soundwire: stream: fix NULL pointer dereference for multi_link

If bus is marked as multi_link, but number of masters in the stream is
not higher than bus->hw_sync_min_links (bus->multi_link && m_rt_count >=
bus->hw_sync_min_links), bank switching should not happen.  The first
part of do_bank_switch() code properly takes these conditions into
account, but second part (sdw_ml_sync_bank_switch()) relies purely on
bus->multi_link property.  This is not balanced and leads to NULL
pointer dereference:

  Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
  ...
  Call trace:
   wait_for_completion_timeout+0x124/0x1f0
   do_bank_switch+0x370/0x6f8
   sdw_prepare_stream+0x2d0/0x438
   qcom_snd_sdw_prepare+0xa0/0x118
   sm8450_snd_prepare+0x128/0x148
   snd_soc_link_prepare+0x5c/0xe8
   __soc_pcm_prepare+0x28/0x1ec
   dpcm_be_dai_prepare+0x1e0/0x2c0
   dpcm_fe_dai_prepare+0x108/0x28c
   snd_pcm_do_prepare+0x44/0x68
   snd_pcm_action_single+0x54/0xc0
   snd_pcm_action_nonatomic+0xe4/0xec
   snd_pcm_prepare+0xc4/0x114
   snd_pcm_common_ioctl+0x1154/0x1cc0
   snd_pcm_ioctl+0x54/0x74

Fixes: ce6e74d ("soundwire: Add support for multi link bank switch")
Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20231124180136.390621-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: krzk

drivers/soundwire/stream.c

@@ -742,14 +742,15 @@ static int sdw_bank_switch(struct sdw_bus *bus, int m_rt_count)
  * sdw_ml_sync_bank_switch: Multilink register bank switch
  *
  * @bus: SDW bus instance
+ * @multi_link: whether this is a multi-link stream with hardware-based sync
  *
  * Caller function should free the buffers on error
  */
-static int sdw_ml_sync_bank_switch(struct sdw_bus *bus)
+static int sdw_ml_sync_bank_switch(struct sdw_bus *bus, bool multi_link)
 {
 	unsigned long time_left;
 
-	if (!bus->multi_link)
+	if (!multi_link)
 		return 0;
 
 	/* Wait for completion of transfer */
@@ -847,7 +848,7 @@ static int do_bank_switch(struct sdw_stream_runtime *stream)
 			bus->bank_switch_timeout = DEFAULT_BANK_SWITCH_TIMEOUT;
 
 		/* Check if bank switch was successful */
-		ret = sdw_ml_sync_bank_switch(bus);
+		ret = sdw_ml_sync_bank_switch(bus, multi_link);
 		if (ret < 0) {
 			dev_err(bus->dev,
 				"multi link bank switch failed: %d\n", ret);