Thanks for looking into this. With 16550A, I'm seeing this weird issue with 3.4 kernel. At random times 8250 driver reads 0xcc out of IIR. I'm not sure why bit 2 is set. #define UART_IIR_BUSY 0x07 /* DesignWare APB Busy Detect */ Soon after this I'm running into "serial8250: too much work for irq4". And this is printed after iterating 512 times in 8250_interrupt handler. This message is printed one more time right after this and it appears that console does not work after those messages. I was suspicious about that 'busy detect' bit. Am trying to reproduce this and see what is in LCR when this hits. Can I (or how do I) reset the device if I see this bit set? On Sat, May 24, 2014 at 5:36 PM, Theodore Ts'o <tytso@xxxxxxx> wrote: > On Fri, May 23, 2014 at 10:08:49AM -0700, Prasad Koya wrote: >> >> I don't see anyone in kernel using UART_IIR_BUSY bit except Designware >> serial driver. We are using 8250 driver for our 16550A and >> occasionally we see UART_IIR_BUSY set and soon after that console is >> hosed. In what situations is this bit set? I don't see much >> documentation for this. > > UART_IIR_BUSY is not a bit, it's a magic bit pattern, which is I > believe a Designware-specific hack. As far as standard > 8250-compatible UART's are concerned, if the low bit (bit 0) is set in > the IIR register, there are no interrupts pending, and so you > shouldn't need to check the 0x06 bits (i.e., bits 1 and 2). > > - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
