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    SubjectRe: [patch] Re: Magic Alt-SysRq change in 2.6.18-rc1
    Dmitry Torokhov wrote:
    > Hi,
    > On 7/12/06, Paulo Marques <> wrote:
    >> Ok, I've tested it this time and this new one works as expected. I can
    >> use any of the sequences discussed and I can produce a SysRq every time.
    >> Still, just pressing SysRq or any sequence that doesn't start with
    >> "press Alt -> press SysRq" seems unaffected.
    > I like this, however:
    >> + if (keycode == KEY_SYSRQ) {
    >> + if (down) {
    >> + if(sysrq_alt)
    >> + sysrq_down = down;
    >> + } else {
    >> + sysrq_down = 0;
    > Are you sure? This will set sysrq_down only if ALT has already been
    > pressed. If SysRq does not autorepeat and it is pressed first we won't
    > ever see sysrq_down. Am I missing something?

    This was done on purpose, yes. My first thought was to allow any order,
    but this could cause strange complications like: if I just press
    PrintScreen when is it ok to forward that keypress before I press Alt?
    The Alt afterwards arrives to late to not have messed the user space
    application already.

    Anyway, none of the sequences posted required this, and it is more
    intuitive to press Alt first anyway.

    >> +
    >> + if (sysrq_down && sysrq_alt)
    >> + sysrq_active = 1;
    >> + else if (!sysrq_down && !sysrq_alt)
    >> + sysrq_active = 0;
    >> +
    >> + if (keycode == KEY_SYSRQ && sysrq_active)
    >> + return;
    > What about alt? I think that "if (...) sysrq_active = 1;" statement
    > should go down, below handle_sysrq block.

    It can not go down, or when you press Alt and then SysRq, the first
    SysRq down event will get through without returning.

    alt is handled a little higher in that function outside the #IFDEF. This
    is because emulate_raw should work even if we don't support magic sysrq.

    The logic here is pretty simple: if we press Alt and then SysRq we enter
    sysrq_active mode. We only come out of that mode when we release both
    keys. Releasing any one of them individually is fine.

    Any KEY_SYSRQ repetitions or releases while on this mode are not
    processed any further.

    Oops, I just realised that if I release Alt first and then SysRq, the
    SysRq release will get through because at that point we're already out
    of sysrq_active mode. This should be easy to fix, though. If this is a
    problem, I can send a new patch tomorrow (with some more comments in
    there, too).

    >> +
    >> + if (sysrq_active && down && !rep) {
    >> handle_sysrq(kbd_sysrq_xlate[keycode], regs, tty);
    >> return;
    >> }
    > We also need to check if emulate_raw() needs to be adjusted...

    I looked at that very quickly, to be honest, but couldn't understand it
    entirely. This code:

    1087 if (keycode == KEY_SYSRQ && sysrq_alt) {
    1088 put_queue(vc, 0x54 | up_flag);
    1089 return 0;
    1090 }

    seems to be supposed to cancel the Alt "press" by sending a Alt
    "release" to handle the sequence press alt -> press sysrq without
    affecting the application. However, this is just a guess. I couldn't
    find out what that magic 0x54 meant or why this would be enough wether
    we pressed the left or the right alt...

    Then there are other things that I don't understand there: I don't see
    any code to filter out the keys we press ('T','P', etc.) while using
    SysRq magic if we are in raw mode. emulate_raw will happily call
    put_queue on them before we have a chance to bail out.

    Maybe we should just stop calling emulate_raw while sysrq_active is
    active. This way, after we press Alt + SysRq, every keypress would be
    processed as a magic sysrq and not handled by any other code until we
    release both keys.

    Does this sound reasonable?

    Paulo Marques -
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