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[Xen-devel] [PATCH v2 2/7] serial: Fix COM1 assumption if pci_uart_config did not find the PCI serial card.

The io_base by default is set to be 0x3f8 for COM1 and 0x2f8 for COM2
in __setup_xen. Then we call 'ns16550_init' which copies those in
the appropiate uart, which then calls 'ns16550_parse_port_config'
to deal with parameter parsing. If the 'pci' or 'amt' parameter
has been specified we further call 'pci_uart_config code' which
scans the PCI bus. If it does not find any relevant devices
it will overwrite io_base with 0x3f8 regardless whether this is
COM1 or COM2. The overwrite is a way to set it back to the
failsafe defaults - except for COM2 of course.

This in theory (as I don't have a machine with two COM ports
readily available) means that if the user specified 'com2=9600,8n1,pci'
and the device did not have an PCI serial card, instead of using 0x2f8
for the io_base it ends up using 0x3f8 - and we don't get the
output on COM2.

Fix it by saving the original io_base before starting the
scan of the PCI bus. If we don't find an serial PCI device then
assign the original io_base value back.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 xen/drivers/char/ns16550.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 2fded08..c2a25da 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -612,10 +612,11 @@ static int __init
 pci_uart_config (struct ns16550 *uart, int skip_amt, int bar_idx)
     uint32_t bar, bar_64 = 0, len, len_64;
-    u64 size, mask;
+    u64 size, mask, orig_base;
     unsigned int b, d, f, nextf, i;
     u16 vendor, device;
+    orig_base = uart->io_base;
     uart->io_base = 0;
     /* NB. Start at bus 1 to avoid AMT: a plug-in card cannot be on bus 0. */
     for ( b = skip_amt ? 1 : 0; b < 0x100; b++ )
@@ -747,7 +748,8 @@ pci_uart_config (struct ns16550 *uart, int skip_amt, int 
     if ( !skip_amt )
         return -1;
-    uart->io_base = 0x3f8;
+    if ( !uart->io_base )
+        uart->io_base = orig_base;
     uart->irq = 0;
     uart->clock_hz  = UART_CLOCK_HZ;

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