bus: mhi: host: Fix up null pointer access in mhi_irq_handler

The irq handler for a shared IRQ ought to be prepared for running
even now it's being freed. So let's check the pointer used by
mhi_irq_handler to avoid null pointer access since it is probably
released before freeing IRQ.

Fixes: 1227d2a20cd7 ("bus: mhi: host: Move IRQ allocation to controller registration phase")
Signed-off-by: Qiang Yu <[e-mail]>
Reviewed-by: Manivannan Sadhasivam <[e-mail]>
Tested-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[mani: added fixes tag]
Signed-off-by: Manivannan Sadhasivam <[e-mail]>

drivers/bus/mhi/host/main.c

···
 {
 	struct mhi_event *mhi_event = dev;
 	struct mhi_controller *mhi_cntrl = mhi_event->mhi_cntrl;
-	struct mhi_event_ctxt *er_ctxt =
-		&mhi_cntrl->mhi_ctxt->er_ctxt[mhi_event->er_index];
+	struct mhi_event_ctxt *er_ctxt;
 	struct mhi_ring *ev_ring = &mhi_event->ring;
-	dma_addr_t ptr = le64_to_cpu(er_ctxt->rp);
+	dma_addr_t ptr;
 	void *dev_rp;
+
+	/*
+	 * If CONFIG_DEBUG_SHIRQ is set, the IRQ handler will get invoked during __free_irq()
+	 * and by that time mhi_ctxt() would've freed. So check for the existence of mhi_ctxt
+	 * before handling the IRQs.
+	 */
+	if (!mhi_cntrl->mhi_ctxt) {
+		dev_dbg(&mhi_cntrl->mhi_dev->dev,
+			"mhi_ctxt has been freed\n");
+		return IRQ_HANDLED;
+	}
+
+	er_ctxt = &mhi_cntrl->mhi_ctxt->er_ctxt[mhi_event->er_index];
+	ptr = le64_to_cpu(er_ctxt->rp);
 
 	if (!is_valid_ring_ptr(ev_ring, ptr)) {
 		dev_err(&mhi_cntrl->mhi_dev->dev,