[PATCH v2] misc: fastrpc: Drop unhandled DSP PD exit notification

[Shawn Guo]

Newer DSP firmware implements a PD (Protection Domain) notification
framework that sends PD state notifications upon request. The PD exit
notification is unconditionally sent by the DSP with a fixed sentinel
0xABCDABCD in the context field.

fastrpc_rpmsg_callback() treats every inbound message as an invoke
response, so the sentinel is masked and shifted like any real response
((0xABCDABCD & 0xFF0) >> 4 == 188) and looked up in the channel's
context idr.

This is not merely cosmetic. In the common case idr slot 188 is empty,
the lookup fails, and the driver only logs a spurious "No context ID
matches response" error on every teardown. But the context idr is shared
by every protection domain and the listener thread on the channel and is
filled cyclically over [1, FASTRPC_CTX_MAX]. If slot 188 holds a live
context when the sentinel arrives, the sentinel's return value is written
into that unrelated in-flight invocation and it is completed early.

Since neither the fastrpc library nor the driver supports the DSP PD
notification framework, it is safe to drop the PD exit notification
before it is ever turned into a context lookup. This removes both the
log spam and the mis-completion race. A genuine response can never be
masked: a real context is (idr_index << 4) | pd (at most 0xFF3) and
can never equal the sentinel.

Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
Signed-off-by: Shawn Guo <shengchao.guo@xxxxxxxxxxxxxxxx>
---
Changes for v2:
- Update per Ekansh's input about PD state notification (Thanks Ekansh!)
- Link to v1: https://lore.kernel.org/all/20260727130940.577721-1-shengchao.guo@xxxxxxxxxxxxxxxx/

 drivers/misc/fastrpc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index d86e79134c68..97c60271f94a 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -38,6 +38,17 @@
 #define FASTRPC_INIT_HANDLE	1
 #define FASTRPC_DSP_UTILITIES_HANDLE	2
 #define FASTRPC_CTXID_MASK (0xFF0)
+
+/*
+ * Newer DSP firmware implements a PD (Protection Domain) notification
+ * framework that sends PD state notifications upon request. The PD exit
+ * notification is unconditionally sent by the DSP with this fixed sentinel
+ * in the context field rather than the context of an outstanding invocation.
+ * Since the fastrpc driver does not support the DSP PD notification framework,
+ * this message must be dropped rather than matched against the context idr.
+ */
+#define FASTRPC_DSP_PD_NOTIFY_CTX	0xABCDABCD
+
 #define INIT_FILELEN_MAX (2 * 1024 * 1024)
 #define INIT_FILE_NAMELEN_MAX (128)
 #define FASTRPC_DEVICE_NAME	"fastrpc"
@@ -2552,6 +2563,14 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
 	if (!cctx)
 		return -ENODEV;
 
+	/*
+	 * A PD exit notification from the DSP PD notification framework carries
+	 * this sentinel rather than a real context. Drop it: a real context is
+	 * (idr_index << 4) | pd and can never collide with this value.
+	 */
+	if (rsp->ctx == FASTRPC_DSP_PD_NOTIFY_CTX)
+		return 0;
+
 	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
 
 	spin_lock_irqsave(&cctx->lock, flags);
-- 
2.43.0