[Qemu-devel] [PATCH] Add 'maxqdepth' as an option to tty character devices.

[John Baboval]

From: "John V. Baboval" <address@hidden>

This parameter will cause writes to tty backed chardevs to return
-EAGAIN if the backing tty has buffered more than the specified
number of characters. When data is sent, the TIOCOUTQ ioctl is invoked
to determine the current TTY output buffer depth.

Background:

Some devices use DTR/DSR as flow control. (eg. Check/Receipt
printers with some POS software). When the device de-asserts
DTR, the guest OS notifies the application and new data is blocked.
When running on a QEMU serial port backed by a TTY, though the guest
stops transmitting, all the characters in the TTY output buffer are
still sent. The device buffer overflows and data is lost. In this
case the user could set maxqdepth=1.

Signed-off-by: John Baboval <address@hidden>
---
 include/sysemu/char.h |    2 ++
 qapi-schema.json      |    5 ++++-
 qemu-char.c           |   40 +++++++++++++++++++++++++++++++++++++++-
 qemu-options.hx       |    4 ++--
 4 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/include/sysemu/char.h b/include/sysemu/char.h
index 5e42c90..a94c1fb 100644
--- a/include/sysemu/char.h
+++ b/include/sysemu/char.h
@@ -43,6 +43,7 @@ typedef struct {
 
 #define CHR_IOCTL_SERIAL_SET_TIOCM   13
 #define CHR_IOCTL_SERIAL_GET_TIOCM   14
+#define CHR_IOCTL_SERIAL_TIOCOUTQ    15
 
 #define CHR_TIOCM_CTS  0x020
 #define CHR_TIOCM_CAR  0x040
@@ -77,6 +78,7 @@ struct CharDriverState {
     int fe_open;
     int explicit_fe_open;
     int avail_connections;
+    uint32_t maxqdepth;
     QemuOpts *opts;
     QTAILQ_ENTRY(CharDriverState) next;
 };
diff --git a/qapi-schema.json b/qapi-schema.json
index 7797400..029e7c9 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -3182,11 +3182,14 @@
 #
 # @device: The name of the special file for the device,
 #          i.e. /dev/ttyS0 on Unix or COM1: on Windows
+# @maxqdepth: The maximum depth of the underlying tty
+              output queue (Unix) 
 # @type: What kind of device this is.
 #
 # Since: 1.4
 ##
-{ 'type': 'ChardevHostdev', 'data': { 'device' : 'str' } }
+{ 'type': 'ChardevHostdev', 'data': { 'device'    : 'str',
+                                      'maxqdepth' : 'int' } }
 
 ##
 # @ChardevSocket:
diff --git a/qemu-char.c b/qemu-char.c
index 64e824d..e2e4217 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -782,6 +782,7 @@ typedef struct FDCharDriver {
     GIOChannel *fd_in, *fd_out;
     guint fd_in_tag;
     int max_size;
+    int tiocoutq_failed;
     QTAILQ_ENTRY(FDCharDriver) node;
 } FDCharDriver;
 
@@ -1260,6 +1261,22 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
     return chr;
 }
 
+static int tty_serial_write(CharDriverState *chr, const uint8_t *buf, int len)
+{
+    FDCharDriver *s = chr->opaque;
+    uint32_t inflight = 0;
+
+    qemu_chr_fe_ioctl(chr, CHR_IOCTL_SERIAL_TIOCOUTQ, &inflight);
+    if (inflight >= chr->maxqdepth)
+        return -EAGAIN;
+
+    if (inflight + len > chr->maxqdepth) {
+        len = chr->maxqdepth - inflight;
+    }
+
+    return io_channel_send(s->fd_out, buf, len);
+}
+
 static void tty_serial_init(int fd, int speed,
                             int parity, int data_bits, int stop_bits)
 {
@@ -1438,6 +1455,16 @@ static int tty_serial_ioctl(CharDriverState *chr, int 
cmd, void *arg)
             ioctl(g_io_channel_unix_get_fd(s->fd_in), TIOCMSET, &targ);
         }
         break;
+    case CHR_IOCTL_SERIAL_TIOCOUTQ:
+        {
+            if (!s->tiocoutq_failed)
+                s->tiocoutq_failed = ioctl(g_io_channel_unix_get_fd(s->fd_in),
+                                           TIOCOUTQ, arg);
+
+            if (s->tiocoutq_failed)
+                *(unsigned int *)arg = 0;            
+        }
+        break;
     default:
         return -ENOTSUP;
     }
@@ -1466,6 +1493,7 @@ static CharDriverState *qemu_chr_open_tty_fd(int fd)
 
     tty_serial_init(fd, 115200, 'N', 8, 1);
     chr = qemu_chr_open_fd(fd, fd);
+    chr->chr_write = tty_serial_write;
     chr->chr_ioctl = tty_serial_ioctl;
     chr->chr_close = qemu_chr_close_tty;
     return chr;
@@ -3172,6 +3200,8 @@ static void qemu_chr_parse_serial(QemuOpts *opts, 
ChardevBackend *backend,
     }
     backend->serial = g_new0(ChardevHostdev, 1);
     backend->serial->device = g_strdup(device);
+    backend->serial->maxqdepth =
+        qemu_opt_get_number(opts, "maxqdepth", -1);
 }
 
 static void qemu_chr_parse_parallel(QemuOpts *opts, ChardevBackend *backend,
@@ -3575,6 +3605,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "size",
             .type = QEMU_OPT_SIZE,
+        },{
+            .name = "maxqdepth",
+            .type = QEMU_OPT_NUMBER,
         },
         { /* end of list */ }
     },
@@ -3653,6 +3686,7 @@ static CharDriverState 
*qmp_chardev_open_serial(ChardevHostdev *serial,
                                                 Error **errp)
 {
 #ifdef HAVE_CHARDEV_TTY
+    CharDriverState *chr;
     int fd;
 
     fd = qmp_chardev_open_file_source(serial->device, O_RDWR, errp);
@@ -3660,7 +3694,11 @@ static CharDriverState 
*qmp_chardev_open_serial(ChardevHostdev *serial,
         return NULL;
     }
     qemu_set_nonblock(fd);
-    return qemu_chr_open_tty_fd(fd);
+    chr = qemu_chr_open_tty_fd(fd);
+    if (chr) {
+        chr->maxqdepth = serial->maxqdepth;
+    }
+    return chr;
 #else
     error_setg(errp, "character device backend type 'serial' not supported");
     return NULL;
diff --git a/qemu-options.hx b/qemu-options.hx
index e86cc24..c522f13 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1792,8 +1792,8 @@ DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
 #endif
 #if defined(__linux__) || defined(__sun__) || defined(__FreeBSD__) \
         || defined(__NetBSD__) || defined(__OpenBSD__) || 
defined(__DragonFly__)
-    "-chardev serial,id=id,path=path[,mux=on|off]\n"
-    "-chardev tty,id=id,path=path[,mux=on|off]\n"
+    "-chardev serial,id=id,path=path[,mux=on|off][,maxqdepth=count]\n"
+    "-chardev tty,id=id,path=path[,mux=on|off][,maxqdepth=count]\n"
 #endif
 #if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
     "-chardev parallel,id=id,path=path[,mux=on|off]\n"
-- 
1.7.9.5