[PATCH v2 2/3] 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, which might put the device into a non-working state.

Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage")
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Adjusted over previous fixes.
---
 xen/drivers/char/ns16550.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index d05dc506ed9c..d16e447c0e76 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -283,14 +283,19 @@ 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;
+
     if ( !uart->ps_bdf_enable )
         return;
 
+    cmd = pci_conf_read16(PCI_SBDF(0, uart->ps_bdf[0], uart->ps_bdf[1],
+                                  uart->ps_bdf[2]), PCI_COMMAND);
+
     if ( 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 +312,7 @@ 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