linux-kernel - [PATCH v1 6/6] x86/boot: Introduce MMIO accessors and their support in earlyprintk

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Message-ID: <20250502123145.4066635-7-andriy.shevchenko@linux.intel.com>
Date: Fri,  2 May 2025 15:29:42 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Ashish Kalra <ashish.kalra@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-kernel@...r.kernel.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>,
	Denis Mukhin <dmukhin@...d.com>
Subject: [PATCH v1 6/6] x86/boot: Introduce MMIO accessors and their support in earlyprintk

If user supplied serial base address via kernel command line and
at the same time the word 'mmio' is present, use MMIO accessors.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 arch/x86/boot/early_serial_console.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/x86/boot/early_serial_console.c b/arch/x86/boot/early_serial_console.c
index 6841f14346c2..fa8eb0b4542e 100644
--- a/arch/x86/boot/early_serial_console.c
+++ b/arch/x86/boot/early_serial_console.c
@@ -33,6 +33,20 @@ static void io_serial_out(unsigned long addr, int offset, int value)
 	outb(value, addr + offset);
 }
 
+static void mem8_serial_out(unsigned long addr, int offset, int value)
+{
+	u8 __iomem *vaddr = (u8 __iomem *)addr;
+	/* shift implied by pointer type */
+	writeb(value, vaddr + offset);
+}
+
+static unsigned int mem8_serial_in(unsigned long addr, int offset)
+{
+	u8 __iomem *vaddr = (u8 __iomem *)addr;
+	/* shift implied by pointer type */
+	return readb(vaddr + offset);
+}
+
 static void early_serial_configure(unsigned long port, int baud)
 {
 	unsigned char c;
@@ -59,6 +73,13 @@ static void early_serial_use_io_accessors(void)
 	serial_out = io_serial_out;
 }
 
+static void early_serial_use_mmio_accessors(void)
+{
+	/* It is memory mapped - assume 8-bit alignment */
+	serial_in = mem8_serial_in;
+	serial_out = mem8_serial_out;
+}
+
 static void early_serial_init(unsigned long port, int baud)
 {
 	early_serial_configure(port, baud);
@@ -101,12 +122,16 @@ static void parse_earlyprintk(void)
 		/*
 		 * make sure we have
 		 *	"serial,0x3f8,115200"
+		 *	"serial,mmio,0xff010180,115200"
 		 *	"serial,ttyS0,115200"
 		 *	"ttyS0,115200"
 		 */
 		if (pos == 7 && !strncmp(arg + pos, "0x", 2)) {
 			port = parse_serial_port(arg, pos + 0, &pos);
 			early_serial_use_io_accessors();
+		} else if (pos == 7 && !strncmp(arg + pos, "mmio,0x", 7)) {
+			port = parse_serial_port(arg, pos + 5, &pos);
+			early_serial_use_mmio_accessors();
 		} else if (!strncmp(arg + pos, "ttyS", 4)) {
 			static const int bases[] = { 0x3f8, 0x2f8 };
 			int idx = 0;
-- 
2.47.2