* David Brownell <david-b@xxxxxxxxxxx> wrote: > On Monday 20 April 2009, Ingo Molnar wrote: > > > The proper approach would be to use one of the > > async_synchronize*() facilities in kernel/async.c to properly > > order the opening of the console with device init. > > Stepping back a moment from "how" to make sure "what" is agreed > on. > > I think I see three scenarios here: > > - Classic PC or server, where there's a meaningful console; > > - Deeply embedded systems, where there isn't; > > - Development stages of "deeply embedded", where there *may* be > one. > > If that's correct, then async_synchronize() isn't a full answer... > > I think a fair number of cases can be papered over with a serial > console with no hardware flow control, which isn't hooked up to > anything. Maybe the board needs a test/development jig to get > one; without it, the "console" bits spill out into the aether. > Linux can dump bits to ttyS0, which will act like /dev/null. > > But the problem case here seems to be one where such un-hooked-up > serial ports are not realistic options. Which is why the > regression in USB console functionality has been troublesome. > > Is that correct? Not sure i understand the complication. The practical issue addressed by this particular patch i've replied to is that if /dev/console is sys_open()-ed by an app "too early", an error code is returned and subsequently user-space (and all its child tasks) remain console-less. Whether the console has flow control or not is immaterial. Initialization sequence ordering details should be transparent as far as sys_open() of a console is concerned. If there is _no console_, then obviously there's nothing to wait for and the async wait will succeed immediately. The console driver still returns an error on sys_open() but that is expected behavior. The solution proposed in the patch i'm replying to is a boot flag that causes the kernel to wait N milliseconds. This is incorrect and wrong on several levels. If the console can be initialized via a proper handshake that should be exposed precisely - we should not wait neither longer nor shorter than needed. _If_ there's no proper handshake and a magic delay is needed (because there's some hardware-internal delay needed at init time) before the console can be used then that should be in the console driver itself and the async-wait will wait for that automatically. But never should we have delays like this in generic code. Regardless of embedded versus server versus desktop issues. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
