Received-SPF: pass (google.com: domain of linux-kernel+bounces-175325-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
From: Nam Cao <namcao@linutronix.de>
To: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv@lists.infradead.org,
	linux-kernel@vger.kernel.org
Cc: Nam Cao <namcao@linutronix.de>
Subject: [PATCH 4/7] riscv: drop the use of XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET
Date: Fri, 10 May 2024 08:28:42 +0200
Message-Id: <a49044c3833e40546f35dd1f821a683e6f4dfc1f.1715286093.git.namcao@linutronix.de>
In-Reply-To: <cover.1715286093.git.namcao@linutronix.de>
References: <cover.1715286093.git.namcao@linutronix.de>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

XIP_OFFSET is the hard-coded offset of writable data section within the
kernel.

By hard-coding this value, the read-only section of the kernel (which is
placed before the writable data section) is restricted in size.

As a preparation to remove this hard-coded macro XIP_OFFSET entirely, stop
using XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET. Instead, use __data_loc and
_sdata to do the same thing.

While at it, also add a description for XIP_FIXUP_FLASH_OFFSET.

Signed-off-by: Nam Cao <namcao@linutronix.de>
---
 arch/riscv/include/asm/xip_fixup.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/include/asm/xip_fixup.h b/arch/riscv/include/asm/xip_fixup.h
index 9ed2cfae09e0..f3d56299bc22 100644
--- a/arch/riscv/include/asm/xip_fixup.h
+++ b/arch/riscv/include/asm/xip_fixup.h
@@ -24,13 +24,21 @@
 	sub \reg, \reg, t0
 .endm
 .macro XIP_FIXUP_FLASH_OFFSET reg
+	/* In linker script, at the transition from read-only section to
+	 * writable section, the VMA is increased while LMA remains the same.
+	 * (See in linker script how _sdata, __data_loc and LOAD_OFFSET is
+	 * changed)
+	 *
+	 * Consequently, early during boot before MMU is up, the generated code
+	 * reads the "writable" section at wrong addresses, because VMA is used
+	 * by compiler to generate code, but the data is located in Flash using
+	 * LMA.
+	 */
+	la t0, _sdata
+	sub \reg, \reg, t0
 	la t0, __data_loc
-	REG_L t1, _xip_phys_offset
-	sub \reg, \reg, t1
 	add \reg, \reg, t0
 .endm
-
-_xip_phys_offset: .dword CONFIG_XIP_PHYS_ADDR + XIP_OFFSET
 #else
 .macro XIP_FIXUP_OFFSET reg
 .endm
-- 
2.39.2