From: Prasad J Pandit <address@hidden>

USB Ehci emulation supports host controller capability registers.
But its mmio '.write' function was missing, which lead to a null
pointer dereference issue. Add a do nothing 'ehci_caps_write'
definition to avoid it; Do nothing because capability registers
are Read Only(RO).

Reported-by: Zuozhi Fzz <address@hidden>
Signed-off-by: Prasad J Pandit <address@hidden>
---
 hw/usb/hcd-ehci.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/usb/hcd-ehci.c b/hw/usb/hcd-ehci.c
index c40013e..b08ff62 100644
--- a/tools/qemu-xen/hw/usb/hcd-ehci.c
+++ b/tools/qemu-xen/hw/usb/hcd-ehci.c
@@ -893,6 +893,11 @@ static uint64_t ehci_caps_read(void *ptr, hwaddr addr,
     return s->caps[addr];
 }
 
+static void ehci_caps_write(void *ptr, hwaddr addr,
+                             uint64_t val, unsigned size)
+{
+}
+
 static uint64_t ehci_opreg_read(void *ptr, hwaddr addr,
                                 unsigned size)
 {
@@ -2313,6 +2318,7 @@ static void ehci_frame_timer(void *opaque)
 
 static const MemoryRegionOps ehci_mmio_caps_ops = {
     .read = ehci_caps_read,
+    .write = ehci_caps_write,
     .valid.min_access_size = 1,
     .valid.max_access_size = 4,
     .impl.min_access_size = 1,
-- 
2.5.0