry's * length is not a multiple of 4 (to overcome hardware limitation) * @tx_spillover: whether @tx_prologue spills over to second TX sglist entry + * @prepare_cs: precalculated CS register value for ->prepare_message() + * (uses slave-specific clock polarity and phase settings) * @debugfs_dir: the debugfs directory - neede to remove debugfs when * unloading the module * @count_transfer_polling: count of how often polling mode is used @@ -116,6 +119,7 @@ struct bcm2835_spi { int tx_prologue; int rx_prologue; unsigned int tx_spillover; + u32 prepare_cs[BCM2835_SPI_NUM_CS]; struct dentry *debugfs_dir; u64 count_transfer_polling; @@ -808,7 +812,7 @@ static int bcm2835_spi_transfer_one(struct spi_controller *ctlr, struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr); unsigned long spi_hz, clk_hz, cdiv, spi_used_hz; unsigned long hz_per_byte, byte_limit; - u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS); + u32 cs = bs->prepare_cs[spi->chip_select]; /* set clock */ spi_hz = tfr->speed_hz; @@ -833,15 +837,6 @@ static int bcm2835_spi_transfer_one(struct spi_controller *ctlr, if (spi->mode & SPI_3WIRE && tfr->rx_buf && tfr->rx_buf != ctlr->dummy_rx) cs |= BCM2835_SPI_CS_REN; - else - cs &= ~BCM2835_SPI_CS_REN; - - /* - * The driver always uses software-controlled GPIO Chip Select. - * Set the hardware-controlled native Chip Select to an invalid - * value to prevent it from interfering. - */ - cs |= BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01; /* set transmit buffers and length */ bs->tx_buf = tfr->tx_buf; @@ -878,7 +873,6 @@ static int bcm2835_spi_prepare_message(struct spi_controller *ctlr, { struct spi_device *spi = msg->spi; struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr); - u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS); int ret; if (ctlr->can_dma) { @@ -893,14 +887,11 @@ static int bcm2835_spi_prepare_message(struct spi_controller *ctlr, return ret; } - cs &= ~(BCM2835_SPI_CS_CPOL | BCM2835_SPI_CS_CPHA); - - if (spi->mode & SPI_CPOL) - cs |= BCM2835_SPI_CS_CPOL; - if (spi->mode & SPI_CPHA) - cs |= BCM2835_SPI_CS_CPHA; - - bcm2835_wr(bs, BCM2835_SPI_CS, cs); + /* + * Set up clock polarity before spi_transfer_one_message() asserts + * chip select to avoid a gratuitous clock signal edge. + */ + bcm2835_wr(bs, BCM2835_SPI_CS, bs->prepare_cs[spi->chip_select]); return 0; } @@ -926,8 +917,23 @@ static int chip_match_name(struct gpio_chip *chip, void *data) static int bcm2835_spi_setup(struct spi_device *spi) { + struct bcm2835_spi *bs = spi_controller_get_devdata(spi->controller); struct gpio_chip *chip; enum gpio_lookup_flags lflags; + u32 cs; + + /* + * Precalculate SPI slave's CS register value for ->prepare_message(): + * The driver always uses software-controlled GPIO chip select, hence + * set the hardware-controlled native chip select to an invalid value + * to prevent it from interfering. + */ + cs = BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01; + if (spi->mode & SPI_CPOL) + cs |= BCM2835_SPI_CS_CPOL; + if (spi->mode & SPI_CPHA) + cs |= BCM2835_SPI_CS_CPHA; + bs->prepare_cs[spi->chip_select] = cs; /* * sanity checking the native-chipselects @@ -1005,7 +1011,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev) ctlr->use_gpio_descriptors = true; ctlr->mode_bits = BCM2835_SPI_MODE_BITS; ctlr->bits_per_word_mask = SPI_BPW_MASK(8); - ctlr->num_chipselect = 3; + ctlr->num_chipselect = BCM2835_SPI_NUM_CS; ctlr->setup = bcm2835_spi_setup; ctlr->transfer_one = bcm2835_spi_transfer_one; ctlr->handle_err = bcm2835_spi_handle_err; -- cgit v1.2.3