xHCI - Check if command ring is full before enqueueing command

kernel/include/drivers/usb/xhci/xhci_rings.h

@@ -14,14 +14,17 @@ class xhci_command_ring {
     inline uintptr_t get_physical_base() const { return m_physical_base; }
     inline uint8_t  get_cycle_bit() const { return m_rcs_bit; }
 
-    void enqueue(xhci_trb_t* trb);
+    bool enqueue(xhci_trb_t* trb);
+    void process_event(xhci_command_completion_trb_t* event);
 
 private:
-    size_t              m_max_trb_count;     // Number of valid TRBs in the ring including the LINK_TRB
-    size_t              m_enqueue_ptr;       // Index in the ring where to enqueue next TRB
-    xhci_trb_t*         m_trbs;              // Base address of the ring buffer
-    uintptr_t           m_physical_base;     // Physical base of the ring
-    uint8_t             m_rcs_bit;           // Ring cycle state
+    size_t              m_max_trb_count;        // Number of valid TRBs in the ring including the LINK_TRB
+    size_t              m_enqueue_ptr;          // Index in the ring where to enqueue next TRB
+    xhci_trb_t*         m_trbs;                 // Base address of the ring buffer
+    uintptr_t           m_physical_base;        // Physical base of the ring
+    uint8_t             m_rcs_bit;              // Ring cycle state
+    size_t              m_dequeue_ptr;          // The xHC's position as it reads the ring
+    bool                m_consumer_cycle_state; // The consumer (xHC)'s ring cycle state
 };
 
 /*

kernel/src/drivers/usb/xhci/xhci.cpp

@@ -472,7 +472,10 @@ xhci_command_completion_trb_t* xhci_driver::_send_command(xhci_trb_t* trb, uint3
     mutex_guard guard(s_xhc_command_lock);
 
     // Enqueue the TRB
-    m_command_ring->enqueue(trb);
+    if (!m_command_ring->enqueue(trb)) {
+        xhci_warn("Failed to enqueue command. Command ring is full.");
+        return nullptr;
+    };
 
     // Ring the command doorbell
     m_doorbell_manager->ring_command_doorbell();
@@ -505,6 +508,9 @@ xhci_command_completion_trb_t* xhci_driver::_send_command(xhci_trb_t* trb, uint3
         return nullptr;
     }
 
+    // Update the command ring dequeue pointer
+    m_command_ring->process_event(completion_trb);
+
     return completion_trb;
 }
 

kernel/src/drivers/usb/xhci/xhci_rings.cpp

@@ -7,6 +7,8 @@ xhci_command_ring::xhci_command_ring(size_t max_trbs) {
     m_max_trb_count = max_trbs;
     m_rcs_bit = XHCI_CRCR_RING_CYCLE_STATE;
     m_enqueue_ptr = 0;
+    m_dequeue_ptr = 0;
+    m_consumer_cycle_state = true;
 
     const uint64_t ring_size = max_trbs * sizeof(xhci_trb_t);
 
@@ -25,7 +27,14 @@ xhci_command_ring::xhci_command_ring(size_t max_trbs) {
         (XHCI_TRB_TYPE_LINK << XHCI_TRB_TYPE_SHIFT) | XHCI_LINK_TRB_TC_BIT | m_rcs_bit;
 }
 
-void xhci_command_ring::enqueue(xhci_trb_t* trb) {
+bool xhci_command_ring::enqueue(xhci_trb_t* trb) {
+    auto can_enqueue = m_consumer_cycle_state == m_rcs_bit 
+        ? m_enqueue_ptr >= m_dequeue_ptr 
+        : m_enqueue_ptr < m_dequeue_ptr;
+    if (!can_enqueue) {
+        return false;
+    }
+
     // Adjust the TRB's cycle bit to the current RCS
     trb->cycle_bit = m_rcs_bit;
 
@@ -43,6 +52,25 @@ void xhci_command_ring::enqueue(xhci_trb_t* trb) {
         m_enqueue_ptr = 0;
         m_rcs_bit = !m_rcs_bit;
     }
+
+    return true;
+}
+
+void xhci_command_ring::process_event(xhci_command_completion_trb_t* event) {
+    // xHCI 4.9.3 Command Ring Management
+    // > The location of the Command Ring Dequeue Pointer is reported on the Event Ring in Command Completion Events.
+    // xHCI 3.3 Command Interface
+    // > Commands are executed by the xHC in the order that they are placed on the Command Ring.
+    auto command_index = (event->command_trb_pointer - m_physical_base) / sizeof(xhci_trb_t);
+    // This could result in the dequeue pointer pointing to a Link TRB, which should be pretty instantly processed.
+    // But we can't assume that the xHC processed the Link TRB and we shouldn't overwrite it until we're sure.
+    // Since commands are executed in order, we don't need to worry about the dequeue pointer getting moved back because of out-of-order events.
+    auto new_dequeue_ptr = command_index + 1;
+    // If the consumer (xHC) looped around, it must have toggled its consumer cycle state
+    if (new_dequeue_ptr < m_dequeue_ptr) {
+        m_consumer_cycle_state = !m_consumer_cycle_state;
+    }
+    m_dequeue_ptr = new_dequeue_ptr;
 }
 
 xhci_event_ring::xhci_event_ring(