Linux Kernel: udelay() returns too early?
delayed-execution, driver, kernel, linux
Solution
I don't know of any bug in that version of the kernel (but that doesn't mean that there isn't one).
`udelay()` isn't "uninterruptible" - it does not disable preemption, so your task can be preempted by a RT task during the delay. However the same is true of your alternate delay implementation, so that is unlikely to be the problem.
Could your actual problem be a DMA coherency / memory ordering issue? Your alternate delay implementation accesses the bus, so this might be hiding the real problem as a side-effect.
Problem
I have a driver which requires microsecond delays. To create this delay, my driver is using the kernel's udelay function. Specifically, there is one call to udelay(90): ``` iowrite32(data, addr + DATA_OFFSET); iowrite32(trig, addr + CONTROL_OFFSET); udelay(30); trig |= 1; iowrite32(trig, addr + CONTROL_OFFSET); udelay(90); // This is the problematic call ``` We had reliability issues with the device. After a lot of debugging, we traced the problem to the driver resuming before 90us has passed. (See "proof" below.) I am running kernel version 2.6.38-11-generic SMP (Kubuntu 11.04, x86_64) on an Intel Pentium Dual Core (E5700). As far as I know, the documentation states that udelay will delay execution for at least the specified delay, and is uninterruptible. Is there a bug is this version of the kernel, or did I misunderstand something about the use of udelay? To convince ourselves that the problem was caused by udelay returning too early, we fed a 100kHz clock to one of the I/O ports and implemented our own delay as follows: ``` // Wait until n number of falling edges // are observed void clk100_delay(void *addr, u32 n) { int i; for (i = 0; i < n; i++) { u32 prev_clk = ioread32(addr); while (1) { u32 clk = ioread32(addr); if (prev_clk && !clk) { break; } else { prev_clk = clk; } } } } ``` ...and the driver now works flawlessly. As a final note, I found a discussion indicating that frequency scaling could be causing the *delay() family of functions to misbehave, but this was on a ARM mailing list - I assuming such problems would be non-existent on a Linux x86 based PC.