5 files changed, 160 insertions, 1 deletions
diff --git a/kernel/filesystem/CMakeLists.txt b/kernel/filesystem/CMakeLists.txt
index 2a93c62..1d74585 100644
--- a/kernel/filesystem/CMakeLists.txt
+++ b/kernel/filesystem/CMakeLists.txt
@@ -3,6 +3,7 @@ add_library("kernel::filesystem" ALIAS "kernel_filesystem")
 
 target_sources("kernel_filesystem" PRIVATE
     "src/file.cpp"
+    "src/device_file.cpp"
     "src/open_file_description.cpp"
     "src/file_descriptor_table.cpp"
 )
@@ -12,6 +13,7 @@ target_sources("kernel_filesystem" PUBLIC
     BASE_DIRS "include"
     FILES
     "include/filesystem/file.hpp"
+    "include/filesystem/device_file.hpp"
     "include/filesystem/open_file_description.hpp"
     "include/filesystem/file_descriptor_table.hpp"
 )
@@ -20,6 +22,8 @@ target_include_directories("kernel_filesystem" PUBLIC
     "include"
 )
 
-target_link_libraries("kernel_filesystem" PRIVATE
+target_link_libraries("kernel_filesystem" PUBLIC
+    "kernel::devices"
+PRIVATE
     "os::kapi"
 )
diff --git a/kernel/filesystem/include/filesystem/device_file.hpp b/kernel/filesystem/include/filesystem/device_file.hpp
new file mode 100644
index 0000000..08d81f6
--- /dev/null
+++ b/kernel/filesystem/include/filesystem/device_file.hpp
@@ -0,0 +1,30 @@
+#ifndef TEACH_OS_KERNEL_FILESYSTEM_DEVICE_FILE_HPP
+#define TEACH_OS_KERNEL_FILESYSTEM_DEVICE_FILE_HPP
+
+#include "devices/block_device.hpp"
+#include "devices/device.hpp"
+#include "filesystem/file.hpp"
+
+#include <cstddef>
+
+namespace filesystem
+{
+  struct device_file : file
+  {
+    device_file(devices::device * device);
+
+    auto open() -> void override;
+
+    auto read(void * buffer, size_t offset, size_t size) const -> size_t override;
+    auto write(void const * buffer, size_t offset, size_t size) -> size_t override;
+
+  private:
+    using block_op = void (*)(size_t idx, size_t off, size_t len, size_t done, devices::block_device * device,
+                              std::byte * scratch, void * buffer);
+    auto process_blocks(size_t offset, size_t size, void * buffer, block_op op) const -> size_t;
+
+    devices::device * m_device;
+  };
+}  // namespace filesystem
+
+#endif
diff --git a/kernel/filesystem/include/filesystem/open_file_description.hpp b/kernel/filesystem/include/filesystem/open_file_description.hpp
index e4febbd..3241ea4 100644
--- a/kernel/filesystem/include/filesystem/open_file_description.hpp
+++ b/kernel/filesystem/include/filesystem/open_file_description.hpp
@@ -9,6 +9,8 @@ namespace filesystem
 {
   struct open_file_description
   {
+    open_file_description(file * file);
+
     ~open_file_description() = default;
 
     auto read(void * buffer, size_t size) -> size_t;
diff --git a/kernel/filesystem/src/device_file.cpp b/kernel/filesystem/src/device_file.cpp
new file mode 100644
index 0000000..a6c234c
--- /dev/null
+++ b/kernel/filesystem/src/device_file.cpp
@@ -0,0 +1,119 @@
+#include "filesystem/device_file.hpp"
+
+#include "kapi/system.hpp"
+
+#include "devices/block_device.hpp"
+#include "devices/device.hpp"
+
+#include <kstd/cstring>
+
+#include <algorithm>
+#include <array>
+#include <cstddef>
+
+namespace filesystem
+{
+  device_file::device_file(devices::device * device)
+      : m_device(device)
+  {
+    if (m_device == nullptr)
+    {
+      kapi::system::panic("[FILESYSTEM] device_file constructed with null device.");
+    }
+  }
+
+  auto device_file::open() -> void
+  {
+    // Hook point for permission checks or lazy metadata loading.
+  }
+
+  auto device_file::read(void * buffer, size_t offset, size_t size) const -> size_t
+  {
+    if (m_device->is_block_device())
+    {
+      return process_blocks(offset, size, buffer,
+                            [](size_t idx, size_t off, size_t len, size_t done, devices::block_device * device,
+                               std::byte * scratch, void * buffer) {
+                              auto * out = static_cast<std::byte *>(buffer);
+                              if (off == 0 && len == device->block_size())
+                              {
+                                device->read_block(idx, out + done);
+                              }
+                              else
+                              {
+                                device->read_block(idx, scratch);
+                                kstd::libc::memcpy(out + done, scratch + off, len);
+                              }
+                            });
+    }
+    else
+    {
+      kapi::system::panic("[FILESYSTEM] device_file::read called on non-block device.");
+    }
+  }
+
+  auto device_file::write(void const * buffer, size_t offset, size_t size) -> size_t
+  {
+    if (m_device->is_block_device())
+    {
+      return process_blocks(offset, size, const_cast<void *>(buffer),
+                            [](size_t idx, size_t off, size_t len, size_t done, devices::block_device * device,
+                               std::byte * scratch, void * buffer) {
+                              auto const * in = static_cast<std::byte const *>(buffer);
+                              if (off == 0 && len == device->block_size())
+                              {
+                                device->write_block(idx, in + done);
+                              }
+                              else
+                              {
+                                device->read_block(idx, scratch);
+                                kstd::libc::memcpy(scratch + off, in + done, len);
+                                device->write_block(idx, scratch);
+                              }
+                            });
+    }
+    else
+    {
+      kapi::system::panic("[FILESYSTEM] device_file::write called on non-block device.");
+    }
+  }
+
+  auto device_file::process_blocks(size_t offset, size_t size, void * buffer, block_op op) const -> size_t
+  {
+    if (buffer == nullptr)
+    {
+      kapi::system::panic("[FILESYSTEM] device_file::write called with null buffer.");
+    }
+
+    if (size == 0)
+    {
+      return 0;
+    }
+
+    auto * block_dev = static_cast<devices::block_device *>(m_device);
+
+    size_t const block_size = block_dev->block_size();
+    size_t const capacity = block_dev->capacity();
+
+    if (offset >= capacity)
+      return 0;
+    size_t const total_to_process = std::min(size, capacity - offset);
+
+    std::array<std::byte, 512> scratch_buffer{};  // TODO BA-FS26 better solution than fixed scratch_buffer ??
+    auto processed = 0uz;
+
+    while (processed < total_to_process)
+    {
+      size_t const absolute_offset = offset + processed;
+      size_t const block_index = absolute_offset / block_size;
+      size_t const in_block_offset = absolute_offset % block_size;
+      size_t const chunk_size = std::min(total_to_process - processed, block_size - in_block_offset);
+
+      op(block_index, in_block_offset, chunk_size, processed, block_dev, scratch_buffer.data(), buffer);
+
+      processed += chunk_size;
+    }
+
+    return processed;
+  }
+}  // namespace filesystem
diff --git a/kernel/filesystem/src/open_file_description.cpp b/kernel/filesystem/src/open_file_description.cpp
index 8c20397..1f0410c 100644
--- a/kernel/filesystem/src/open_file_description.cpp
+++ b/kernel/filesystem/src/open_file_description.cpp
@@ -6,6 +6,10 @@
 
 namespace filesystem
 {
+  open_file_description::open_file_description(file * file)
+      : m_file(file)
+  {}
+
   auto open_file_description::read(void * buffer, size_t size) -> size_t
   {
     // TODO BA-FS26 nullptr check