The only way to _assume_ a printer is online is to attempt a dummy poll for information. Again note that this is a strong assumption as only some new printers return data for a poll, and legacy printers control of the data port are undefined. The poll btw needs to be done in userspace because printers respond differently with different polls. Thus a 'printer driver' interface needs developed. Oddly enough, WinPrinters won't suffer this problem simply because of their bidirectional design :( -d Jesse Pollard wrote: > --------- Received message begins Here --------- > Tim Waugh <[EMAIL PROTECTED]>: > > On Thu, Jan 04, 2001 at 03:39:10PM +0100, Andrea Arcangeli wrote: > > > > > As noted yesterday falling into parport_write will silenty lose data when the > > > printer is off. > > > > (Actually it depends; I think FIFO/DMA paths are fine, but yes, the > > software implementation can lose data.) > > > > > If it's not feasible to make parport_write reliable against > > > power-off printer, then I recommend to loop in interruptible mode > > > before entering the main loop (waiting the printer to power-on) like > > > in latest patch from Peter. > > > > Have I missed a patch? How do you know whether or not the printer is > > on yet? > > > > As I understand it, you can't guarantee anything about any of the > > signals when the printer is off, so all you can do is look for > > 'suspicous' things (like 'no error' and 'paper out'). But some > > printers do this during normal operation, and hence the LP_CAREFUL > > switch. > > > > Return -EIO when the printer is on and off-line is a bug, sure enough. > > That's what the -EAGAIN patch was for, and Peter's patch fixes this > > too. > > > > But if you want to avoid losing data when your printer is off you need > > to use LP_CAREFUL, and hope printing still works at all (depends on > > your printer). > > > > If this goes away: > > > > if ((status & LP_PERRORP) && !(LP_F(minor) & LP_CAREFUL)) > > /* No error. */ > > last = 0; > > > > then some people might not be able to print at all. > > I don't believe this is a problem that CAN be fixe, either by hardware > or software. > > Originally, (wayback machine on) this was handled by a pull-up resistor > in the parallel interface, on the "off-line" signal. ANY time the printer > was powered off, set offline, or cable unplugged, the "off-line" signal > was raised by the pull-up. No data lost. > > Now the parallel interface is bidirectional, and can have multiple devices > attached - this "fix" cannot be used. The interface is now more of a > buss than a single attached interface, and signals from a missing device > (powered off or disconnected) are floating. They may float high or low, > and depending on the environment (and which end of the cable is unplugged) > any thing in between. > > Yes, I've lost printouts this way, but there's nothing I can really do > about it other than than tell the users "don't do that - make sure the > printer is turned on before you print". > > ------------------------------------------------------------------------- > Jesse I Pollard, II > Email: [EMAIL PROTECTED] > > Any opinions expressed are solely my own. > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [EMAIL PROTECTED] > Please read the FAQ at http://www.tux.org/lkml/
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