[PATCH v3 2/5] kdb: Simplify code to fetch characters from console

From: Daniel Thompson
Date: Mon Oct 14 2019 - 11:46:38 EST


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@xxxxxxxxxx>
Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---
kernel/debug/kdb/kdb_io.c | 38 ++++++++++++++------------------------
1 file changed, 14 insertions(+), 24 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 68e2c29f14f5..78cb6e339408 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -122,25 +122,18 @@ 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';
@@ -148,28 +141,25 @@ 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_read_handle_escape(escape_data,
ped - escape_data);
- if (key > 0) {
- escape_data[0] = key;
- escape_data[1] = '\0';
- }
- if (key)
- escape_delay = 2;
- continue;
+ if (key < 0)
+ return '\e';
+ if (key == 0)
+ continue;
}
+
break; /* A key to process */
}
return key;
--
2.21.0