4.4-stable review patch. If anyone has any objections, please let me know.
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From: Phil Elwell phil@raspberrypi.org
[ Upstream commit 8344498721059754e09d30fe255a12dab8fb03ef ]
The SC16IS752 is a dual-channel device. The two channels are largely independent, but the IRQ signals are wired together as an open-drain, active low signal which will be driven low while either of the channels requires attention, which can be for significant periods of time until operations complete and the interrupt can be acknowledged. In that respect it is should be treated as a true level-sensitive IRQ.
The kernel, however, needs to be able to exit interrupt context in order to use I2C or SPI to access the device registers (which may involve sleeping). Therefore the interrupt needs to be masked out or paused in some way.
The usual way to manage sleeping from within an interrupt handler is to use a threaded interrupt handler - a regular interrupt routine does the minimum amount of work needed to triage the interrupt before waking the interrupt service thread. If the threaded IRQ is marked as IRQF_ONESHOT the kernel will automatically mask out the interrupt until the thread runs to completion. The sc16is7xx driver used to use a threaded IRQ, but a patch switched to using a kthread_worker in order to set realtime priorities on the handler thread and for other optimisations. The end result is non-threaded IRQ that schedules some work then returns IRQ_HANDLED, making the kernel think that all IRQ processing has completed.
The work-around to prevent a constant stream of interrupts is to mark the interrupt as edge-sensitive rather than level-sensitive, but interpreting an active-low source as a falling-edge source requires care to prevent a total cessation of interrupts. Whereas an edge-triggering source will generate a new edge for every interrupt condition a level-triggering source will keep the signal at the interrupting level until it no longer requires attention; in other words, the host won't see another edge until all interrupt conditions are cleared. It is therefore vital that the interrupt handler does not exit with an outstanding interrupt condition, otherwise the kernel will not receive another interrupt unless some other operation causes the interrupt state on the device to be cleared.
The existing sc16is7xx driver has a very simple interrupt "thread" (kthread_work job) that processes interrupts on each channel in turn until there are no more. If both channels are active and the first channel starts interrupting while the handler for the second channel is running then it will not be detected and an IRQ stall ensues. This could be handled easily if there was a shared IRQ status register, or a convenient way to determine if the IRQ had been deasserted for any length of time, but both appear to be lacking.
Avoid this problem (or at least make it much less likely to happen) by reducing the granularity of per-channel interrupt processing to one condition per iteration, only exiting the overall loop when both channels are no longer interrupting.
Signed-off-by: Phil Elwell phil@raspberrypi.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/sc16is7xx.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-)
--- a/drivers/tty/serial/sc16is7xx.c +++ b/drivers/tty/serial/sc16is7xx.c @@ -648,7 +648,7 @@ static void sc16is7xx_handle_tx(struct u uart_write_wakeup(port); }
-static void sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno) +static bool sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno) { struct uart_port *port = &s->p[portno].port;
@@ -657,7 +657,7 @@ static void sc16is7xx_port_irq(struct sc
iir = sc16is7xx_port_read(port, SC16IS7XX_IIR_REG); if (iir & SC16IS7XX_IIR_NO_INT_BIT) - break; + return false;
iir &= SC16IS7XX_IIR_ID_MASK;
@@ -685,16 +685,23 @@ static void sc16is7xx_port_irq(struct sc port->line, iir); break; } - } while (1); + } while (0); + return true; }
static void sc16is7xx_ist(struct kthread_work *ws) { struct sc16is7xx_port *s = to_sc16is7xx_port(ws, irq_work); - int i;
- for (i = 0; i < s->devtype->nr_uart; ++i) - sc16is7xx_port_irq(s, i); + while (1) { + bool keep_polling = false; + int i; + + for (i = 0; i < s->devtype->nr_uart; ++i) + keep_polling |= sc16is7xx_port_irq(s, i); + if (!keep_polling) + break; + } }
static irqreturn_t sc16is7xx_irq(int irq, void *dev_id)