ql: optimize USB data flowfixes

Write column data into a buffer, and only write
to USB once the buffer is filled past threshould.

This reduces the amount of data frames on the USB bus,
in a 429x336 sample image it was reduced from 1000 frames
to about 100 frames, where the image data is only 20 frames,
while the rest is mainly polling for status reponses.

1 files changed, 29 insertions, 3 deletions

diff --git a/src/ql.c b/src/ql.c
index 1023dbe..31bd24b 100644
--- a/src/ql.c
+++ b/src/ql.c
@@ -13,21 +13,29 @@
 #include <string.h>
 #include <time.h>
 
+#define QL_XMIT_SIZE 4096
+#define QL_BUF_SIZE (QL_XMIT_SIZE * 2)
+
 struct ql_ctx {
 	char *printer;
 	int fd;
+	ssize_t pos;
+	char buf[QL_BUF_SIZE];
 };
 
 #define ESC 0x1b
 
 #define STATUS_READ_TIMEOUT 5	/* seconds */
 
-#define full_write(ctx, buf) (retry_write(ctx, buf, sizeof(buf)) == (ssize_t)sizeof(buf))
+#define partial_write(ctx, buf) (do_write(ctx, buf, sizeof(buf), 0) == (ssize_t)sizeof(buf))
+#define full_write(ctx, buf) (do_write(ctx, buf, sizeof(buf), 1) == (ssize_t)sizeof(buf))
 
-ssize_t retry_write(ql_ctx_t ctx, const char *buf, size_t len)
+ssize_t retry_write(ql_ctx_t ctx)
 {
 	int fd = ctx->fd;
 	size_t written = 0;
+	size_t len = ctx->pos;
+	char *buf = (char *)&ctx->buf;
 
 	int retval;
 	fd_set wfds;
@@ -54,9 +62,27 @@ ssize_t retry_write(ql_ctx_t ctx, const char *buf, size_t len)
 				written += n;
 		}
 	}
+	if (written == len) {
+		ctx->pos = 0;
+	}
 	return written;
 }
 
+static ssize_t do_write(ql_ctx_t ctx, const char *buf, ssize_t len, int flush_buffer)
+{
+	if (ctx->pos + len >= QL_BUF_SIZE)
+		return 0;
+
+	memcpy(ctx->buf + ctx->pos, buf, len);
+	ctx->pos += len;
+
+	ssize_t pos = ctx->pos;
+	if (flush_buffer || pos > QL_XMIT_SIZE) {
+		return retry_write(ctx) == pos ? len : 0;
+	}
+	return len;
+}
+
 static void get_mono_time(struct timespec *tp)
 {
 	if (clock_gettime(CLOCK_MONOTONIC, tp) == -1) {
@@ -240,7 +266,7 @@ bool ql_print_raster_image(ql_ctx_t ctx, const ql_status_t * status,
 		block[1] = 0;
 		block[2] = dn;
 		pack_column((uint8_t *) block + 3, dn, w, img, cfg->threshold);
-		if (!full_write(ctx, block))
+		if (!partial_write(ctx, block))
 			return false;
 	}