Kernel: Do not reset AC'97 PCM out on buffer completion

We now only reset the PCM out channel during initialization, and handle
the case where the channel's current index has passed the last valid
index properly.

This fixes issues with stuttering audio between multiple subsequent
`aplay` invocations, for example.

Kernel/Devices/Audio/AC97.cpp

@@ -44,9 +44,10 @@ bool AC97::handle_irq(RegisterState const&)
 {
     auto pcm_out_status_register = m_pcm_out_channel.reg(AC97Channel::Register::Status);
     auto pcm_out_status = pcm_out_status_register.in<u16>();
-    dbgln_if(AC97_DEBUG, "AC97 @ {}: interrupt received - stat: {:#b}", pci_address(), pcm_out_status);
+    dbgln_if(AC97_DEBUG, "AC97 @ {}: interrupt received - status: {:#05b}", pci_address(), pcm_out_status);
 
     bool is_dma_halted = (pcm_out_status & AudioStatusRegisterFlag::DMAControllerHalted) > 0;
+    bool current_equals_last_valid = (pcm_out_status & AudioStatusRegisterFlag::CurrentEqualsLastValid) > 0;
     bool is_completion_interrupt = (pcm_out_status & AudioStatusRegisterFlag::BufferCompletionInterruptStatus) > 0;
     bool is_fifo_error = (pcm_out_status & AudioStatusRegisterFlag::FIFOError) > 0;
 
@@ -62,11 +63,14 @@ bool AC97::handle_irq(RegisterState const&)
         | AudioStatusRegisterFlag::FIFOError;
     pcm_out_status_register.out(pcm_out_status);
 
-    // Stop the DMA engine if we're through with the buffer and no one is waiting
-    if (is_dma_halted && m_irq_queue.is_empty())
-        reset_pcm_out();
-    else
+    if (is_dma_halted) {
+        VERIFY(current_equals_last_valid);
+        m_pcm_out_channel.handle_dma_stopped();
+    }
+
+    if (!m_irq_queue.is_empty())
         m_irq_queue.wake_all();
+
     return true;
 }
 
@@ -115,17 +119,11 @@ UNMAP_AFTER_INIT ErrorOr<void> AC97::initialize()
     set_master_output_volume(0, 0, Muted::No);
     set_pcm_output_volume(0, 0, Muted::No);
 
-    reset_pcm_out();
+    m_pcm_out_channel.reset();
     enable_irq();
     return {};
 }
 
-void AC97::reset_pcm_out()
-{
-    m_pcm_out_channel.reset();
-    m_buffer_descriptor_list_index = 0;
-}
-
 void AC97::set_master_output_volume(u8 left_channel, u8 right_channel, Muted mute)
 {
     u16 volume_value = ((right_channel & 63) << 0)
@@ -225,20 +223,26 @@ ErrorOr<void> AC97::write_single_buffer(UserOrKernelBuffer const& data, size_t o
     cli();
     do {
         auto pcm_out_status = m_pcm_out_channel.reg(AC97Channel::Register::Status).in<u16>();
-        auto is_dma_controller_halted = (pcm_out_status & AudioStatusRegisterFlag::DMAControllerHalted) > 0;
         auto current_index = m_pcm_out_channel.reg(AC97Channel::Register::CurrentIndexValue).in<u8>();
-        auto last_valid_index = m_pcm_out_channel.reg(AC97Channel::Register::LastValidIndex).in<u8>();
+        int last_valid_index = m_pcm_out_channel.reg(AC97Channel::Register::LastValidIndex).in<u8>();
 
-        auto head_distance = static_cast<int>(last_valid_index) - current_index;
+        auto head_distance = last_valid_index - current_index;
         if (head_distance < 0)
             head_distance += buffer_descriptor_list_max_entries;
-        if (!is_dma_controller_halted)
+        if (m_pcm_out_channel.dma_running())
             ++head_distance;
 
+        // Current index has _passed_ last valid index - move our list index up
+        if (head_distance > m_output_buffer_page_count) {
+            m_buffer_descriptor_list_index = current_index + 1;
+            break;
+        }
+
+        // There is room for our data
         if (head_distance < m_output_buffer_page_count)
             break;
 
-        dbgln_if(AC97_DEBUG, "AC97 @ {}: waiting on interrupt - stat: {:#b} CI: {} LVI: {}", pci_address(), pcm_out_status, current_index, last_valid_index);
+        dbgln_if(AC97_DEBUG, "AC97 @ {}: waiting on interrupt - status: {:#05b} CI: {} LVI: {}", pci_address(), pcm_out_status, current_index, last_valid_index);
         m_irq_queue.wait_forever("AC97"sv);
     } while (m_pcm_out_channel.dma_running());
     sti();
@@ -246,9 +250,6 @@ ErrorOr<void> AC97::write_single_buffer(UserOrKernelBuffer const& data, size_t o
     // Copy data from userspace into one of our buffers
     TRY(data.read(m_output_buffer->vaddr_from_page_index(m_output_buffer_page_index).as_ptr(), offset, length));
 
-    if (!m_pcm_out_channel.dma_running())
-        reset_pcm_out();
-
     // Write the next entry to the buffer descriptor list
     u16 number_of_samples = length / sizeof(u16);
     auto list_entries = reinterpret_cast<BufferDescriptorListEntry*>(m_buffer_descriptor_list->vaddr().get());
@@ -268,9 +269,16 @@ ErrorOr<void> AC97::write_single_buffer(UserOrKernelBuffer const& data, size_t o
     return {};
 }
 
+void AC97::AC97Channel::handle_dma_stopped()
+{
+    dbgln_if(AC97_DEBUG, "AC97 @ {}: channel {}: DMA engine has stopped", m_device.pci_address(), name());
+    VERIFY(m_dma_running);
+    m_dma_running = false;
+}
+
 void AC97::AC97Channel::reset()
 {
-    dbgln("AC97 @ {}: channel {}: resetting", m_device.pci_address(), name());
+    dbgln_if(AC97_DEBUG, "AC97 @ {}: channel {}: resetting", m_device.pci_address(), name());
 
     auto control_register = reg(Register::Control);
     control_register.out(AudioControlRegisterFlag::ResetRegisters);
@@ -283,7 +291,7 @@ void AC97::AC97Channel::reset()
 
 void AC97::AC97Channel::set_last_valid_index(u32 buffer_address, u8 last_valid_index)
 {
-    dbgln_if(AC97_DEBUG, "AC97 @ {}: setting LVI - address: {:#x} LVI: {}", m_device.pci_address(), buffer_address, last_valid_index);
+    dbgln_if(AC97_DEBUG, "AC97 @ {}: channel {}: setting buffer address: {:#x} LVI: {}", m_device.pci_address(), name(), buffer_address, last_valid_index);
 
     reg(Register::BufferDescriptorListBaseAddress).out(buffer_address);
     reg(Register::LastValidIndex).out(last_valid_index);
@@ -291,7 +299,7 @@ void AC97::AC97Channel::set_last_valid_index(u32 buffer_address, u8 last_valid_i
 
 void AC97::AC97Channel::start_dma()
 {
-    dbgln("AC97 @ {}: channel {}: starting DMA engine", m_device.pci_address(), name());
+    dbgln_if(AC97_DEBUG, "AC97 @ {}: channel {}: starting DMA engine", m_device.pci_address(), name());
 
     auto control_register = reg(Register::Control);
     auto control = control_register.in<u8>();

Kernel/Devices/Audio/AC97.h

@@ -128,6 +128,7 @@ private:
         }
 
         bool dma_running() const { return m_dma_running; }
+        void handle_dma_stopped();
         StringView name() const { return m_name; }
         IOAddress reg(Register reg) const { return m_channel_base.offset(reg); }
         void reset();
@@ -148,7 +149,6 @@ private:
 
     AC97Channel channel(StringView name, NativeAudioBusChannel channel) { return AC97Channel(*this, name, m_io_bus_base.offset(channel)); }
     ErrorOr<void> initialize();
-    void reset_pcm_out();
     void set_master_output_volume(u8, u8, Muted);
     ErrorOr<void> set_pcm_output_sample_rate(u32);
     void set_pcm_output_volume(u8, u8, Muted);