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

[patchbot@xxxxxxx]

commit ebf543972560e45b838def3a54859331cc92c970
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Mar 25 14:32:59 2026 +0100
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Mon Mar 30 16:43:14 2026 +0200

    xen/uart: be more careful with changes to the PCI command register
    
    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.
    
    Xen being the owner of the PCI device it's legitimately allowed to change
    command registers bits.  Be careful however in case buggy devices require
    certain command register bits to be enabled, while not strictly required
    from Xen's usage.
    
    Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage")
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 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 45ac089193..293fc74d63 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
 }
 
--
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