Re: UART_IIR_BUSY set for 16550A

[Prasad Koya <prasad.koya@gmail.com>]

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@mit.edu> 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