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serial: imx-serial - update UART IMX driver to use cyclic DMA

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The IMX UART has a 32 bytes HW buffer which can be filled up in
2777us at 115200 baud or 80us at 4Mbaud (supported by IMX53).
Taking this in consideration there is a good probability to lose
data because of the DMA startup latency.
Our tests (explained below) indicates a latency up to 4400us when
creating interrupt load and ~70us without. When creating interrupt
load I was able to see continuous overrun errors by checking serial
driver statistics using the command:
`cat /proc/tty/driver/IMX-uart`.

Replace manual restart of DMA with cyclic DMA to eliminate data loss
due to DMA engine startup latency (similar approch to atmel_serial.c
driver). As result the DMA engine will start on the first serial data
transfer and stops only when serial port is closed.

Tests environment:
 Using the m53evk board I have used a GPIO for profiling the IMX
 serial driver.
  - The RX line and GPIO were connected to oscilloscope.
  - Run a small test program on the m53evk board that will only open
    and read data from ttymxc2 port.
  - Connect the ttymxc2 port to my laptop using a USB serial converter
    where another test program is running, able to send configurable
    packet lengths and intervals.
  - Serial ports configured at 115200 8N1.
  - Interrupts load created by disconnecting/connecting (3s interval)
    a USB hub, using a digital switch, with 4 USB devices (USB-Serial
    converter, USB SD card, etc) connected.
    (around 160 interrupts/second generated)
  - The GPIO was toggled HI in the `imx_int` when USR1_RRDY or USR1_AGTIM
    events are received and toggled back, once the DMA configuration
    is finalized, at the end of `imx_dma_rxint`.

Measurements:
The measurements were done from the end of the last byte (RX line) until
the GPIO was toggled back LOW.

Note: The GPIO toggling was done using `gpiod_set_value` method.

Tests performed:
   1. Sending 9 bytes packets at 8ms interval. Having the 9 bytes packets
      will activate the RRDY threshold event and IMX serial interrupt
      called.
      Results:
        - DMA start latency (interrupt start latency +
           DMA configuration) consistently 70us when system not loaded.
        - DMA start latency up to 4400us when system loaded.
   2. Sending 40 bytes packet at 8mS interval.
      Results with load:
        - Able to observe overruns by running:
           `watch -n1 cat /proc/tty/driver/IMX-uart`

Tested-by: Peter Senna Tschudin <peter.senna@collabora.com>
Acked-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Nandor Han <nandor.han@ge.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Nandor Han 2016-08-08 15:38:27 +03:00 committed by Greg Kroah-Hartman
parent 5881826ded
commit 9d297239b8
1 changed files with 82 additions and 67 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

137
drivers/tty/serial/imx.c
View file

@ -222,6 +222,9 @@ struct imx_port {
struct dma_chan *dma_chan_rx, *dma_chan_tx;
struct scatterlist rx_sgl, tx_sgl[2];
void *rx_buf;
struct circ_buf rx_ring;
unsigned int rx_periods;
dma_cookie_t rx_cookie;
unsigned int tx_bytes;
unsigned int dma_tx_nents;
wait_queue_head_t dma_wait;
@ -932,30 +935,6 @@ static void imx_timeout(unsigned long data)
}
#define RX_BUF_SIZE (PAGE_SIZE)
static void imx_rx_dma_done(struct imx_port *sport)
{
unsigned long temp;
unsigned long flags;
spin_lock_irqsave(&sport->port.lock, flags);
/* re-enable interrupts to get notified when new symbols are incoming */
temp = readl(sport->port.membase + UCR1);
temp |= UCR1_RRDYEN;
writel(temp, sport->port.membase + UCR1);
temp = readl(sport->port.membase + UCR2);
temp |= UCR2_ATEN;
writel(temp, sport->port.membase + UCR2);
sport->dma_is_rxing = 0;
/* Is the shutdown waiting for us? */
if (waitqueue_active(&sport->dma_wait))
wake_up(&sport->dma_wait);
spin_unlock_irqrestore(&sport->port.lock, flags);
}
/*
* There are two kinds of RX DMA interrupts(such as in the MX6Q):
@ -972,42 +951,74 @@ static void dma_rx_callback(void *data)
struct scatterlist *sgl = &sport->rx_sgl;
struct tty_port *port = &sport->port.state->port;
struct dma_tx_state state;
struct circ_buf *rx_ring = &sport->rx_ring;
enum dma_status status;
unsigned int count;
/* unmap it first */
dma_unmap_sg(sport->port.dev, sgl, 1, DMA_FROM_DEVICE);
unsigned int w_bytes = 0;
unsigned int r_bytes;
unsigned int bd_size;
status = dmaengine_tx_status(chan, (dma_cookie_t)0, &state);
count = RX_BUF_SIZE - state.residue;
dev_dbg(sport->port.dev, "We get %d bytes.\n", count);
if (status == DMA_ERROR) {
dev_err(sport->port.dev, "DMA transaction error.\n");
return;
}
if (count) {
if (!(sport->port.ignore_status_mask & URXD_DUMMY_READ)) {
int bytes = tty_insert_flip_string(port, sport->rx_buf,
count);
if (bytes != count)
sport->port.icount.buf_overrun++;
}
tty_flip_buffer_push(port);
sport->port.icount.rx += count;
}
/*
* Restart RX DMA directly if more data is available in order to skip
* the roundtrip through the IRQ handler. If there is some data already
* in the FIFO, DMA needs to be restarted soon anyways.
*
* Otherwise stop the DMA and reactivate FIFO IRQs to restart DMA once
* data starts to arrive again.
* The state-residue variable represents the empty space
* relative to the entire buffer. Taking this in consideration
* the head is always calculated base on the buffer total
* length - DMA transaction residue. The UART script from the
* SDMA firmware will jump to the next buffer descriptor,
* once a DMA transaction if finalized (IMX53 RM - A.4.1.2.4).
* Taking this in consideration the tail is always at the
* beginning of the buffer descriptor that contains the head.
*/
if (readl(sport->port.membase + USR2) & USR2_RDR)
start_rx_dma(sport);
else
imx_rx_dma_done(sport);
/* Calculate the head */
rx_ring->head = sg_dma_len(sgl) - state.residue;
/* Calculate the tail. */
bd_size = sg_dma_len(sgl) / sport->rx_periods;
rx_ring->tail = ((rx_ring->head-1) / bd_size) * bd_size;
if (rx_ring->head <= sg_dma_len(sgl) &&
rx_ring->head > rx_ring->tail) {
/* Move data from tail to head */
r_bytes = rx_ring->head - rx_ring->tail;
/* CPU claims ownership of RX DMA buffer */
dma_sync_sg_for_cpu(sport->port.dev, sgl, 1,
DMA_FROM_DEVICE);
w_bytes = tty_insert_flip_string(port,
sport->rx_buf + rx_ring->tail, r_bytes);
/* UART retrieves ownership of RX DMA buffer */
dma_sync_sg_for_device(sport->port.dev, sgl, 1,
DMA_FROM_DEVICE);
if (w_bytes != r_bytes)
sport->port.icount.buf_overrun++;
sport->port.icount.rx += w_bytes;
} else {
WARN_ON(rx_ring->head > sg_dma_len(sgl));
WARN_ON(rx_ring->head <= rx_ring->tail);
}
}
if (w_bytes) {
tty_flip_buffer_push(port);
dev_dbg(sport->port.dev, "We get %d bytes.\n", w_bytes);
}
}
/* RX DMA buffer periods */
#define RX_DMA_PERIODS 4
static int start_rx_dma(struct imx_port *sport)
{
@ -1017,14 +1028,21 @@ static int start_rx_dma(struct imx_port *sport)
struct dma_async_tx_descriptor *desc;
int ret;
sport->rx_ring.head = 0;
sport->rx_ring.tail = 0;
sport->rx_periods = RX_DMA_PERIODS;
sg_init_one(sgl, sport->rx_buf, RX_BUF_SIZE);
ret = dma_map_sg(dev, sgl, 1, DMA_FROM_DEVICE);
if (ret == 0) {
dev_err(dev, "DMA mapping error for RX.\n");
return -EINVAL;
}
desc = dmaengine_prep_slave_sg(chan, sgl, 1, DMA_DEV_TO_MEM,
DMA_PREP_INTERRUPT);
desc = dmaengine_prep_dma_cyclic(chan, sg_dma_address(sgl),
sg_dma_len(sgl), sg_dma_len(sgl) / sport->rx_periods,
DMA_DEV_TO_MEM, DMA_PREP_INTERRUPT);
if (!desc) {
dma_unmap_sg(dev, sgl, 1, DMA_FROM_DEVICE);
dev_err(dev, "We cannot prepare for the RX slave dma!\n");
@ -1034,7 +1052,7 @@ static int start_rx_dma(struct imx_port *sport)
desc->callback_param = sport;
dev_dbg(dev, "RX: prepare for the DMA.\n");
dmaengine_submit(desc);
sport->rx_cookie = dmaengine_submit(desc);
dma_async_issue_pending(chan);
return 0;
}
@ -1058,14 +1076,16 @@ static void imx_setup_ufcr(struct imx_port *sport,
static void imx_uart_dma_exit(struct imx_port *sport)
{
if (sport->dma_chan_rx) {
dmaengine_terminate_all(sport->dma_chan_rx);
dma_release_channel(sport->dma_chan_rx);
sport->dma_chan_rx = NULL;
sport->rx_cookie = -EINVAL;
kfree(sport->rx_buf);
sport->rx_buf = NULL;
}
if (sport->dma_chan_tx) {
dmaengine_terminate_all(sport->dma_chan_tx);
dma_release_channel(sport->dma_chan_tx);
sport->dma_chan_tx = NULL;
}
@ -1103,6 +1123,7 @@ static int imx_uart_dma_init(struct imx_port *sport)
ret = -ENOMEM;
goto err;
}
sport->rx_ring.buf = sport->rx_buf;
/* Prepare for TX : */
sport->dma_chan_tx = dma_request_slave_channel(dev, "tx");
@ -1283,17 +1304,11 @@ static void imx_shutdown(struct uart_port *port)
unsigned long flags;
if (sport->dma_is_enabled) {
int ret;
/* We have to wait for the DMA to finish. */
ret = wait_event_interruptible(sport->dma_wait,
!sport->dma_is_rxing && !sport->dma_is_txing);
if (ret != 0) {
sport->dma_is_rxing = 0;
sport->dma_is_txing = 0;
dmaengine_terminate_all(sport->dma_chan_tx);
dmaengine_terminate_all(sport->dma_chan_rx);
}
spin_lock_irqsave(&sport->port.lock, flags);
imx_stop_tx(port);
imx_stop_rx(port);