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SubjectRe: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx
On Wed, Sep 10, 2014 at 09:30:04PM +0200, Sebastian Andrzej Siewior wrote:
> At least on AM335x the following problem exists: Even if the TX FIFO is
> empty and a TX transfer is programmed (and started) the UART does not
> trigger the DMA transfer.
> After $TRESHOLD number of bytes have been written to the FIFO manually the
> UART reevaluates the whole situation and decides that now there is enough
> room in the FIFO and so the transfer begins.
> This problem has not been seen on DRA7 or beagle board xm (OMAP3). I am not
> sure if this is UART-IP core specific or DMA engine.
>
> The workaround is to use a threshold of one byte, program the DMA
> transfer minus one byte and then to put the first byte into the FIFO to
> kick start the transfer.
>
> v7…v8:
> - fix the problem when get invoked and the FIFO is full.
>
> Reviewed-by: Tony Lindgren <tony@atomide.com>
> Tested-by: Tony Lindgren <tony@atomide.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> drivers/tty/serial/8250/8250.h | 3 +++
> drivers/tty/serial/8250/8250_dma.c | 39 +++++++++++++++++++++++++++++++++++---
> include/uapi/linux/serial_reg.h | 1 +
> 3 files changed, 40 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index fbed1636e9c4..09489b391568 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -82,6 +82,9 @@ struct serial8250_config {
> #define UART_BUG_PARITY (1 << 4) /* UART mishandles parity if FIFO enabled */
> #define UART_BUG_DMA_RX (1 << 5) /* UART needs DMA RX req before there is
> data in FIFO */
> +#define UART_BUG_DMA_TX (1 << 6) /* UART needs one byte in FIFO for
> + kickstart */

I don't think we should go ahead with this patch. I'm pretty sure
this is AM335 specific problem, or at least limited to only few
platforms. And I don't think we should take any more "BUG" flags.

We should add hooks like tx_dma and rx_dma to struct uart_8250_dma so
that the probe drivers can replace serial8250_tx_dma and
seria8250_rx_dma, like I think Alan already suggested.

Let's keep serial8250_tx_dma/rx_dma as the default, and not add any
quirks to them. Only if there is a very common case should it be
handled in those. The case of RX req needing to be sent before data in
FIFO maybe one of those, but I'm no sure.


> #define PROBE_RSA (1 << 0)
> #define PROBE_ANY (~0)
>
> diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
> index 3674900a1f14..48dc57aad0dd 100644
> --- a/drivers/tty/serial/8250/8250_dma.c
> +++ b/drivers/tty/serial/8250/8250_dma.c
> @@ -83,6 +83,7 @@ int serial8250_tx_dma(struct uart_8250_port *p)
> struct uart_8250_dma *dma = p->dma;
> struct circ_buf *xmit = &p->port.state->xmit;
> struct dma_async_tx_descriptor *desc;
> + unsigned int skip_byte = 0;
> int ret;
>
> if (uart_tx_stopped(&p->port) || dma->tx_running ||
> @@ -91,10 +92,40 @@ int serial8250_tx_dma(struct uart_8250_port *p)
>
> dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
>
> + if (p->bugs & UART_BUG_DMA_TX) {
> + u8 tx_lvl;
> +
> + /*
> + * We need to put the first byte into the FIFO in order to start
> + * the DMA transfer. For transfers smaller than four bytes we
> + * don't bother doing DMA at all. It seem not matter if there
> + * are still bytes in the FIFO from the last transfer (in case
> + * we got here directly from __dma_tx_complete()). Bytes leaving
> + * the FIFO seem not to trigger the DMA transfer. It is really
> + * the byte that we put into the FIFO.
> + * If the FIFO is already full then we most likely got here from
> + * __dma_tx_complete(). And this means the DMA engine just
> + * completed its work. We don't have to wait the complete 86us
> + * at 115200,8n1 but around 60us (not to mention lower
> + * baudrates). So in that case we take the interrupt and try
> + * again with an empty FIFO.
> + */
> + tx_lvl = serial_in(p, UART_OMAP_TX_LVL);
> + if (tx_lvl == p->tx_loadsz) {
> + ret = -EBUSY;
> + goto err;
> + }
> + if (dma->tx_size < 4) {
> + ret = -EINVAL;
> + goto err;
> + }
> + skip_byte = 1;
> + }
> +
> desc = dmaengine_prep_slave_single(dma->txchan,
> - dma->tx_addr + xmit->tail,
> - dma->tx_size, DMA_MEM_TO_DEV,
> - DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
> + dma->tx_addr + xmit->tail + skip_byte,
> + dma->tx_size - skip_byte, DMA_MEM_TO_DEV,
> + DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
> if (!desc) {
> ret = -EBUSY;
> goto err;
> @@ -118,6 +149,8 @@ int serial8250_tx_dma(struct uart_8250_port *p)
> serial_out(p, UART_IER, p->ier);
> }
> }
> + if (skip_byte)
> + serial_out(p, UART_TX, xmit->buf[xmit->tail]);
> return 0;
> err:
> dma->tx_err = 1;
> diff --git a/include/uapi/linux/serial_reg.h b/include/uapi/linux/serial_reg.h
> index df6c9ab6b0cd..53af3b790129 100644
> --- a/include/uapi/linux/serial_reg.h
> +++ b/include/uapi/linux/serial_reg.h
> @@ -359,6 +359,7 @@
> #define UART_OMAP_SYSC 0x15 /* System configuration register */
> #define UART_OMAP_SYSS 0x16 /* System status register */
> #define UART_OMAP_WER 0x17 /* Wake-up enable register */
> +#define UART_OMAP_TX_LVL 0x1a /* TX FIFO level register */
>
> /*
> * These are the definitions for the MDR1 register
> --
> 2.1.0

Cheers,

--
heikki
--
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