From 5aa046245391af3dcc606f21ea64e6e866f79e21 Mon Sep 17 00:00:00 2001
From: Ingvar Stepanyan <me@rreverser.com>
Date: Tue, 25 Oct 2022 23:26:12 +0100
Subject: [PATCH 1/2] Move serial cache from Canon to port library

---
 camlibs/canon/serial.c        | 24 +++---------------------
 libgphoto2_port/serial/unix.c | 24 ++++++++++++++++++++----
 2 files changed, 23 insertions(+), 25 deletions(-)

diff --git a/camlibs/canon/serial.c b/camlibs/canon/serial.c
index 7d6b0f0fcd..24151d064c 100644
--- a/camlibs/canon/serial.c
+++ b/camlibs/canon/serial.c
@@ -218,28 +218,10 @@ serial_set_timeout (GPPort *gdev, int to)
 static int
 canon_serial_get_byte (GPPort *gdev)
 {
-	static unsigned char cache[512];
-	static unsigned char *cachep = cache;
-	static unsigned char *cachee = cache;
-	int recv;
-
-	/* if still data in cache, get it */
-	if (cachep < cachee) {
-		return (int) *cachep++;
-	}
-
-	recv = gp_port_read (gdev, (char *)cache, 1);
-	if (recv < 0)		/* An error occurred */
+	char byte;
+	if (gp_port_read (gdev, (char *)&byte, 1) < 0)
 		return -1;
-
-	cachep = cache;
-	cachee = cache + recv;
-
-	if (recv) {
-		return (int) *cachep++;
-	}
-
-	return -1;
+	return byte;
 }
 
 /* ------------------------- Frame-level processing ------------------------- */
diff --git a/libgphoto2_port/serial/unix.c b/libgphoto2_port/serial/unix.c
index d751ed4d2a..5e033d21c0 100644
--- a/libgphoto2_port/serial/unix.c
+++ b/libgphoto2_port/serial/unix.c
@@ -184,6 +184,11 @@
 struct _GPPortPrivateLibrary {
 	int fd;       /* Device handle */
 	int baudrate; /* Current speed */
+
+	/* Internal cache for faster reading */
+	char cache[512];
+	const char *cachep;
+	const char *cachee;
 };
 
 static int gp_port_serial_check_speed (GPPort *dev);
@@ -560,10 +565,21 @@ gp_port_serial_read (GPPort *dev, char *bytes, int size)
 			/* FALLTHROUGH */
 		    }
 		} else {
-		    /* Just read the bytes */
-		    now = read (dev->pl->fd, bytes, size - readen);
-		    if (now < 0)
-			    return GP_ERROR_IO_READ;
+			if (dev->pl->cachep == dev->pl->cachee) {
+				/* Cache is empty, fill it up. */
+				now = read (dev->pl->fd, dev->pl->cache, sizeof(dev->pl->cache));
+				if (now < 0) {
+					/* nothing more to read from the actual port */
+					return GP_ERROR_IO_READ;
+				}
+				/* Reset cache pointers */
+				dev->pl->cachep = dev->pl->cache;
+				dev->pl->cachee = dev->pl->cache + now;
+			}
+			/* read up to the required chunk size from cache */
+			now = MIN(size - readen, dev->pl->cachee - dev->pl->cachep);
+			memcpy(bytes, dev->pl->cachep, now);
+			dev->pl->cachep += now;
 		}
 		bytes += now;
 		readen += now;

From f895c6f398ca0f0b67ca3cdc037c649dae1c5510 Mon Sep 17 00:00:00 2001
From: Ingvar Stepanyan <me@rreverser.com>
Date: Sun, 30 Oct 2022 17:54:33 +0000
Subject: [PATCH 2/2] Add fast branch that skips serial cache entirely

---
 libgphoto2_port/serial/unix.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/libgphoto2_port/serial/unix.c b/libgphoto2_port/serial/unix.c
index 5e033d21c0..3fa5233012 100644
--- a/libgphoto2_port/serial/unix.c
+++ b/libgphoto2_port/serial/unix.c
@@ -517,7 +517,7 @@ gp_port_serial_read (GPPort *dev, char *bytes, int size)
         FD_ZERO (&readfs);
         FD_SET (dev->pl->fd, &readfs);
 
-        while (readen < size) {
+        while (size > 0) {
 
 		/* Set timeout value within input loop */
                 timeout.tv_usec = (dev->timeout % 1000) * 1000;
@@ -564,25 +564,30 @@ gp_port_serial_read (GPPort *dev, char *bytes, int size)
 			/* Ok, we read 1 byte and it is 0xff */
 			/* FALLTHROUGH */
 		    }
+		} else if (dev->pl->cachep == dev->pl->cachee && size >= sizeof(dev->pl->cache)) {
+			/* We're trying to read a chunk larger than the cache and the cache is empty.
+			   In this case, skip the cache entirely and read as much as we can directly into the destination. */
+			now = read (dev->pl->fd, bytes, size);
+			if (now < 0)
+				return GP_ERROR_IO_READ;
 		} else {
 			if (dev->pl->cachep == dev->pl->cachee) {
-				/* Cache is empty, fill it up. */
+				/* We're reading only a few bytes and the cache is empty; fill it up. */
 				now = read (dev->pl->fd, dev->pl->cache, sizeof(dev->pl->cache));
-				if (now < 0) {
-					/* nothing more to read from the actual port */
+				if (now < 0)
 					return GP_ERROR_IO_READ;
-				}
 				/* Reset cache pointers */
 				dev->pl->cachep = dev->pl->cache;
 				dev->pl->cachee = dev->pl->cache + now;
 			}
 			/* read up to the required chunk size from cache */
-			now = MIN(size - readen, dev->pl->cachee - dev->pl->cachep);
+			now = MIN(size, dev->pl->cachee - dev->pl->cachep);
 			memcpy(bytes, dev->pl->cachep, now);
 			dev->pl->cachep += now;
 		}
 		bytes += now;
 		readen += now;
+		size -= readen;
         }
 
         return readen;