When submitting a DMA request fails, the driver is supposed to fall back
to PIO. However, this never really worked due to various reasons
(sh-sci driver issues and dmaengine framework limitations).
There are three places where DMA submission can fail, and the driver
should fall back to PIO:
1. sci_dma_rx_complete(),
2. sci_submit_rx(),
3. work_fn_tx().
This RFC fixes fallback to PIO in the receive path (cases 1 and 2).
Fallback to PIO in the transmit path (case 3) already works fine.
Case 1: sci_dma_rx_complete()
A. PIO cannot take over on SCIF if any DMA transactions are pending,
hence they must be terminated first.
B. The active cookies must be invalidated, else rx_timer_fn() may
trigger a NULL pointer dereference.
C. Restarting the port is not needed, as it is already running, but
serial port interrupts must be directed back from the DMA engine to
the CPU.
Case 2: sci_submit_rx()
D. Some callers of sci_submit_rx() hold the port spinlock, others
don't. During fallback to PIO, the driver needs to obtain the port
spinlock. If the lock was already held, spinlock recursion is
detected, causing a deadlock: BUG: spinlock recursion on CPU#0.
Fix this by adding a flag parameter to sci_submit_rx() for the
caller to indicate the port spinlock is already held, so spinlock
recursion can be avoided.
Move the spin_lock_irqsave() up, so all DMA disable steps are
protected, which is safe as the recently introduced
dmaengine_terminate_async() can be called in atomic context.
E. When falling back to PIO, active_rx must be set to a different
value than cookie_rx[i], else sci_dma_rx_find_active() will
incorrectly find a match, leading to a NULL pointer dereference in
rx_timer_fn() later.
F. On (H)SCIF, sci_submit_rx() is called in the receive interrupt
handler. Hence if DMA submission fails, the interrupt handler
should handle reception using PIO.
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
---
Against tty/next + "serial: sh-sci: Fix receive on SCIFA/SCIFB variants
with DMA".
All testing was done on r8a7791/koelsch using SCIF1 on debug serial 1,
and SCIFA3 on EXIO-B, by introducing random failures in DMA submission
code.
Thanks for your comments!
v2:
- Fix fallback in sci_dma_rx_complete(),
- Fallback in the transmit path already works fine,
- Widen audience, but keep RFC.
---
drivers/tty/serial/sh-sci.c | 34 +++++++++++++++++++++++++---------
1 file changed, 25 insertions(+), 9 deletions(-)
diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 3aad48e64b9b71ff..30b2728c20d6e982 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1272,6 +1272,7 @@ static void sci_dma_rx_complete(void *arg)
struct dma_async_tx_descriptor *desc;
unsigned long flags;
int active, count = 0;
+ unsigned int i;
dev_dbg(port->dev, "%s(%d) active cookie %d\n", __func__, port->line,
s->active_rx);
@@ -1309,12 +1310,22 @@ static void sci_dma_rx_complete(void *arg)
return;
fail:
+ dmaengine_terminate_async(chan);
spin_unlock_irqrestore(&port->lock, flags);
dev_warn(port->dev, "Failed submitting Rx DMA descriptor\n");
/* Switch to PIO */
spin_lock_irqsave(&port->lock, flags);
+ for (i = 0; i < 2; i++)
+ s->cookie_rx[i] = -EINVAL;
+ s->active_rx = 0;
s->chan_rx = NULL;
- sci_start_rx(port);
+ /* Direct new serial port interrupts back to CPU */
+ scr = serial_port_in(port, SCSCR);
+ if (port->type == PORT_SCIFA || port->type == PORT_SCIFB) {
+ scr &= ~SCSCR_RDRQE;
+ enable_irq(s->irqs[SCIx_RXI_IRQ]);
+ }
+ serial_port_out(port, SCSCR, scr | SCSCR_RIE);
spin_unlock_irqrestore(&port->lock, flags);
}
@@ -1331,7 +1342,7 @@ static void sci_tx_dma_release(struct sci_port *s)
dma_release_channel(chan);
}
-static void sci_submit_rx(struct sci_port *s)
+static int sci_submit_rx(struct sci_port *s, bool port_lock_held)
{
struct dma_chan *chan = s->chan_rx;
struct uart_port *port = &s->port;
@@ -1359,19 +1370,22 @@ static void sci_submit_rx(struct sci_port *s)
s->active_rx = s->cookie_rx[0];
dma_async_issue_pending(chan);
- return;
+ return 0;
fail:
+ if (!port_lock_held)
+ spin_lock_irqsave(&port->lock, flags);
if (i)
dmaengine_terminate_async(chan);
for (i = 0; i < 2; i++)
s->cookie_rx[i] = -EINVAL;
- s->active_rx = -EINVAL;
+ s->active_rx = 0;
/* Switch to PIO */
- spin_lock_irqsave(&port->lock, flags);
s->chan_rx = NULL;
sci_start_rx(port);
- spin_unlock_irqrestore(&port->lock, flags);
+ if (!port_lock_held)
+ spin_unlock_irqrestore(&port->lock, flags);
+ return -1;
}
static void work_fn_tx(struct work_struct *work)
@@ -1491,7 +1505,7 @@ static enum hrtimer_restart rx_timer_fn(struct hrtimer *t)
}
if (port->type == PORT_SCIFA || port->type == PORT_SCIFB)
- sci_submit_rx(s);
+ sci_submit_rx(s, true);
/* Direct new serial port interrupts back to CPU */
scr = serial_port_in(port, SCSCR);
@@ -1617,7 +1631,7 @@ static void sci_request_dma(struct uart_port *port)
s->chan_rx_saved = s->chan_rx = chan;
if (port->type == PORT_SCIFA || port->type == PORT_SCIFB)
- sci_submit_rx(s);
+ sci_submit_rx(s, false);
}
}
@@ -1667,7 +1681,8 @@ static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
scr |= SCSCR_RDRQE;
} else {
scr &= ~SCSCR_RIE;
- sci_submit_rx(s);
+ if (sci_submit_rx(s, false))
+ goto handle_pio;
}
serial_port_out(port, SCSCR, scr);
/* Clear current interrupt */
@@ -1681,6 +1696,7 @@ static irqreturn_t sci_rx_interrupt(int irq, void *ptr)
}
#endif
+handle_pio:
if (s->rx_trigger > 1 && s->rx_fifo_timeout > 0) {
if (!scif_rtrg_enabled(port))
scif_set_rtrg(port, s->rx_trigger);
--
2.17.1
