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: https://linaro.atlassian.net/browse/TRS-149?focusedCommentId=149574 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). Due to the addition of the TX FIFO in the instance state, increase the migration stream version. Reported-by: Mikko Rapeli <mikko.rap...@linaro.org> Suggested-by: Alex Bennée <alex.ben...@linaro.org> Signed-off-by: Philippe Mathieu-Daudé <phi...@linaro.org> --- RFC because I'm pretty sure I got the migration code wrong. After writing this I noticed the hw/char/cmsdk-apb-uart.c model is much more complete. Instead of copy/pasting its code, I'd rather try to extract some generic/bstract "FIFO based chardev" QOM class; but this is beyond the scope of this series. --- include/hw/char/pl011.h | 2 + hw/char/pl011.c | 105 +++++++++++++++++++++++++++++++++++++--- hw/char/trace-events | 4 ++ 3 files changed, 105 insertions(+), 6 deletions(-) diff --git a/include/hw/char/pl011.h b/include/hw/char/pl011.h index d853802132..20898f43a6 100644 --- a/include/hw/char/pl011.h +++ b/include/hw/char/pl011.h @@ -18,6 +18,7 @@ #include "hw/sysbus.h" #include "chardev/char-fe.h" #include "qom/object.h" +#include "qemu/fifo8.h" #define TYPE_PL011 "pl011" OBJECT_DECLARE_SIMPLE_TYPE(PL011State, PL011) @@ -53,6 +54,7 @@ struct PL011State { Clock *clk; bool migrate_clk; const unsigned char *id; + Fifo8 xmit_fifo; }; DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr); diff --git a/hw/char/pl011.c b/hw/char/pl011.c index 03c006199e..a957138405 100644 --- a/hw/char/pl011.c +++ b/hw/char/pl011.c @@ -57,6 +57,9 @@ DeviceState *pl011_create(hwaddr addr, qemu_irq irq, Chardev *chr) /* Data Register, UARTDR */ #define DR_BE (1 << 10) +/* Receive Status Register/Error Clear Register, UARTRSR/UARTECR */ +#define RSR_OE (1 << 3) + /* Interrupt status bits in UARTRIS, UARTMIS, UARTIMSC */ #define INT_OE (1 << 10) #define INT_BE (1 << 9) @@ -152,19 +155,94 @@ static inline void pl011_reset_tx_fifo(PL011State *s) /* Reset FIFO flags */ s->flags &= ~PL011_FLAG_TXFF; s->flags |= PL011_FLAG_TXFE; + + fifo8_reset(&s->xmit_fifo); +} + +static gboolean pl011_drain_tx(PL011State *s) +{ + trace_pl011_fifo_tx_drain(fifo8_num_used(&s->xmit_fifo)); + pl011_reset_tx_fifo(s); + s->rsr &= ~RSR_OE; + return FALSE; +} + +static gboolean pl011_xmit(void *do_not_use, GIOCondition cond, void *opaque) +{ + PL011State *s = opaque; + int ret; + const uint8_t *buf; + uint32_t buflen; + uint32_t count; + + if (!qemu_chr_fe_backend_connected(&s->chr)) { + /* Instant drain the fifo when there's no back-end */ + return pl011_drain_tx(s); + } + + count = fifo8_num_used(&s->xmit_fifo); + if (!count) { + return FALSE; + } + if (!(s->cr & CR_UARTEN)) { + /* Allow completing the current FIFO character before stopping. */ + count = 1; + } + + /* Transmit as much data as we can */ + buf = fifo8_peek_buf(&s->xmit_fifo, count, &buflen); + ret = qemu_chr_fe_write(&s->chr, buf, buflen); + if (ret >= 0) { + /* Pop the data we could transmit */ + trace_pl011_fifo_tx_xmit(ret); + fifo8_pop_buf(&s->xmit_fifo, ret, NULL); + s->int_level |= INT_TX; + } + + if ((s->cr & CR_UARTEN) && !fifo8_is_empty(&s->xmit_fifo)) { + /* Reschedule another transmission if we couldn't transmit all */ + guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP, + pl011_xmit, s); + if (!r) { + return pl011_drain_tx(s); + } + } + + pl011_update(s); + + return FALSE; } static void pl011_write_tx(PL011State *s, const uint8_t *buf, int length) { if (!(s->cr & (CR_UARTEN | CR_TXE))) { + if (!fifo8_is_empty(&s->xmit_fifo)) { + /* + * If the UART is disabled in the middle of transmission + * or reception, it completes the current character before + * stopping. + */ + pl011_xmit(NULL, G_IO_OUT, s); + } return; } - /* XXX this blocks entire thread. Rewrite to use - * qemu_chr_fe_write and background I/O callbacks */ - qemu_chr_fe_write_all(&s->chr, buf, 1); - s->int_level |= INT_TX; - pl011_update(s); + if (length > fifo8_num_free(&s->xmit_fifo)) { + /* + * The FIFO contents remain valid because no more data is + * written when the FIFO is full, only the contents of the + * shift register are overwritten. The CPU must now read + * the data, to empty the FIFO. + */ + trace_pl011_fifo_tx_overrun(); + s->rsr |= RSR_OE; + return; + } + + trace_pl011_fifo_tx_put(length); + fifo8_push_all(&s->xmit_fifo, buf, length); + + pl011_xmit(NULL, G_IO_OUT, s); } static uint64_t pl011_read(void *opaque, hwaddr offset, @@ -444,12 +522,17 @@ static int pl011_post_load(void *opaque, int version_id) s->read_pos = 0; } + if (version_id >= 3 && !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); + } + return 0; } static const VMStateDescription vmstate_pl011 = { .name = "pl011", - .version_id = 2, + .version_id = 3, .minimum_version_id = 2, .post_load = pl011_post_load, .fields = (VMStateField[]) { @@ -462,6 +545,7 @@ static const VMStateDescription vmstate_pl011 = { VMSTATE_UINT32(int_enabled, PL011State), VMSTATE_UINT32(int_level, PL011State), VMSTATE_UINT32_ARRAY(read_fifo, PL011State, PL011_FIFO_DEPTH), + VMSTATE_FIFO8(xmit_fifo, PL011State), VMSTATE_UINT32(ilpr, PL011State), VMSTATE_UINT32(ibrd, PL011State), VMSTATE_UINT32(fbrd, PL011State), @@ -505,10 +589,18 @@ static void pl011_realize(DeviceState *dev, Error **errp) { PL011State *s = PL011(dev); + fifo8_create(&s->xmit_fifo, PL011_FIFO_DEPTH); qemu_chr_fe_set_handlers(&s->chr, pl011_can_receive, pl011_receive, pl011_event, NULL, s, NULL, true); } +static void pl011_unrealize(DeviceState *dev) +{ + PL011State *s = PL011(dev); + + fifo8_destroy(&s->xmit_fifo); +} + static void pl011_reset(DeviceState *dev) { PL011State *s = PL011(dev); @@ -534,6 +626,7 @@ static void pl011_class_init(ObjectClass *oc, void *data) DeviceClass *dc = DEVICE_CLASS(oc); dc->realize = pl011_realize; + dc->unrealize = pl011_unrealize; dc->reset = pl011_reset; dc->vmsd = &vmstate_pl011; device_class_set_props(dc, pl011_properties); diff --git a/hw/char/trace-events b/hw/char/trace-events index 9fd40e3aae..4c25564066 100644 --- a/hw/char/trace-events +++ b/hw/char/trace-events @@ -60,6 +60,10 @@ pl011_write(uint32_t addr, uint32_t value, const char *regname) "addr 0x%03x val pl011_can_receive(uint32_t lcr, int read_count, int r) "LCR 0x%08x read_count %d returning %d" 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(int count) "TX FIFO push %d" +pl011_fifo_tx_xmit(int count) "TX FIFO pop %d" +pl011_fifo_tx_overrun(void) "TX FIFO overrun" +pl011_fifo_tx_drain(unsigned drained) "TX FIFO draining %u" pl011_baudrate_change(unsigned int baudrate, uint64_t clock, uint32_t ibrd, uint32_t fbrd) "new baudrate %u (clk: %" PRIu64 "hz, ibrd: %" PRIu32 ", fbrd: %" PRIu32 ")" # cmsdk-apb-uart.c -- 2.38.1
