Messages in this thread | | | Date | Fri, 6 Jun 2008 21:23:38 +0200 | From | "Leon Woestenberg" <> | Subject | Re: Locking in the (now generic) GPIO infrastructure? |
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Hello David, all,
On Fri, Jun 6, 2008 at 2:53 PM, David Brownell <david-b@pacbell.net> wrote: > On Wednesday 04 June 2008, Leon Woestenberg wrote: >> include/asm-arm/arch-ixp4xx/platform.h: >> static inline void gpio_line_set(u8 line, int value) >> { >> if (value == IXP4XX_GPIO_HIGH) >> *IXP4XX_GPIO_GPOUTR |= (1 << line); >> else if (value == IXP4XX_GPIO_LOW) >> *IXP4XX_GPIO_GPOUTR &= ~(1 << line); >> } >> >> Under a Linux kernel where multiple drivers are accessing GPIO, the >> latter does not seem safe against preemption (assuming the memory >> read-modify-write is not atomic). >> >> Shouldn't GPIO access be protected against concurrent access here? > > Well, against an IRQ in the middle of those read/modify/write > sequences hidden by the "|=" and "&=" syntax. Last I knew, > no XScale CPUs support on-chip SMP. > Indeed, however, I used a kernel with -rt patch (and using PREEMPT RT) as mentioned in my original e-mail. For completeness I should have stated this:
The interrupt handlers become kernel threads. As such they become preemptable (to reduce latencies for any higher priority threads, such as those from other interrupts or even RT user tasks).
>> Documentation/gpio.txt does not really mention the locking mechanism >> assumed to modify GPIO lines. > > That function isn't part of the GPIO interface, despite > its gpio_* prefix, so that's not relevant. > > It resembles gpio_set_value() though. That can use at > In fact, on the IXP4xx, gpio_set_value() is just gpio_line_set(), so I think it is valid to understand where the locking should occur (lowest level, higher level?)
> most spinlocks to establish its atomicity guarantee; it's > described as "spinlock-safe", and in distinction to the > gpio_set_value_cansleep() call which could use a mutex or > other sleeping synch primitive. >
So, the solution (for the upstream work on -rt) would be to add spinlock protection to gpio_line_set(), mutex protection for _cansleep() variants?
Regards, -- Leon
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