Re: [Qemu-devel] PATCH: serial.c

[Tristan Gingold]

On May 4, 2007, at 3:34 PM, Wessel, Jason wrote:

> Tristan,
>
> I do not believe that is actually the problem, and arguably QEMU is
> correct with timing accuracy set aside.  The root of the issue is a  
> more
> of a timing problem because the clear/set can happen atomically in  
> QEMU
> with respect to the cpu clock vs the real hardware which takes  
> multiple
> clock cycles for this to happen.  Even Linux can fall victim to this
> with heavy printk (if you have ever seen the message "too much work  
> for
> interrupt....").
I think this is a different issue: QEMU serial baudrate is almost  
infinite and linux interrupt
handler detects IRQ storm IIRC.

> You should instead consider fixing the BSP i8250Sio.c, because it  
> clears
> reads the iir and jumps out of the main serial loop if there is if it
> has done enough work, vs checking if it has done enough work and  
> jumping
> out of the loop before reading the iir.
I don't agree because IIR is not cleared when read.
Yes the handler first reads IIR and then jump out of the loop if it  
has done enough work but
the interrupt is not cleared (because the xmit register is still  
empty) and cpu should enter into the interrupt
function as soon as the interrupts are still enabled.

IMHO changing the code (ie inverting the tests) is a (valuable) work- 
around.

Tristan.