Re: [Xen-devel] [PATCH v3 1/2] arm/xen: vpl011: Fix the slow early console SBSA UART output

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 24 Oct 2017, Andre Przywara wrote:
> From: Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>
> 
> The early console output uses pl011_early_write() to write data. This
> function waits for BUSY bit to get cleared before writing the next byte.
> 
> In the SBSA UART emulation logic, the BUSY bit was set as soon one
> byte was written in the FIFO and it remained set until the FIFO was
> emptied. This meant that the output was delayed as each character needed
> the BUSY to get cleared.
> 
> Since the SBSA UART is getting emulated in Xen using ring buffers, it
> ensures that once the data is enqueued in the FIFO, it will be received
> by xenconsole so it is safe to set the BUSY bit only when FIFO becomes
> full. This will ensure that pl011_early_write() is not delayed unduly
> to write the data.
> 
> Signed-off-by: Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>
> Reviewed-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>  xen/arch/arm/vpl011.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
> index f7ddccb42a..0b0743679f 100644
> --- a/xen/arch/arm/vpl011.c
> +++ b/xen/arch/arm/vpl011.c
> @@ -159,9 +159,15 @@ static void vpl011_write_data(struct domain *d, uint8_t 
> data)
>      {
>          vpl011->uartfr |= TXFF;
>          vpl011->uartris &= ~TXI;
> -    }
>  
> -    vpl011->uartfr |= BUSY;
> +        /*
> +         * This bit is set only when FIFO becomes full. This ensures that
> +         * the SBSA UART driver can write the early console data as fast as
> +         * possible, without waiting for the BUSY bit to get cleared before
> +         * writing each byte.
> +         */
> +        vpl011->uartfr |= BUSY;
> +    }
>  
>      vpl011->uartfr &= ~TXFE;
>  
> @@ -371,11 +377,16 @@ static void vpl011_data_avail(struct domain *d)
>      {
>          vpl011->uartfr &= ~TXFF;
>          vpl011->uartris |= TXI;
> +
> +        /*
> +         * Clear the BUSY bit as soon as space becomes available
> +         * so that the SBSA UART driver can start writing more data
> +         * without any further delay.
> +         */
> +        vpl011->uartfr &= ~BUSY;
> +
>          if ( out_ring_qsize == 0 )
> -        {
> -            vpl011->uartfr &= ~BUSY;
>              vpl011->uartfr |= TXFE;
> -        }
>      }
>  
>      vpl011_update_interrupt_status(d);
> -- 
> 2.14.1
> 

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