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

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Date:   Mon, 14 Oct 2019 16:46:23 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        patches@...aro.org
Subject: [PATCH v3 2/5] 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@...aro.org>
Reviewed-by: Douglas Anderson <dianders@...omium.org>
---
 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