[xen staging] ns16550: do not override fifo size if explicitly set

[patchbot@xxxxxxx]

commit 60a9d8d2fc9c4a524c7342499580a88aaa3a2b55
Author:     Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
AuthorDate: Mon Aug 16 15:14:37 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Aug 16 15:14:37 2021 +0200

    ns16550: do not override fifo size if explicitly set
    
    If fifo size is already set via uart_params, do not force it to 16 - which
    may not match the actual hardware. Specifically Exar cards have fifo of
    256 bytes.
    
    Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/drivers/char/ns16550.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 16a73d0c0e..97b85b0225 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -357,7 +357,8 @@ static void __init ns16550_init_preirq(struct serial_port 
*port)
     ns16550_setup_preirq(uart);
 
     /* Check this really is a 16550+. Otherwise we have no FIFOs. */
-    if ( ((ns_read_reg(uart, UART_IIR) & 0xc0) == 0xc0) &&
+    if ( uart->fifo_size <= 1 &&
+         ((ns_read_reg(uart, UART_IIR) & 0xc0) == 0xc0) &&
          ((ns_read_reg(uart, UART_FCR) & UART_FCR_TRG14) == UART_FCR_TRG14) )
         uart->fifo_size = 16;
 }
--
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