Re: [Qemu-devel] [PATCH v2 4/7] hw/net/stellaris_enet: Correctly implement the TR and THR registers

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> Packet transmission for the stellaris ethernet controller can be triggered
> in one of two ways:
>  * by setting a threshold value in the THR register; when the FIFO
>    fill level reaches the threshold, the h/w starts transmitting.
>    Software has to finish filling the FIFO before the transmit
>    process completes to avoid a (silent) underrun
>  * by software writing to the TR register to explicitly trigger
>    transmission
> 
> Since QEMU transmits packets instantaneously (from the guest's
> point of view), implement "transmit based on threshold" with
> our existing mechanism of "transmit as soon as we have the whole
> packet", with the additional wrinkle that we don't transmit if
> the packet size is below the specified threshold, and implement
> "transmit by specific request" properly.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  hw/net/stellaris_enet.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/net/stellaris_enet.c b/hw/net/stellaris_enet.c
> index 47787fd..db6e43e 100644
> --- a/hw/net/stellaris_enet.c
> +++ b/hw/net/stellaris_enet.c
> @@ -109,6 +109,15 @@ static inline bool stellaris_txpacket_complete(stellaris_enet_state *s)
>      return s->tx_fifo_len >= framelen;
>  }
>  
> +/* Return true if the TX FIFO threshold is enabled and the FIFO
> + * has filled enough to reach it.
> + */
> +static inline bool stellaris_tx_thr_reached(stellaris_enet_state *s)
> +{
> +    return (s->thr < 0x3f &&
> +            (s->tx_fifo_len >= 4 * (s->thr * 8 + 1)));
> +}
> +
>  /* Send the packet currently in the TX FIFO */
>  static void stellaris_enet_send(stellaris_enet_state *s)
>  {
> @@ -309,7 +318,7 @@ static void stellaris_enet_write(void *opaque, hwaddr offset,
>              s->tx_fifo[s->tx_fifo_len++] = value >> 24;
>          }
>  
> -        if (stellaris_txpacket_complete(s)) {
> +        if (stellaris_tx_thr_reached(s) && stellaris_txpacket_complete(s)) {
>              stellaris_enet_send(s);
>          }
>          break;
> @@ -338,9 +347,13 @@ static void stellaris_enet_write(void *opaque, hwaddr offset,
>      case 0x2c: /* MTXD */
>          s->mtxd = value & 0xff;
>          break;
> +    case 0x38: /* TR */
> +        if (value & 1) {
> +            stellaris_enet_send(s);
> +        }
> +        break;
>      case 0x30: /* MRXD */
>      case 0x34: /* NP */
> -    case 0x38: /* TR */
>          /* Ignored.  */
>      case 0x3c: /* Undocuented: Timestamp? */
>          /* Ignored.  */
> -- 
> 1.9.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK