File_Sys: Add a size dependend delay for each file read

src/core/file_sys/file_backend.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <algorithm>
 #include <cstddef>
 #include "common/common_types.h"
 #include "core/hle/result.h"
@@ -37,6 +38,24 @@ public:
      */
     virtual ResultVal<size_t> Write(u64 offset, size_t length, bool flush, const u8* buffer) = 0;
 
+    /**
+     * Get the amount of time a 3ds needs to read those data
+     * @param length Length in bytes of data read from file
+     * @return Nanoseconds for the delay
+     */
+    virtual u64 GetReadDelayNs(size_t length) const {
+        // Return the default delay for the case that the subclass backend didn't
+        // implement one. We take the one measured for romfs reads
+        // This should be removed as soon as every subclass backend
+        // has one implemented
+        LOG_WARNING(Service_FS, "Using default delay for read");
+        static constexpr u64 slope(94);
+        static constexpr u64 offset(582778);
+        static constexpr u64 minimum(663124);
+        u64 IPCDelayNanoseconds = std::max<u64>(static_cast<u64>(length) * slope + offset, minimum);
+        return IPCDelayNanoseconds;
+    }
+
     /**
      * Get the size of the file in bytes
      * @return Size of the file in bytes