Re: [ECOS] Blocking design

[Jonathan Larmour <jifl at eCosCentric dot com>]

Gary Thomas wrote:
> On Mon, 2004-02-09 at 15:37, Matt Jerdonek wrote:
> > I'd like to have my device driver block while waiting
> > for the hardware to complete its task.  My basic
> > design is this:
> >
> > void invoke_hardware(void)
> > {
> >     cyg_drv_mutex_lock(&mutex);
> >     done = false;
> >     start_hardware();
> >     while (!done)
> >        cyg_drv_cond_wait(&cond);
> >     cyg_drv_mutex_unlock(&mutex);
> > }
> >
> > void hardware_complete_dsr(void)
> > {
> >      done = true;
> >      cyg_drv_cond_signal(&cond);
> > }
> >
> > Here's my concern ... The code appears to work fine,
> > but I think there is a window (albeit small) between
> > the start_hardware() and the cyg_drv_cond_wait() where
> > the hardware could complete, trigger the DSR, and
> > signal the condition variable before the thread has
> > done the cyg_drv_cond_wait(). [snip]
>
> Yes, but your concerns are a bit unfounded.  Use "cyg_drv_dsr_lock()"
> and "cyg_drv_dsr_unlock()" [which are just calls to lock/unlock the
> scheduler, but it's best to follow the same API].  When your thread
> blocks by calling 'cyg_drv_cond_wait()', the scheduler will be unlocked
> as necessary.

An alternative is to use a semaphore. Although that's not in the driver API 
(I can't remember why not, as this is an example of when it's useful) so 
you'd have to use the kernel API.

Jifl
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