[PATCH] spi: bcm2835: enable dma modes for transfers meeting certain conditions
kernel at martin.sperl.org
kernel at martin.sperl.org
Sun May 10 13:47:28 PDT 2015
From: Martin Sperl <kernel at martin.sperl.org>
Conditions per spi_transfer are:
* transfer.len >= 96 bytes (to avoid mapping overhead costs)
* transfer.len < 65536 bytes (limitaion by spi-hw block - could get extended)
* an individual scatter/gather transfer length must be a multiple of 4
for anything but the last transfer - spi-hw block limit.
(some shortcut has been taken in can_dma to avoid unnecessary mapping of
pages which, for which there is a chance that there is a split with a
transfer length not a multiple of 4)
If it becomes a necessity these restrictions can get removed by additional
code.
Note that this patch requires a patch to dma-bcm2835.c by Noralf to
enable scatter-gather mode inside the dmaengine, which has not been
merged yet.
That is why no patch to arch/arm/boot/dts/bcm2835.dtsi is included - the
code works as before without dma when tx/rx are not set, but it writes
a message warning about dma not used:
spi-bcm2835 20204000.spi: no tx-dma configuration found - not using dma mode
To enable dma-mode add the following lines to the device-tree:
dmas = <&dma 6>, <&dma 7>;
dma-names = "tx", "rx";
Tested-by: Noralf Trønnes <noralf at tronnes.org>
(private communication)
Signed-off-by: Martin Sperl <kernel at martin.sperl.org>
---
drivers/spi/spi-bcm2835.c | 303 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 301 insertions(+), 2 deletions(-)
Tested with:
* 2x mcp251x
* 1x enc28j60
* 1x fb_st7735r
Note that to make it work it requires the patch to dma-engine by
Noralf Tronnes for scatter/gather DMA.
Also it is recommended that the patch for the race-condition in
spi_finalize_current_message is applied to avoid a free before unmap
situations (this only happens only in some configurations)
diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 830d99c..6ab43c8 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -23,15 +23,18 @@
#include <linux/clk.h>
#include <linux/completion.h>
#include <linux/delay.h>
+#include <linux/dma-mapping.h>
+#include <linux/dmaengine.h>
#include <linux/err.h>
#include <linux/interrupt.h>
#include <linux/io.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/of.h>
-#include <linux/of_irq.h>
-#include <linux/of_gpio.h>
+#include <linux/of_address.h>
#include <linux/of_device.h>
+#include <linux/of_gpio.h>
+#include <linux/of_irq.h>
#include <linux/spi/spi.h>
/* SPI register offsets */
@@ -70,6 +73,7 @@
#define BCM2835_SPI_POLLING_LIMIT_US 30
#define BCM2835_SPI_POLLING_JIFFIES 2
+#define BCM2835_SPI_DMA_MIN_LENGTH 96
#define BCM2835_SPI_MODE_BITS (SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
| SPI_NO_CS | SPI_3WIRE)
@@ -83,6 +87,7 @@ struct bcm2835_spi {
u8 *rx_buf;
int tx_len;
int rx_len;
+ bool dma_pending;
};
static inline u32 bcm2835_rd(struct bcm2835_spi *bs, unsigned reg)
@@ -128,12 +133,15 @@ static void bcm2835_spi_reset_hw(struct spi_master *master)
/* Disable SPI interrupts and transfer */
cs &= ~(BCM2835_SPI_CS_INTR |
BCM2835_SPI_CS_INTD |
+ BCM2835_SPI_CS_DMAEN |
BCM2835_SPI_CS_TA);
/* and reset RX/TX FIFOS */
cs |= BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX;
/* and reset the SPI_HW */
bcm2835_wr(bs, BCM2835_SPI_CS, cs);
+ /* as well as DLEN */
+ bcm2835_wr(bs, BCM2835_SPI_DLEN, 0);
}
static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
@@ -193,6 +201,279 @@ static int bcm2835_spi_transfer_one_irq(struct spi_master *master,
return 1;
}
+/*
+ * DMA support
+ *
+ * this implementation has currently a few issues in so far as it does
+ * not work arrount limitations of the HW.
+ *
+ * the main one being that DMA transfers are limited to 16 bit
+ * (so 0 to 65535 bytes) by the SPI HW due to BCM2835_SPI_DLEN
+ *
+ * also we currently assume that the scatter-gather fragments are
+ * all multiple of 4 (except the last) - otherwise we would need
+ * to reset the FIFO before subsequent transfers...
+ * this also means that tx/rx transfers sg's need to be of equal size!
+ *
+ * there may be a few more border-cases we may need to address as well
+ * but unfortunately this would mean splitting up the scatter-gather
+ * list making it slightly unpractical...
+ */
+static void bcm2835_spi_dma_done(void *data)
+{
+ struct spi_master *master = data;
+ struct bcm2835_spi *bs = spi_master_get_devdata(master);
+
+ /* reset fifo and HW */
+ bcm2835_spi_reset_hw(master);
+
+ /* and terminate tx-dma as we do not have an irq for it
+ * because when the rx dma will terminate and this callback
+ * is called the tx-dma must have finished - can't get to this
+ * situation otherwise...
+ */
+ dmaengine_terminate_all(master->dma_tx);
+
+ /* mark as no longer pending */
+ bs->dma_pending = 0;
+
+ /* and mark as completed */;
+ complete(&master->xfer_completion);
+}
+
+static int bcm2835_spi_prepare_sg(struct spi_master *master,
+ struct spi_transfer *tfr,
+ bool is_tx)
+{
+ struct dma_chan *chan;
+ struct scatterlist *sgl;
+ unsigned int nents;
+ enum dma_transfer_direction dir;
+ unsigned long flags;
+
+ struct dma_async_tx_descriptor *desc;
+ dma_cookie_t cookie;
+
+ if (is_tx) {
+ dir = DMA_MEM_TO_DEV;
+ chan = master->dma_tx;
+ nents = tfr->tx_sg.nents;
+ sgl = tfr->tx_sg.sgl;
+ flags = 0 /* no tx interrupt */;
+
+ } else {
+ dir = DMA_DEV_TO_MEM;
+ chan = master->dma_rx;
+ nents = tfr->rx_sg.nents;
+ sgl = tfr->rx_sg.sgl;
+ flags = DMA_PREP_INTERRUPT;
+ }
+ /* prepare the channel */
+ desc = dmaengine_prep_slave_sg(chan, sgl, nents, dir, flags);
+ if (!desc)
+ return -EINVAL;
+
+ /* set callback for rx */
+ if (!is_tx) {
+ desc->callback = bcm2835_spi_dma_done;
+ desc->callback_param = master;
+ }
+
+ /* submit it to DMA-engine */
+ cookie = dmaengine_submit(desc);
+
+ return dma_submit_error(cookie);
+}
+
+static inline int bcm2835_check_sg_length(struct sg_table *sgt)
+{
+ int i;
+ struct scatterlist *sgl;
+
+ /* check that the sg entries are word-sized (except for last) */
+ for_each_sg(sgt->sgl, sgl, (int)sgt->nents - 1, i) {
+ if (sg_dma_len(sgl) % 4)
+ return -EFAULT;
+ }
+
+ return 0;
+}
+
+static int bcm2835_spi_transfer_one_dma(struct spi_master *master,
+ struct spi_device *spi,
+ struct spi_transfer *tfr,
+ u32 cs)
+{
+ struct bcm2835_spi *bs = spi_master_get_devdata(master);
+ int ret;
+
+ /* check that the scatter gather segments are all a multiple of 4 */
+ if (bcm2835_check_sg_length(&tfr->tx_sg) ||
+ bcm2835_check_sg_length(&tfr->rx_sg)) {
+ dev_warn_once(&spi->dev,
+ "scatter gather segment length is not a multiple of 4 - falling back to interrupt mode\n");
+ return bcm2835_spi_transfer_one_irq(master, spi, tfr, cs);
+ }
+
+ /* setup tx-DMA */
+ ret = bcm2835_spi_prepare_sg(master, tfr, true);
+ if (ret)
+ return ret;
+
+ /* start TX early */
+ dma_async_issue_pending(master->dma_tx);
+
+ /* mark as dma pending */
+ bs->dma_pending = 1;
+
+ /* set the DMA length */
+ bcm2835_wr(bs, BCM2835_SPI_DLEN, tfr->len);
+
+ /* start the HW */
+ bcm2835_wr(bs, BCM2835_SPI_CS,
+ cs | BCM2835_SPI_CS_TA | BCM2835_SPI_CS_DMAEN);
+
+ /* setup rx-DMA late - to run transfers while
+ * mapping of the rx buffers still takes place
+ * this saves 10us or more.
+ */
+ ret = bcm2835_spi_prepare_sg(master, tfr, false);
+ if (ret) {
+ /* need to reset on errors */
+ dmaengine_terminate_all(master->dma_tx);
+ bcm2835_spi_reset_hw(master);
+ return ret;
+ }
+
+ /* start rx dma late */
+ dma_async_issue_pending(master->dma_rx);
+
+ /* wait for wakeup in framework */
+ return 1;
+}
+
+static bool bcm2835_spi_can_dma(struct spi_master *master,
+ struct spi_device *spi,
+ struct spi_transfer *tfr)
+{
+ /* only run for gpio_cs */
+ if (!gpio_is_valid(spi->cs_gpio))
+ return false;
+
+ /* we start DMA efforts only on bigger transfers */
+ if (tfr->len < BCM2835_SPI_DMA_MIN_LENGTH)
+ return false;
+
+ /* BCM2835_SPI_DLEN has defined a max transfer size as
+ * 16 bit, so max is 65535
+ * we can revisit this by using an alternative transfer
+ * method - ideally this would get done without any more
+ * interaction...
+ */
+ if (tfr->len > 65535) {
+ dev_warn_once(&spi->dev,
+ "transfer size of %d too big for dma-transfer\n",
+ tfr->len);
+ return false;
+ }
+
+ /* if we run rx/tx_buf with word aligned addresses then we are OK */
+ if (((u32)tfr->tx_buf % 4 == 0) && ((u32)tfr->tx_buf % 4 == 0))
+ return true;
+
+ /* otherwise we only allow transfers within the same page
+ * to avoid wasting time on dma_mapping when it is not practical
+ */
+ if (((u32)tfr->tx_buf % SZ_4K) + tfr->len > SZ_4K) {
+ dev_warn_once(&spi->dev,
+ "Unaligned spi tx-transfer bridging page\n");
+ return false;
+ }
+ if (((u32)tfr->rx_buf % SZ_4K) + tfr->len > SZ_4K) {
+ dev_warn_once(&spi->dev,
+ "Unaligned spi tx-transfer bridging page\n");
+ return false;
+ }
+
+ /* return OK */
+ return true;
+}
+
+void bcm2835_dma_release(struct spi_master *master)
+{
+ if (master->dma_tx) {
+ dmaengine_terminate_all(master->dma_tx);
+ dma_release_channel(master->dma_tx);
+ master->dma_tx = NULL;
+ }
+ if (master->dma_rx) {
+ dmaengine_terminate_all(master->dma_rx);
+ dma_release_channel(master->dma_rx);
+ master->dma_rx = NULL;
+ }
+}
+
+void bcm2835_dma_init(struct spi_master *master, struct device *dev)
+{
+ struct dma_slave_config slave_config;
+ const __be32 *addr;
+ dma_addr_t dma_reg_base;
+ int ret;
+
+ /* base address in dma-space */
+ addr = of_get_address(master->dev.of_node, 0, NULL, NULL);
+ if (!addr) {
+ dev_err(dev, "could not get DMA-register address - not using dma mode\n");
+ goto err;
+ }
+ dma_reg_base = be32_to_cpup(addr);
+
+ /* get tx/rx dma */
+ master->dma_tx = dma_request_slave_channel(dev, "tx");
+ if (!master->dma_tx) {
+ dev_err(dev, "no tx-dma configuration found - not using dma mode\n");
+ goto err;
+ }
+ master->dma_rx = dma_request_slave_channel(dev, "rx");
+ if (!master->dma_rx) {
+ dev_err(dev, "no rx-dma configuration found - not using dma mode\n");
+ goto err_release;
+ }
+
+ /* configure DMAs */
+ slave_config.direction = DMA_MEM_TO_DEV;
+ slave_config.dst_addr = (u32)(dma_reg_base + BCM2835_SPI_FIFO);
+ slave_config.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
+
+ ret = dmaengine_slave_config(master->dma_tx, &slave_config);
+ if (ret)
+ goto err_config;
+
+ slave_config.direction = DMA_DEV_TO_MEM;
+ slave_config.src_addr = (u32)(dma_reg_base + BCM2835_SPI_FIFO);
+ slave_config.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
+
+ ret = dmaengine_slave_config(master->dma_rx, &slave_config);
+ if (ret)
+ goto err_config;
+
+ /* all went well, so set can_dma */
+ master->can_dma = bcm2835_spi_can_dma;
+ master->max_dma_len = 65535; /* limitation by BCM2835_SPI_DLEN */
+ /* need to do TX AND RX DMA, so we need dummy buffers */
+ master->flags = SPI_MASTER_MUST_RX | SPI_MASTER_MUST_TX;
+
+ return;
+
+err_config:
+ dev_err(dev, "issue configuring dma: %d - not using DMA mode\n",
+ ret);
+err_release:
+ bcm2835_dma_release(master);
+err:
+ return;
+}
+
static int bcm2835_spi_transfer_one_poll(struct spi_master *master,
struct spi_device *spi,
struct spi_transfer *tfr,
@@ -301,12 +582,26 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
return bcm2835_spi_transfer_one_poll(master, spi, tfr,
cs, xfer_time_us);
+ /* run in dma mode if conditions are right */
+ if (master->can_dma && bcm2835_spi_can_dma(master, spi, tfr))
+ return bcm2835_spi_transfer_one_dma(master, spi, tfr, cs);
+
+ /* run in interrupt-mode */
return bcm2835_spi_transfer_one_irq(master, spi, tfr, cs);
}
static void bcm2835_spi_handle_err(struct spi_master *master,
struct spi_message *msg)
{
+ struct bcm2835_spi *bs = spi_master_get_devdata(master);
+
+ /* if an error occurred and we have an active dma, then terminate */
+ if (bs->dma_pending) {
+ dmaengine_terminate_all(master->dma_tx);
+ dmaengine_terminate_all(master->dma_rx);
+ bs->dma_pending = 0;
+ }
+ /* and reset */
bcm2835_spi_reset_hw(master);
}
@@ -476,6 +771,8 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
goto out_clk_disable;
}
+ bcm2835_dma_init(master, &pdev->dev);
+
/* initialise the hardware with the default polarities */
bcm2835_wr(bs, BCM2835_SPI_CS,
BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX);
@@ -506,6 +803,8 @@ static int bcm2835_spi_remove(struct platform_device *pdev)
clk_disable_unprepare(bs->clk);
+ bcm2835_dma_release(master);
+
return 0;
}
--
1.7.10.4
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