ALSA: ctxfi: Limit PTP to a single page

ALSA: ctxfi: Limit PTP to a single page

Commit 391e69143d0a increased CT_PTP_NUM from 1 to 4 to support 256
playback streams, but the additional pages are not used by the card
correctly. The CT20K2 hardware already has multiple VMEM_PTPAL
registers, but using them separately would require refactoring the
entire virtual memory allocation logic.

ct_vm_map() always uses PTEs in vm->ptp[0].area regardless of
CT_PTP_NUM. On AMD64 systems, a single PTP covers 512 PTEs (2M). When
aggregate memory allocations exceed this limit, ct_vm_map() tries to
access beyond the allocated space and causes a page fault:

  BUG: unable to handle page fault for address: ffffd4ae8a10a000
  Oops: Oops: 0002 [#1] SMP PTI
  RIP: 0010:ct_vm_map+0x17c/0x280 [snd_ctxfi]
  Call Trace:
  atc_pcm_playback_prepare+0x225/0x3b0
  ct_pcm_playback_prepare+0x38/0x60
  snd_pcm_do_prepare+0x2f/0x50
  snd_pcm_action_single+0x36/0x90
  snd_pcm_action_nonatomic+0xbf/0xd0
  snd_pcm_ioctl+0x28/0x40
  __x64_sys_ioctl+0x97/0xe0
  do_syscall_64+0x81/0x610
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

Revert CT_PTP_NUM to 1. The 256 SRC_RESOURCE_NUM and playback_count
remain unchanged.

Fixes: 391e69143d0a ("ALSA: ctxfi: Bump playback substreams to 256")
Cc: stable@vger.kernel.org
Signed-off-by: Harin Lee <me@harin.net>
Link: https://patch.msgid.link/20260406074857.216034-1-me@harin.net
Signed-off-by: Takashi Iwai <tiwai@suse.de>