Re: [PATCH v2 2/2] hw/char: sifive_uart: Print uart charecters async

[Thomas Huth]

On 19/08/2024 13.31, Alistair Francis wrote:
> The current approach of using qemu_chr_fe_write() and ignoring the
> return values results in dropped charecters [1].

s/charecters/characters/g

(also in the subject of the patch)

> Let's update the SiFive UART to use a async sifive_uart_xmit() function
> to transmit the charecters and apply back preassure to the guest with

s/preassure/pressure/

> the SIFIVE_UART_TXFIFO_FULL status.
>
> This should avoid dropped charecters and more realisiticly model the

s/realisiticly/realisticly/

> hardware.
>
> 1: https://gitlab.com/qemu-project/qemu/-/issues/2114
>
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>   include/hw/char/sifive_uart.h | 17 ++++++-
>   hw/char/sifive_uart.c         | 88 +++++++++++++++++++++++++++++++++--
>   2 files changed, 99 insertions(+), 6 deletions(-)
>
> diff --git a/include/hw/char/sifive_uart.h b/include/hw/char/sifive_uart.h
> index 7f6c79f8bd..b43109bb8b 100644
> --- a/include/hw/char/sifive_uart.h
> +++ b/include/hw/char/sifive_uart.h
> @@ -24,6 +24,7 @@
>   #include "hw/qdev-properties.h"
>   #include "hw/sysbus.h"
>   #include "qom/object.h"
> +#include "qemu/fifo8.h"
>   
>   enum {
>       SIFIVE_UART_TXFIFO        = 0,
> @@ -48,9 +49,13 @@ enum {
>       SIFIVE_UART_IP_RXWM       = 2  /* Receive watermark interrupt pending */
>   };
>   
> +#define SIFIVE_UART_TXFIFO_FULL    0x80000000
> +
>   #define SIFIVE_UART_GET_TXCNT(txctrl)   ((txctrl >> 16) & 0x7)
>   #define SIFIVE_UART_GET_RXCNT(rxctrl)   ((rxctrl >> 16) & 0x7)
> +
>   #define SIFIVE_UART_RX_FIFO_SIZE 8
> +#define SIFIVE_UART_TX_FIFO_SIZE 8
>   
>   #define TYPE_SIFIVE_UART "riscv.sifive.uart"
>   OBJECT_DECLARE_SIMPLE_TYPE(SiFiveUARTState, SIFIVE_UART)
> @@ -63,13 +68,21 @@ struct SiFiveUARTState {
>       qemu_irq irq;
>       MemoryRegion mmio;
>       CharBackend chr;
> -    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> -    uint8_t rx_fifo_len;
> +
> +    uint32_t txfifo;
>       uint32_t ie;
>       uint32_t ip;
>       uint32_t txctrl;
>       uint32_t rxctrl;
>       uint32_t div;
> +
> +    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> +    uint8_t rx_fifo_len;
> +
> +    Fifo8 tx_fifo;
> +
> +    QEMUTimer *fifo_trigger_handle;
> +    uint64_t char_tx_time;
>   };
>   
>   SiFiveUARTState *sifive_uart_create(MemoryRegion *address_space, hwaddr base,
> diff --git a/hw/char/sifive_uart.c b/hw/char/sifive_uart.c
> index 7fc6787f69..07730e241c 100644
> --- a/hw/char/sifive_uart.c
> +++ b/hw/char/sifive_uart.c
> @@ -64,6 +64,72 @@ static void sifive_uart_update_irq(SiFiveUARTState *s)
>       }
>   }
>   
> +static gboolean sifive_uart_xmit(void *do_not_use, GIOCondition cond,
> +                                 void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +    int ret;
> +    const uint8_t *charecters;
> +    uint32_t numptr = 0;
> +
> +    /* instant drain the fifo when there's no back-end */
> +    if (!qemu_chr_fe_backend_connected(&s->chr)) {
> +        fifo8_reset(&s->tx_fifo);
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    if (fifo8_is_empty(&s->tx_fifo)) {
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    /* Don't pop the FIFO incase the write fails */

s/incase/in case/

> +    charecters = fifo8_peek_bufptr(&s->tx_fifo,
> +                                   fifo8_num_used(&s->tx_fifo), &numptr);
> +    ret = qemu_chr_fe_write(&s->chr, charecters, numptr);
> +
> +    if (ret >= 0) {
> +        /* We wrote the data, actuallly pop the fifo */

s/actuallly/actually/

I recommend running checkpatch.pl with the --codespell flag ;-)

Anyway, this indeed seems to fix the problem with the avocado test, thank 
you very much for tackling this!

Tested-by: Thomas Huth <thuth@redhat.com>