[PATCH v4 03/10] serial: sh-sci: Submit RX DMA from RX interrupt on (H)SCIF

[Geert Uytterhoeven <geert+renesas@glider.be>]

For DMA receive requests, the driver is only notified by DMA completion
after the whole DMA request has been transferred.  If less data is
received, it will stay stuck until more data arrives.  The driver
handles this by setting up a timer handler from the receive interrupt,
after reception of the first character.

Unlike SCIFA and SCIFB, SCIF and HSCIF don't issue receive interrupts on
reception of individual characters if a receive DMA request is in
progress, so the timer is never set up.

To fix receive DMA on SCIF and HSCIF, submit the receive DMA request
from the receive interrupt handler instead.
In some sense this is similar to the SCIFA/SCIFB behavior, where the
RDRQE (Rx Data Transfer Request Enable) bit is also set from the receive
interrupt handler.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
v4:
  - Dropped RFC status,
  - Rebased on top of "[PATCH] serial: sh-sci: Shuffle functions
    around", hence the forward declaration of sci_submit_rx() is no
    longer needed, and the declared-but-never-defined compiler warning
    if CONFIG_SERIAL_SH_SCI_DMA=n is gone.

v3:
  - New, this replaces the one-byte DMA transfer from "[PATCH/RFC v2
    28/29] serial: sh-sci: Add (H)SCIF DMA support".
---
 drivers/tty/serial/sh-sci.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index eb2b369b1cf1be0b..02aaf4d213d9c280 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1317,7 +1317,8 @@ static void rx_timer_fn(unsigned long arg)
 
 	spin_unlock_irqrestore(&port->lock, flags);
 
-	sci_submit_rx(s);
+	if (port->type = PORT_SCIFA || port->type = PORT_SCIFB)
+		sci_submit_rx(s);
 }
 
 static void sci_request_dma(struct uart_port *port)
@@ -1403,7 +1404,8 @@ static void sci_request_dma(struct uart_port *port)
 
 		setup_timer(&s->rx_timer, rx_timer_fn, (unsigned long)s);
 
-		sci_submit_rx(s);
+		if (port->type = PORT_SCIFA || port->type = PORT_SCIFB)
+			sci_submit_rx(s);
 	}
 }
 
@@ -1442,6 +1444,7 @@ static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
 			scr |= SCSCR_RDRQE;
 		} else {
 			scr &= ~SCSCR_RIE;
+			sci_submit_rx(s);
 		}
 		serial_port_out(port, SCSCR, scr);
 		/* Clear current interrupt */
-- 
1.9.1