riscv: allow resume after exception

If CSRs like seed are readable by S-mode, may not be determinable by
S-mode. For safe driver probing allow to resume via a longjmp after an
exception.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

include/interrupt.h

@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include <asm/setjmp.h>
+
+/**
+ * struct resume_data - data for resume after interrupt
+ */
+struct resume_data {
+	/** @jump: longjmp buffer */
+	jmp_buf jump;
+	/** @code: exception code */
+	ulong code;
+};
+
+/**
+ * set_resume() - set longjmp buffer for resuming after exception
+ *
+ * By calling this function it is possible to use a long jump to catch an
+ * exception. The caller sets the long jump buffer with set_resume() and then
+ * executes setjmp(). If an exception occurs, the code will return to the
+ * setjmp caller(). The exception code will be returned in @data->code.
+ *
+ * After the critical operation call set_resume(NULL) so that an exception in
+ * another part of the code will not accidently invoke the long jump.
+ *
+ * .. code-block:: c
+ *
+ *     // This example shows how to use set_resume().
+ *
+ *     struct resume_data resume;
+ *     int ret;
+ *
+ *     set_resume(&resume);
+ *     ret = setjmp(resume.jump);
+ *     if (ret) {
+ *          printf("An exception %ld occurred\n", resume.code);
+ *     } else {
+ *          // Do what might raise an exception here.
+ *     }
+ *     set_resume(NULL);
+ *
+ * @data:	pointer to structure with longjmp address
+ * Return:	0 before an exception, 1 after an exception occurred
+ */
+void set_resume(struct resume_data *data);