diff --git a/opm/simulators/linalg/mixed/bsr.h b/opm/simulators/linalg/mixed/bsr.h
index 00b53f44e1..f58a022179 100644
--- a/opm/simulators/linalg/mixed/bsr.h
+++ b/opm/simulators/linalg/mixed/bsr.h
@@ -4,6 +4,7 @@
 extern "C" {
 #endif
 
+// bsr matrix struct
 typedef
 struct bsr_matrix
 {
@@ -19,16 +20,82 @@ struct bsr_matrix
 
 } bsr_matrix;
 
+/**
+ * @brief Create empty bsr matrix.
+ *
+ * @return Pointer to bsr matrix.
+ */
 bsr_matrix* bsr_alloc();
+
+/**
+ * @brief Delete bsr matrix.
+ *
+ * @param A Pointer to bsr matrix.
+ */
 void bsr_free(bsr_matrix *A);
 
+/**
+ * @brief Initialize bsr matrix.
+ *
+ * @param A Pointer to bsr matrix.
+ * @param nrows Number of rows.
+ * @apram nnz Number of nonzero blocks.
+ * @param b Block size.
+ */
 void bsr_init(bsr_matrix *A, int nrows, int nnz, int b);
+
+/**
+ * @brief Sparse matrix-vector multiplication in mixed precision.
+ *
+ * @note Function is specialized for 3x3 block-sparse matrices.
+ * @note Function uses AVX2 intrinsics.
+ *
+ * @param A Pointer to bsr matrix.
+ * @param x Pointer to input vector.
+ * @param y Pointer to output vector.
+ */
 void bsr_vmspmv3(bsr_matrix *A, const double *x, double *y);
+
+/**
+ * @brief Sparse matrix-vector multiplication in double precision.
+ *
+ * @note Function is specialized for 3x3 block-sparse matrices.
+ * @note Function uses AVX2 intrinsics.
+ *
+ * @param A Pointer to bsr matrix.
+ * @param x Pointer to input vector.
+ * @param y Pointer to output vector.
+ */
 void bsr_vdspmv3(bsr_matrix *A, const double *x, double *y);
+
+/**
+ * @brief Make single-precision copy of double-precision values.
+ *
+ * @param M Pointer to bsr matrix
+ */
 void bsr_downcast(bsr_matrix *M);
 
+/**
+ * @brief Display matrix statistics.
+ *
+ * @param A Pointer to bsr matrix
+ */
 void bsr_info(bsr_matrix *A);
+
+/**
+ * @brief Display spasity pattern of first few rows.
+ *
+ * @param A Pointer to bsr matrix
+ * @param name String with desired name of bsr_matrix
+ */
 void bsr_sparsity(const bsr_matrix *A, const char *name);
+
+/**
+ * @brief Display nonzero blocks of first few rows.
+ *
+ * @param A Pointer to bsr matrix
+ * @param name String with desired name of bsr_matrix
+ */
 void bsr_nonzeros(bsr_matrix *A, const char *name);
 
 #ifdef __cplusplus