Commit d04213af authored Oct 25, 2019 by Daniel Thompson

kdb: Simplify code to fetch characters from console



Currently kdb_read_get_key() contains complex control flow that, on
close inspection, turns out to be unnecessary. In particular:

1. It is impossible to enter the branch conditioned on (escape_delay == 1)
   except when the loop enters with (escape_delay == 2) allowing us to
   combine the branches.

2. Most of the code conditioned on (escape_delay == 2) simply modifies
   local data and then breaks out of the loop causing the function to
   return escape_data[0].

3. Based on #2 there is not actually any need to ever explicitly set
   escape_delay to 2 because we it is much simpler to directly return
   escape_data[0] instead.

4. escape_data[0] is, for all but one exit path, known to be '\e'.

Simplify the code based on these observations.

There is a subtle (and harmless) change of behaviour resulting from this
simplification: instead of letting the escape timeout after ~1998
milliseconds we now timeout after ~2000 milliseconds

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191025073328.643-3-daniel.thompson@linaro.org

parent 53b63136

kernel/debug/kdb/kdb_io.c

+14 −24

Original line number	Diff line number	Diff line
		@@ -124,25 +124,18 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
		touch_nmi_watchdog();
		f = &kdb_poll_funcs[0];
		}
		if (escape_delay == 2) {
		*ped = '\0';
		ped = escape_data;
		--escape_delay;
		}
		if (escape_delay == 1) {
		key = *ped++;
		if (!*ped)
		--escape_delay;
		break;
		}

		key = (*f)();

		if (key == -1) {
		if (escape_delay) {
		udelay(ESCAPE_UDELAY);
		--escape_delay;
		if (--escape_delay == 0)
		return '\e';
		}
		continue;
		}

		if (bufsize <= 2) {
		if (key == '\r')
		key = '\n';
		@@ -150,27 +143,24 @@ static int kdb_read_get_key(char *buffer, size_t bufsize)
		*buffer = '\0';
		return -1;
		}

		if (escape_delay == 0 && key == '\e') {
		escape_delay = ESCAPE_DELAY;
		ped = escape_data;
		f_escape = f;
		}
		if (escape_delay) {
		*ped++ = key;
		if (f_escape != f) {
		escape_delay = 2;
		continue;
		}
		if (f_escape != f)
		return '\e';

		*ped++ = key;
		key = kdb_handle_escape(escape_data, ped - escape_data);
		if (key > 0) {
		escape_data[0] = key;
		escape_data[1] = '\0';
		}
		if (key)
		escape_delay = 2;
		if (key < 0)
		return '\e';
		if (key == 0)
		continue;
		}

		break; /* A key to process */
		}
		return key;

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