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]"