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path: root/drivers/spi/spi-bcm2835.c
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-rw-r--r--drivers/spi/spi-bcm2835.c93
1 files changed, 82 insertions, 11 deletions
diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 3fe823891861..8a0ea465cbe0 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -111,6 +111,9 @@ MODULE_PARM_DESC(polling_limit_us,
* @tx_dma_active: whether a TX DMA descriptor is in progress
* @rx_dma_active: whether a RX DMA descriptor is in progress
* (used by bcm2835_spi_dma_tx_done() to handle a race)
+ * @fill_tx_desc: preallocated TX DMA descriptor used for RX-only transfers
+ * (cyclically copies from zero page to TX FIFO)
+ * @fill_tx_addr: bus address of zero page
* @clear_rx_desc: preallocated RX DMA descriptor used for TX-only transfers
* (cyclically clears RX FIFO by writing @clear_rx_cs to CS register)
* @clear_rx_addr: bus address of @clear_rx_cs
@@ -140,6 +143,8 @@ struct bcm2835_spi {
u8 chip_select;
unsigned int tx_dma_active;
unsigned int rx_dma_active;
+ struct dma_async_tx_descriptor *fill_tx_desc;
+ dma_addr_t fill_tx_addr;
struct dma_async_tx_descriptor *clear_rx_desc[BCM2835_SPI_NUM_CS];
dma_addr_t clear_rx_addr;
u32 clear_rx_cs[BCM2835_SPI_NUM_CS] ____cacheline_aligned;
@@ -469,14 +474,14 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
bs->rx_prologue = 0;
bs->tx_spillover = false;
- if (!sg_is_last(&tfr->tx_sg.sgl[0]))
+ if (bs->tx_buf && !sg_is_last(&tfr->tx_sg.sgl[0]))
bs->tx_prologue = sg_dma_len(&tfr->tx_sg.sgl[0]) & 3;
if (bs->rx_buf && !sg_is_last(&tfr->rx_sg.sgl[0])) {
bs->rx_prologue = sg_dma_len(&tfr->rx_sg.sgl[0]) & 3;
if (bs->rx_prologue > bs->tx_prologue) {
- if (sg_is_last(&tfr->tx_sg.sgl[0])) {
+ if (!bs->tx_buf || sg_is_last(&tfr->tx_sg.sgl[0])) {
bs->tx_prologue = bs->rx_prologue;
} else {
bs->tx_prologue += 4;
@@ -508,6 +513,9 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
sg_dma_len(&tfr->rx_sg.sgl[0]) -= bs->rx_prologue;
}
+ if (!bs->tx_buf)
+ return;
+
/*
* Write remaining TX prologue. Adjust first entry in TX sglist.
* Also adjust second entry if prologue spills over to it.
@@ -552,6 +560,9 @@ static void bcm2835_spi_undo_prologue(struct bcm2835_spi *bs)
sg_dma_len(&tfr->rx_sg.sgl[0]) += bs->rx_prologue;
}
+ if (!bs->tx_buf)
+ goto out;
+
if (likely(!bs->tx_spillover)) {
sg_dma_address(&tfr->tx_sg.sgl[0]) -= bs->tx_prologue;
sg_dma_len(&tfr->tx_sg.sgl[0]) += bs->tx_prologue;
@@ -560,7 +571,7 @@ static void bcm2835_spi_undo_prologue(struct bcm2835_spi *bs)
sg_dma_address(&tfr->tx_sg.sgl[1]) -= 4;
sg_dma_len(&tfr->tx_sg.sgl[1]) += 4;
}
-
+out:
bs->tx_prologue = 0;
}
@@ -575,10 +586,7 @@ static void bcm2835_spi_dma_rx_done(void *data)
struct spi_controller *ctlr = data;
struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
- /* reset fifo and HW */
- bcm2835_spi_reset_hw(ctlr);
-
- /* and terminate tx-dma as we do not have an irq for it
+ /* 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...
@@ -588,6 +596,9 @@ static void bcm2835_spi_dma_rx_done(void *data)
bs->rx_dma_active = false;
bcm2835_spi_undo_prologue(bs);
+ /* reset fifo and HW */
+ bcm2835_spi_reset_hw(ctlr);
+
/* and mark as completed */;
complete(&ctlr->xfer_completion);
}
@@ -715,6 +726,24 @@ static int bcm2835_spi_prepare_sg(struct spi_controller *ctlr,
* register.) Reading 32 bytes from the RX FIFO would normally require 8 bus
* accesses, whereas clearing it requires only 1 bus access. So an 8-fold
* reduction in bus traffic and thus energy consumption is achieved.
+ *
+ * For *RX-only* transfers (tx_buf is %NULL), fill the TX FIFO by cyclically
+ * copying from the zero page. The DMA descriptor to do this is preallocated
+ * in bcm2835_dma_init(). It must be terminated once the RX DMA channel is
+ * done and can then be reused.
+ *
+ * The BCM2835 DMA driver autodetects when a transaction copies from the zero
+ * page and utilizes the DMA controller's ability to synthesize zeroes instead
+ * of copying them from memory. This reduces traffic on the memory bus. The
+ * feature is not available on so-called "lite" channels, but normally TX DMA
+ * is backed by a full-featured channel.
+ *
+ * Zero-filling the TX FIFO is paced by the DREQ signal. Unfortunately the
+ * BCM2835 SPI controller continues to assert DREQ even after the DLEN register
+ * has been counted down to zero (hardware erratum). Thus, when the transfer
+ * has finished, the DMA engine zero-fills the TX FIFO until it is half full.
+ * (Tuneable with the DC register.) So up to 9 gratuitous bus accesses are
+ * performed at the end of an RX-only transfer.
*/
static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr,
struct spi_device *spi,
@@ -735,7 +764,12 @@ static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr,
bcm2835_spi_transfer_prologue(ctlr, tfr, bs, cs);
/* setup tx-DMA */
- ret = bcm2835_spi_prepare_sg(ctlr, spi, tfr, bs, true);
+ if (bs->tx_buf) {
+ ret = bcm2835_spi_prepare_sg(ctlr, spi, tfr, bs, true);
+ } else {
+ cookie = dmaengine_submit(bs->fill_tx_desc);
+ ret = dma_submit_error(cookie);
+ }
if (ret)
goto err_reset_hw;
@@ -812,6 +846,16 @@ static void bcm2835_dma_release(struct spi_controller *ctlr,
if (ctlr->dma_tx) {
dmaengine_terminate_sync(ctlr->dma_tx);
+
+ if (bs->fill_tx_desc)
+ dmaengine_desc_free(bs->fill_tx_desc);
+
+ if (bs->fill_tx_addr)
+ dma_unmap_page_attrs(ctlr->dma_tx->device->dev,
+ bs->fill_tx_addr, sizeof(u32),
+ DMA_TO_DEVICE,
+ DMA_ATTR_SKIP_CPU_SYNC);
+
dma_release_channel(ctlr->dma_tx);
ctlr->dma_tx = NULL;
}
@@ -862,7 +906,11 @@ static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev,
goto err_release;
}
- /* configure DMAs */
+ /*
+ * The TX DMA channel either copies a transfer's TX buffer to the FIFO
+ * or, in case of an RX-only transfer, cyclically copies from the zero
+ * page to the FIFO using a preallocated, reusable descriptor.
+ */
slave_config.dst_addr = (u32)(dma_reg_base + BCM2835_SPI_FIFO);
slave_config.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
@@ -870,6 +918,31 @@ static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev,
if (ret)
goto err_config;
+ bs->fill_tx_addr = dma_map_page_attrs(ctlr->dma_tx->device->dev,
+ ZERO_PAGE(0), 0, sizeof(u32),
+ DMA_TO_DEVICE,
+ DMA_ATTR_SKIP_CPU_SYNC);
+ if (dma_mapping_error(ctlr->dma_tx->device->dev, bs->fill_tx_addr)) {
+ dev_err(dev, "cannot map zero page - not using DMA mode\n");
+ bs->fill_tx_addr = 0;
+ goto err_release;
+ }
+
+ bs->fill_tx_desc = dmaengine_prep_dma_cyclic(ctlr->dma_tx,
+ bs->fill_tx_addr,
+ sizeof(u32), 0,
+ DMA_MEM_TO_DEV, 0);
+ if (!bs->fill_tx_desc) {
+ dev_err(dev, "cannot prepare fill_tx_desc - not using DMA mode\n");
+ goto err_release;
+ }
+
+ ret = dmaengine_desc_set_reuse(bs->fill_tx_desc);
+ if (ret) {
+ dev_err(dev, "cannot reuse fill_tx_desc - not using DMA mode\n");
+ goto err_release;
+ }
+
/*
* The RX DMA channel is used bidirectionally: It either reads the
* RX FIFO or, in case of a TX-only transfer, cyclically writes a
@@ -913,8 +986,6 @@ static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev,
/* all went well, so set can_dma */
ctlr->can_dma = bcm2835_spi_can_dma;
- /* need to do TX DMA, so we need a dummy buffer */
- ctlr->flags = SPI_CONTROLLER_MUST_TX;
return;
generated by cgit v1.2.3 (git 2.39.1) at 2024-12-18 05:00:54 +0100