Re: [Xen-devel] [PATCH] xen/arm: initialize vpl011 flag register

[Julien Grall <julien@xxxxxxx>]

Hi,

On 25/11/2019 18:35, Jeff Kubascik wrote:
> The tx/rx fifo flags were not set when the vpl011 is initialized. This
> is a problem for certain guests that are operating in polled mode, as a
> guest will generally check the rx fifo empty flag to determine if there
> is data before doing a read. The result is a continuous spam of the
> message "vpl011: Unexpected IN ring buffer empty" before the first valid
> character is received. This initializes the flag status register to the
> default specified in the PL011 technical reference manual.
Note that the vpl011 is not meant to emulate a full PL011. Instead it
emulates the SBSA UART which is a subset of the PL011. They have some 
differences and I would be cautious to try to drive it as a PL011.
> Signed-off-by: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
> ---
>   xen/arch/arm/vpl011.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/xen/arch/arm/vpl011.c b/xen/arch/arm/vpl011.c
> index 7bc5eeb207..31b7d56d7d 100644
> --- a/xen/arch/arm/vpl011.c
> +++ b/xen/arch/arm/vpl011.c
> @@ -626,6 +626,8 @@ int domain_vpl011_init(struct domain *d, struct 
> vpl011_init_info *info)
>       if ( vpl011->backend.dom.ring_buf )
>           return -EINVAL;
>   
> +    vpl011->uartfr = TXFE | RXFE;
I know that it does not make much difference, but I would prefer if 
uartfr is initialized once we know nothing else can fail.
With or without this suggestion:

Acked-by: Julien Gral <julien@xxxxxxx>

Cheers,

--
Julien Grall

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