Re: [PATCH v2 1/2] xen/ns16550: detect and preserve UART_IER_UUE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.10.2026 11:20, Oleksii Kurochko wrote:
> On 10/10/26 5:14 AM, Zhang Zheng wrote:
>> XScale-compatible UARTs require UART_IER_UUE to remain set.  Probe the
>> bit during setup and preserve it when enabling receive interrupts, so
>> the xscale/PXA UART remains operational after ns16550 takes over.
>>
>> Signed-off-by: Zhang Zheng <zhangzheng@xxxxxxxxxxx>
>> ---
>>   xen/drivers/char/ns16550.c  | 12 +++++++++++-
>>   xen/include/xen/8250-uart.h |  1 +
>>   2 files changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
>> index cdda28879e..c174465059 100644
>> --- a/xen/drivers/char/ns16550.c
>> +++ b/xen/drivers/char/ns16550.c
>> @@ -64,6 +64,7 @@ static struct ns16550 {
>>       bool intr_works;
>>       bool force_polling;
>>       bool dw_usr_bsy;
>> +    bool xscale;
>>   #ifdef NS16550_PCI
>>       /* PCI card parameters. */
>>       bool pb_bdf_enable;     /* if =1, pb-bdf effective, port behind bridge 
>> */
>> @@ -352,6 +353,14 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
>>       /* No interrupts. */
>>       ns_write_reg(uart, UART_IER, 0);
>>   
>> +    uart->xscale = false;
>> +    if ( !(ns_read_reg(uart, UART_IER) & UART_IER_UUE) )
>> +    {
>> +        ns_write_reg(uart, UART_IER, UART_IER_UUE);
>> +        if ( ns_read_reg(uart, UART_IER) & UART_IER_UUE )
>> +            uart->xscale = true;
>> +    }
>> +
> 
> This probe only checks whether IER bit 6 is writable, which isn't
> specific to XScale. On UARTs with an EFR (e.g. ST16C650, TI TL16C752,
> Exar XR17V35x) the upper IER bits become writable once EFR[4] (ECB) is
> set, and bit 6 is then the RTS interrupt enable. If ECB is already set
> when the probe runs, it would report XScale, and the driver would then
> keep the RTS interrupt enabled.
> 
> Could the flag be derived from the compatible in ns16550_uart_dt_init()
> instead, the same way dw_usr_bsy is?
> 
>      uart->xscale = dt_device_is_compatible(dev, "intel,xscale-uart");
> 
> That compatible would also need adding to ns16550_dt_match[]. The
> SpacemiT K1 nodes, for example, have no "ns16550a" fallback.

Are you suggesting that Linux'es XScale detection is flawed then, too?

>> @@ -446,7 +455,8 @@ static void ns16550_setup_postirq(struct ns16550 *uart)
>>                        UART_MCR, UART_MCR_OUT2 | UART_MCR_DTR | 
>> UART_MCR_RTS);
>>   
>>           /* Enable receive interrupts. */
>> -        ns_write_reg(uart, UART_IER, UART_IER_ERDAI);
>> +        ns_write_reg(uart, UART_IER,
>> +                     UART_IER_ERDAI | (uart->xscale ? UART_IER_UUE : 0));
> 
> It seems like not all cases are covered. For example, 
> ns16550_interrupt() writes 0 to IER when it switches to polling, which 
> would turn off the UART on XScale. Am I missing something? If not then 
> something like:
> 
> static void ns_write_ier(const struct ns16550 *uart, uint8_t ier)
> {
>      if ( uart->xscale )
>          ier |= UART_IER_UUE;
> 
>      ns_write_reg(uart, UART_IER, ier);
> }
> 
> would be needed, used in ns16550_setup_preirq(), ns16550_setup_postirq()

Yes, the more that as per Linux'es driver source, bit 4 may also need keeping
permanently set (not only for XScale, but apparently also for Tegra).

Jan