[PATCH v3 5/6] xen/riscv: introduce early_printk basic stuff

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

From: Bobby Eshleman <bobby.eshleman@xxxxxxxxx>

The patch introduces a basic stuff of early_printk functionality
which will be enough to print 'hello from C environment".
early_printk() function was changed in comparison with original as
common isn't being built now so there is no vscnprintf.

Because printk() relies on a serial driver (like the ns16550 driver)
and drivers require working virtual memory (ioremap()) there is not
print functionality early in Xen boot.

This commit adds early printk implementation built on the putc SBI call.

As sbi_console_putchar() is being already planned for deprecation
it is used temporary now and will be removed or reworked after
real uart will be ready.

Signed-off-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxx>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in V3:
    - reorder "include <header>" in erly_printk.c
    - change an author of the commit
---
Changes in V2:
    - add license to early_printk.c
    - add signed-off-by Bobby
    - add RISCV_32 to Kconfig.debug to EARLY_PRINTK config
    - update commit message
    - order the files alphabetically in Makefile
---
 xen/arch/riscv/Kconfig.debug              |  7 +++++
 xen/arch/riscv/Makefile                   |  1 +
 xen/arch/riscv/early_printk.c             | 33 +++++++++++++++++++++++
 xen/arch/riscv/include/asm/early_printk.h | 12 +++++++++
 xen/arch/riscv/setup.c                    |  4 +++
 5 files changed, 57 insertions(+)
 create mode 100644 xen/arch/riscv/early_printk.c
 create mode 100644 xen/arch/riscv/include/asm/early_printk.h

diff --git a/xen/arch/riscv/Kconfig.debug b/xen/arch/riscv/Kconfig.debug
index e69de29bb2..e21649781d 100644
--- a/xen/arch/riscv/Kconfig.debug
+++ b/xen/arch/riscv/Kconfig.debug
@@ -0,0 +1,7 @@
+config EARLY_PRINTK
+    bool "Enable early printk"
+    default DEBUG
+    depends on RISCV_64 || RISCV_32
+    help
+
+      Enables early printk debug messages
diff --git a/xen/arch/riscv/Makefile b/xen/arch/riscv/Makefile
index fd916e1004..1a4f1a6015 100644
--- a/xen/arch/riscv/Makefile
+++ b/xen/arch/riscv/Makefile
@@ -1,3 +1,4 @@
+obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
 obj-$(CONFIG_RISCV_64) += riscv64/
 obj-y += sbi.o
 obj-y += setup.o
diff --git a/xen/arch/riscv/early_printk.c b/xen/arch/riscv/early_printk.c
new file mode 100644
index 0000000000..348c21bdaa
--- /dev/null
+++ b/xen/arch/riscv/early_printk.c
@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * RISC-V early printk using SBI
+ *
+ * Copyright (C) 2021 Bobby Eshleman <bobbyeshleman@xxxxxxxxx>
+ */
+#include <asm/early_printk.h>
+#include <asm/sbi.h>
+
+/*
+ * TODO:
+ *   sbi_console_putchar is already planned for deprecation
+ *   so it should be reworked to use UART directly.
+*/
+void early_puts(const char *s, size_t nr)
+{
+    while ( nr-- > 0 )
+    {
+        if (*s == '\n')
+            sbi_console_putchar('\r');
+        sbi_console_putchar(*s);
+        s++;
+    }
+}
+
+void early_printk(const char *str)
+{
+    while (*str)
+    {
+        early_puts(str, 1);
+        str++;
+    }
+}
diff --git a/xen/arch/riscv/include/asm/early_printk.h 
b/xen/arch/riscv/include/asm/early_printk.h
new file mode 100644
index 0000000000..05106e160d
--- /dev/null
+++ b/xen/arch/riscv/include/asm/early_printk.h
@@ -0,0 +1,12 @@
+#ifndef __EARLY_PRINTK_H__
+#define __EARLY_PRINTK_H__
+
+#include <xen/early_printk.h>
+
+#ifdef CONFIG_EARLY_PRINTK
+void early_printk(const char *str);
+#else
+static inline void early_printk(const char *s) {};
+#endif
+
+#endif /* __EARLY_PRINTK_H__ */
diff --git a/xen/arch/riscv/setup.c b/xen/arch/riscv/setup.c
index 13e24e2fe1..d09ffe1454 100644
--- a/xen/arch/riscv/setup.c
+++ b/xen/arch/riscv/setup.c
@@ -1,12 +1,16 @@
 #include <xen/compile.h>
 #include <xen/init.h>
 
+#include <asm/early_printk.h>
+
 /* Xen stack for bringing up the first CPU. */
 unsigned char __initdata cpu0_boot_stack[STACK_SIZE]
     __aligned(STACK_SIZE);
 
 void __init noreturn start_xen(void)
 {
+    early_printk("Hello from C env\n");
+
     for ( ;; )
         asm volatile ("wfi");
 
-- 
2.38.1