[microblaze-uclinux] Re: [uClinux-dev] Regarding the xmbserial driver

[John Williams <jwilliams@xxxxxxxxxxxxxx>]

Hi Anand,

anand_12 wrote:

> I am running uclinux on a spartan 2E microblaze board.
> Sometime back I had posted to the groups regarding a problem
> I was facing.The microblaze-uclinux system used to crash(atleast apparently) if the user typed commands or text at a very fast rate at the shell prompt over the serial console(hyperterm in my case). 

This is a known bug in the UARTlite driver (xmbserial.c)

> As suspected,the slow response of the serial driver was because of some buffering issue.The interrupt service routine invokes a "receive_chars" function in its handler..
> that function basically receives every character and puts it into a buffer(if i got it right)..called a "flip buffer"..which as shown below
> (uclinux-dist/linux-2.4.x/include/linux/tty.h)
> 
> has a size # defined to 512 bytes...i changed this to 1024 bytes(also shown below)..and now the problem is solved..I am able to type and issue commands at a very fast rate without the system crashing...
> 
> the problem seems to be solved but i do need to look at the implications of having increased the size of this buffer..but as it is..there is big difference now when i issue commands or just type some text and a very fast rate..it does not crash at all!!...

This is probably not an acceptable fix, because that header file is 
global to all uClinux targets.  There's nothing stopping you or anyone 
else applying this fix to their local source tree, but I wouldn't try to 
push it into the CVS.

Much better would be for us to identify the source of the bug in 
xmbserial (we've got some good ideas) and fix it there.

The problem seems to be related to a code path in the interrupt handler 
of xmbserial.c - from memory if the flip_buffer is full, the handler 
exits without stripping the incoming chars in the hardware buffer.  This 
causes an endless interrupt loop because the interrupt is still 
asserted, but there's no room in the filp_buf.  Or something like that.

Unfortunately this issue is intertwined with a general unresolved 
question about the behaviour of the interrupt controller regarding 
level-sensitive interrupts.  There was much discussion about this on the 
microblaze list a while back, a search of the archives should track it down.

Cheers,

John
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