[RFC PATCH v5 16/16] hw/char/pl011: Implement TX FIFO

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

If the UART back-end chardev doesn't drain data as fast as stdout
does or blocks, buffer in the TX FIFO to try again later.

This avoids having the IO-thread busy waiting on chardev back-ends,
reported recently when testing the Trusted Reference Stack and
using the socket backend.

Implement registering a front-end 'watch' callback on back-end
events, so we can resume transmitting when the back-end is writable
again, not blocking the main loop.

Similarly to the RX FIFO path, FIFO level selection is not
implemented (interrupt is triggered when a single byte is available
in the FIFO).

Reported-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
RFC: Something is still broken, some characters are emitted async...
---
 hw/char/pl011.c      | 60 ++++++++++++++++++++++++++++++++++++--------
 hw/char/trace-events |  1 +
 2 files changed, 51 insertions(+), 10 deletions(-)

diff --git a/hw/char/pl011.c b/hw/char/pl011.c
index cfa3fd3da4..9f72b6a765 100644
--- a/hw/char/pl011.c
+++ b/hw/char/pl011.c
@@ -240,7 +240,9 @@ static gboolean pl011_xmit(void *do_not_use, GIOCondition cond, void *opaque)
 {
     PL011State *s = opaque;
     int bytes_consumed;
-    uint8_t data;
+    const uint8_t *buf;
+    uint32_t buflen;
+    uint32_t count;
 
     if (!(s->cr & CR_UARTEN)) {
         qemu_log_mask(LOG_GUEST_ERROR, "PL011 data written to disabled UART\n");
@@ -249,25 +251,40 @@ static gboolean pl011_xmit(void *do_not_use, GIOCondition cond, void *opaque)
         qemu_log_mask(LOG_GUEST_ERROR, "PL011 data written to disabled TX UART\n");
     }
 
+    count = fifo8_num_used(&s->xmit_fifo);
+    if (count < 1) {
+        /* FIFO empty */
+        return G_SOURCE_REMOVE;
+    }
+
     if (!qemu_chr_fe_backend_connected(&s->chr)) {
         /* Instant drain the fifo when there's no back-end. */
         pl011_drain_tx(s);
         return G_SOURCE_REMOVE;
     }
 
-    data = fifo8_pop(&s->xmit_fifo);
-    bytes_consumed = 1;
+    buf = fifo8_peek_buf(&s->xmit_fifo, count, &buflen);
 
-    /*
-     * XXX this blocks entire thread. Rewrite to use
-     * qemu_chr_fe_write and background I/O callbacks
-     */
-    qemu_chr_fe_write_all(&s->chr, &data, bytes_consumed);
+    /* Transmit as much data as we can. */
+    bytes_consumed = qemu_chr_fe_write(&s->chr, buf, buflen);
     trace_pl011_fifo_tx_xmit(bytes_consumed);
+    if (bytes_consumed < 0) {
+        /* Error in back-end: drain the fifo. */
+        pl011_drain_tx(s);
+        return G_SOURCE_REMOVE;
+    }
+
+    /* Pop the data we could transmit. */
+    fifo8_pop_buf(&s->xmit_fifo, bytes_consumed, NULL);
     s->int_level |= INT_TX;
 
     pl011_update(s);
 
+    if (!fifo8_is_empty(&s->xmit_fifo)) {
+        /* Reschedule another transmission if we couldn't transmit all. */
+        return G_SOURCE_CONTINUE;
+    }
+
     return G_SOURCE_REMOVE;
 }
 
@@ -290,6 +307,10 @@ static void pl011_write_txdata(PL011State *s, uint8_t data)
     trace_pl011_fifo_tx_put(data);
     pl011_loopback_tx(s, data);
     fifo8_push(&s->xmit_fifo, data);
+    if (fifo8_is_full(&s->xmit_fifo)) {
+        s->flags |= PL011_FLAG_TXFF;
+    }
+
     pl011_xmit(NULL, G_IO_OUT, s);
 }
 
@@ -488,10 +509,24 @@ static void pl011_write(void *opaque, hwaddr offset,
         pl011_trace_baudrate_change(s);
         break;
     case 11: /* UARTLCR_H */
-        /* Reset the FIFO state on FIFO enable or disable */
         if ((s->lcr ^ value) & LCR_FEN) {
-            pl011_reset_rx_fifo(s);
+            bool fifo_enabled = value & LCR_FEN;
+
+            trace_pl011_fifo_enable(fifo_enabled);
+            if (fifo_enabled) {
+                /* Transmit and receive FIFO buffers are enabled (FIFO mode). */
+                fifo8_change_capacity(&s->xmit_fifo, PL011_FIFO_DEPTH);
+            } else {
+                /*
+                 * FIFOs are disabled (character mode) that is, the FIFOs
+                 * become 1-byte-deep holding registers.
+                 */
+                pl011_drain_tx(s);
+                fifo8_change_capacity(&s->xmit_fifo, 1);
+            }
+            /* Reset the FIFO state on FIFO enable or disable */
             pl011_reset_tx_fifo(s);
+            pl011_reset_rx_fifo(s);
         }
         if ((s->lcr ^ value) & LCR_BRK) {
             int break_enable = value & LCR_BRK;
@@ -636,6 +671,11 @@ static int pl011_post_load(void *opaque, int version_id)
         s->read_pos = 0;
     }
 
+    if (!fifo8_is_empty(&s->xmit_fifo)) {
+        /* Reschedule another transmission */
+        qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP, pl011_xmit, s);
+    }
+
     s->ibrd &= IBRD_MASK;
     s->fbrd &= FBRD_MASK;
 
diff --git a/hw/char/trace-events b/hw/char/trace-events
index bf586ba664..2405819812 100644
--- a/hw/char/trace-events
+++ b/hw/char/trace-events
@@ -58,6 +58,7 @@ pl011_read(uint32_t addr, uint32_t value, const char *regname) "addr 0x%03x valu
 pl011_read_fifo(int read_count) "FIFO read, read_count now %d"
 pl011_write(uint32_t addr, uint32_t value, const char *regname) "addr 0x%03x value 0x%08x reg %s"
 pl011_can_receive(uint32_t lcr, int read_count, int r) "LCR 0x%08x read_count %d returning %d"
+pl011_fifo_enable(bool enable) "enable:%u"
 pl011_fifo_rx_put(uint32_t c, int read_count) "new char 0x%02x read_count now %d"
 pl011_fifo_rx_full(void) "RX FIFO now full, RXFF set"
 pl011_fifo_tx_put(uint8_t byte) "TX FIFO push char [0x%02x]"
-- 
2.41.0