[PATCH 1/4] xen/uart: be more careful with changes to the PCI command register

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Read the existing PCI command register and only add the required bits to
it, as to avoid clearing bits that might be possibly set by the firmware
already.

This fixes serial output when booting with `com1=device=amt` on a system
using an "Alder Lake AMT SOL Redirection" PCI device (Vendor ID 0x8086 and
Device ID 0x51e3).  That device has both IO and memory decoding enabled by
the firmware, and disabling memory decoding causes the serial to stop
working (even when the serial register BAR is in the IO space).

Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage")
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/drivers/char/ns16550.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index df7fff7f81df..41d6380367ca 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -283,11 +283,17 @@ static int cf_check ns16550_getc(struct serial_port 
*port, char *pc)
 static void pci_serial_early_init(struct ns16550 *uart)
 {
 #ifdef NS16550_PCI
+    uint16_t cmd = 0;
+
+    if ( uart->ps_bdf_enable )
+        cmd = pci_conf_read16(PCI_SBDF(0, uart->ps_bdf[0], uart->ps_bdf[1],
+                                       uart->ps_bdf[2]), PCI_COMMAND);
+
     if ( uart->bar && uart->io_base >= 0x10000 )
     {
         pci_conf_write16(PCI_SBDF(0, uart->ps_bdf[0], uart->ps_bdf[1],
                                   uart->ps_bdf[2]),
-                         PCI_COMMAND, PCI_COMMAND_MEMORY);
+                         PCI_COMMAND, cmd | PCI_COMMAND_MEMORY);
         return;
     }
 
@@ -307,7 +313,7 @@ static void pci_serial_early_init(struct ns16550 *uart)
                      uart->io_base | PCI_BASE_ADDRESS_SPACE_IO);
     pci_conf_write16(PCI_SBDF(0, uart->ps_bdf[0], uart->ps_bdf[1],
                               uart->ps_bdf[2]),
-                     PCI_COMMAND, PCI_COMMAND_IO);
+                     PCI_COMMAND, cmd | PCI_COMMAND_IO);
 #endif
 }
 
-- 
2.51.0