linux-kernel - [RFC PATCH v4 19/20] x86/kexec: Add 8250 serial port output

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Message-ID: <20241127190343.44916-20-dwmw2@infradead.org>
Date: Wed, 27 Nov 2024 19:00:33 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: kexec@...ts.infradead.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	linux-kernel@...r.kernel.org,
	Simon Horman <horms@...nel.org>,
	Dave Young <dyoung@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	jpoimboe@...nel.org,
	bsz@...zon.de
Subject: [RFC PATCH v4 19/20] x86/kexec: Add 8250 serial port output

From: David Woodhouse <dwmw@...zon.co.uk>

If a serial port was configured for early_printk, use it for debug output
from the relocate_kernel exception handler too.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/include/asm/kexec.h         |  1 +
 arch/x86/kernel/early_printk.c       |  6 +++++
 arch/x86/kernel/relocate_kernel_64.S | 37 +++++++++++++++++++++++-----
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index c14b0a2704c0..f37d2d9fda3f 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -63,6 +63,7 @@ extern unsigned long kexec_pa_table_page;
 extern unsigned long kexec_pa_swap_page;
 extern gate_desc kexec_debug_idt[];
 extern unsigned char kexec_debug_exc_vectors[];
+extern uint16_t kexec_debug_8250_port;
 #endif
 
 /*
diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index 44f937015e1e..bf06866ee90a 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/console.h>
 #include <linux/kernel.h>
+#include <linux/kexec.h>
 #include <linux/init.h>
 #include <linux/string.h>
 #include <linux/screen_info.h>
@@ -141,6 +142,11 @@ static __init void early_serial_hw_init(unsigned divisor)
 	serial_out(early_serial_base, DLL, divisor & 0xff);
 	serial_out(early_serial_base, DLH, (divisor >> 8) & 0xff);
 	serial_out(early_serial_base, LCR, c & ~DLAB);
+
+#ifdef CONFIG_KEXEC_DEBUG
+	if (serial_in == io_serial_in)
+		kexec_debug_8250_port = early_serial_base;
+#endif
 }
 
 #define DEFAULT_BAUD 9600
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index cd349b6d34a7..01a31e4a0664 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -38,6 +38,7 @@ SYM_DATA(kexec_va_control_page, .quad 0)
 SYM_DATA(kexec_pa_table_page, .quad 0)
 SYM_DATA(kexec_pa_swap_page, .quad 0)
 SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
+SYM_DATA(kexec_debug_8250_port, .word 0)
 
 #ifdef CONFIG_KEXEC_DEBUG
 	.balign 16
@@ -364,23 +365,47 @@ SYM_CODE_END(swap_pages)
 
 #ifdef CONFIG_KEXEC_DEBUG
 /*
- * Generic 'print character' routine (as yet unimplemented)
+ * Generic 'print character' routine
  *  - %al: Character to be printed (may clobber %rax)
  *  - %rdx: MMIO address or port.
  */
-SYM_CODE_START_LOCAL_NOALIGN(pr_char)
+#define XMTRDY          0x20
+
+#define TXR             0       /*  Transmit register (WRITE) */
+#define LSR             5       /*  Line Status               */
+
+SYM_CODE_START_LOCAL_NOALIGN(pr_char_8250)
 	UNWIND_HINT_FUNC
+	addw	$LSR, %dx
+	xchg	%al, %ah
+.Lxmtrdy_loop:
+	inb	%dx, %al
+	testb	$XMTRDY, %al
+	jnz	.Lready
+	rep nop
+	jmp .Lxmtrdy_loop
+
+.Lready:
+	subw	$LSR, %dx
+	xchg	%al, %ah
+	outb	%al, %dx
+pr_char_null:
 	ANNOTATE_UNRET_SAFE
 	ret
-SYM_CODE_END(pr_char)
+SYM_CODE_END(pr_char_8250)
 
 /*
  * Load pr_char function pointer into %rsi and load %rdx with whatever
  * that function wants to see there (typically port/MMIO address).
  */
-.macro	pr_setup
-	/* No output; pr_char just returns */
-	leaq	pr_char(%rip), %rsi
+.macro pr_setup
+	leaq	pr_char_8250(%rip), %rsi
+	movw	kexec_debug_8250_port(%rip), %dx
+	testw	%dx, %dx
+	jnz	1f
+
+	leaq	pr_char_null(%rip), %rsi
+1:
 .endm
 
 /* Print the nybble in %bl, clobber %rax */
-- 
2.47.0