Currently we see a kernel-oops reported on Power-9 while attaching a
context to an AFU, with radix-mode and sysfs attr 'prefault_mode' set
to anything other than 'none'. The backtrace of the oops is of this
form:

  Unable to handle kernel paging request for data at address 0x00000080
  Faulting instruction address: 0xc00800000bcf3b20
  cpu 0x1: Vector: 300 (Data Access) at [c00000037f003800]
      pc: c00800000bcf3b20: cxl_load_segment+0x178/0x290 [cxl]
      lr: c00800000bcf39f0: cxl_load_segment+0x48/0x290 [cxl]
      sp: c00000037f003a80
     msr: 9000000000009033
     dar: 80
   dsisr: 40000000
    current = 0xc00000037f280000
    paca    = 0xc0000003ffffe600   softe: 3        irq_happened: 0x01
      pid   = 3529, comm = afp_no_int
  <snip>
  cxl_prefault+0xfc/0x248 [cxl]
  process_element_entry_psl9+0xd8/0x1a0 [cxl]
  cxl_attach_dedicated_process_psl9+0x44/0x130 [cxl]
  native_attach_process+0xc0/0x130 [cxl]
  afu_ioctl+0x3f4/0x5e0 [cxl]
  do_vfs_ioctl+0xdc/0x890
  ksys_ioctl+0x68/0xf0
  sys_ioctl+0x40/0xa0
  system_call+0x58/0x6c

The issue is caused as on Power-8 the AFU attr 'prefault_mode' was
used to improve initial storage fault performance by prefaulting
process segments. However on Power-9 with radix mode we don't have
Storage-Segments that we can prefault. Also prefaulting process Pages
will be too costly and fine-grained.

Hence, since the prefaulting mechanism doesn't makes sense of
radix-mode, this patch updates prefault_mode_store() to not allow any
other value apart from CXL_PREFAULT_NONE when radix mode is enabled.

Fixes: f24be42a ("cxl: Add psl9 specific code")
Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>