[xen stable-4.20] xen/uart: be more careful with changes to the PCI command register

[patchbot@xxxxxxx]

commit 0359ff3565b40f8692c9a986d6d04a61b03e6d4f
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Mon Apr 20 12:34:52 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 20 12:34:52 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>
    master commit: ebf543972560e45b838def3a54859331cc92c970
    master date: 2026-03-30 16:43:14 +0200
---
 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 1dce415cb2..9507af299f 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -273,14 +273,19 @@ 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;
     }
 
@@ -297,7 +302,7 @@ 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
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.20