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117 lines
4.6 KiB
Diff
117 lines
4.6 KiB
Diff
From dbca2148f4c21454da461f1208b05d3a5887d133 Mon Sep 17 00:00:00 2001
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From: Phil Elwell <phil@raspberrypi.org>
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Date: Tue, 20 Feb 2018 11:48:15 +0000
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Subject: [PATCH 246/277] sc16is7xx: Fix for multi-channel stall
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The SC16IS752 is a dual-channel device. The two channels are largely
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independent, but the IRQ signals are wired together as an open-drain,
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active low signal which will be driven low while either of the
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channels requires attention, which can be for significant periods of
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time until operations complete and the interrupt can be acknowledged.
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In that respect it is should be treated as a true level-sensitive IRQ.
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The kernel, however, needs to be able to exit interrupt context in
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order to use I2C or SPI to access the device registers (which may
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involve sleeping). Therefore the interrupt needs to be masked out or
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paused in some way.
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The usual way to manage sleeping from within an interrupt handler
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is to use a threaded interrupt handler - a regular interrupt routine
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does the minimum amount of work needed to triage the interrupt before
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waking the interrupt service thread. If the threaded IRQ is marked as
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IRQF_ONESHOT the kernel will automatically mask out the interrupt
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until the thread runs to completion. The sc16is7xx driver used to
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use a threaded IRQ, but a patch switched to using a kthread_worker
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in order to set realtime priorities on the handler thread and for
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other optimisations. The end result is non-threaded IRQ that
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schedules some work then returns IRQ_HANDLED, making the kernel
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think that all IRQ processing has completed.
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The work-around to prevent a constant stream of interrupts is to
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mark the interrupt as edge-sensitive rather than level-sensitive,
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but interpreting an active-low source as a falling-edge source
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requires care to prevent a total cessation of interrupts. Whereas
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an edge-triggering source will generate a new edge for every interrupt
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condition a level-triggering source will keep the signal at the
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interrupting level until it no longer requires attention; in other
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words, the host won't see another edge until all interrupt conditions
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are cleared. It is therefore vital that the interrupt handler does not
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exit with an outstanding interrupt condition, otherwise the kernel
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will not receive another interrupt unless some other operation causes
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the interrupt state on the device to be cleared.
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The existing sc16is7xx driver has a very simple interrupt "thread"
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(kthread_work job) that processes interrups on each channel in turn
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until there are no more. If both channels are active and the first
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channel starts interrupting while the handler for the second channel
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is running then it will not be detected and an IRQ stall ensues. This
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could be handled easily if there was a shared IRQ status register, or
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a convenient way to determine if the IRQ had been deasserted for any
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length of time, but both appear to be lacking.
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Avoid this problem (or at least make it much less likely to happen)
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by reducing the granularity of per-channel interrupt processing
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to one condition per iteration, only exiting the overall loop when
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both channels are no longer interrupting.
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Signed-off-by: Phil Elwell <phil@raspberrypi.org>
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---
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drivers/tty/serial/sc16is7xx.c | 22 ++++++++++++++++------
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1 file changed, 16 insertions(+), 6 deletions(-)
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diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
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index ca54ce074a5f..e8b928877e66 100644
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--- a/drivers/tty/serial/sc16is7xx.c
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+++ b/drivers/tty/serial/sc16is7xx.c
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@@ -662,7 +662,7 @@ static void sc16is7xx_handle_tx(struct uart_port *port)
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uart_write_wakeup(port);
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}
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-static void sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
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+static bool sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
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{
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struct uart_port *port = &s->p[portno].port;
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@@ -671,7 +671,7 @@ static void sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
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iir = sc16is7xx_port_read(port, SC16IS7XX_IIR_REG);
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if (iir & SC16IS7XX_IIR_NO_INT_BIT)
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- break;
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+ return false;
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iir &= SC16IS7XX_IIR_ID_MASK;
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@@ -693,16 +693,26 @@ static void sc16is7xx_port_irq(struct sc16is7xx_port *s, int portno)
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port->line, iir);
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break;
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}
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- } while (1);
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+ } while (0);
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+ return true;
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}
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static void sc16is7xx_ist(struct kthread_work *ws)
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{
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struct sc16is7xx_port *s = to_sc16is7xx_port(ws, irq_work);
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- int i;
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- for (i = 0; i < s->devtype->nr_uart; ++i)
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- sc16is7xx_port_irq(s, i);
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+ while (1)
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+ {
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+ bool keep_polling = false;
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+ int i;
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+
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+ for (i = 0; i < s->devtype->nr_uart; ++i)
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+ {
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+ keep_polling |= sc16is7xx_port_irq(s, i);
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+ }
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+ if (!keep_polling)
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+ break;
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+ }
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}
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static irqreturn_t sc16is7xx_irq(int irq, void *dev_id)
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--
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2.16.1
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