Re: console spin_lock

[Andrew Morton <andrewm@uow.edu.au>]

James Simmons wrote:
> 
> Some time ago a intel i810 framebuffer driver was written. It only worked
> for 2.2.X. With 2.4.X a spinlock is used in the upper layers of the
> console system. Sooner or later we are going to run into the situtation
> where we will have graphics hardware which has no vga core and wih be
> purely DMA/irq based (i.e i810). In this case using the current
> console_lock will block the driver itself. I have thought about a
> possible solution. A semaphore can't be used since their is a spin_lock
> in the console_softirq. Since this is in a interrupt context a
> semaphore can't be used. Another idea was to do a
> 
> void get_vc_lock(void)
> {
>         while (test_and_set_bit(0, &vc_var))
>                 ;
> }
> 
> Any better ideas?
> 

heh.

I'm actually planning on grabbing console_lock and thoroughly strangling
it
next week.  It can block interrupts for up to a second. That just isn't
civil.

- Use a semaphore for serialisation.
- For printk in interrupt context, grab the
  semaphore (yes, you can do this).
- If it couldn't be acquired from interrupt context,
  buffer the text in the log buffer and return.  The text will be
  printed by whoever holds the semaphore before they
  drop it.
- Special "system booting" mode which bypasses all this
  stuff.
- Special "oops in progress" mode which just
  punches through everything.
- Get rid of the special printk buffer - share the
  log buffer.  (Implies writes to console
  devices will be broken into two writes when they
  wrap around).
- Teach vsprintf to print into a circular buffer
  (snprintf thus comes for free).
- Get rid of all the printk deadlock opportunities (fourth
  attempt).
- Get rid of console_tasklet.  Do it in process context callback
  or just do it synchronously.

Assumption:
- Once the system is up and running, it's always safe to
  call down() when in_interrupt() returns false - probably
  not the case in parts of the exit path - tough.


Anyway, that's the thoughtware.  Sound sane?

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