From: Felix Fietkau <nbd@nbd.name>
Subject: mtd: implement write support for partitions covering only a part of an eraseblock (buffer data that would otherwise be erased)

lede-commit: 87a8e8ac1067f58ba831c4aae443f3655c31cd80
Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
 drivers/mtd/mtdpart.c   | 90 ++++++++++++++++++++++++++++++++++++++++++++-----
 include/linux/mtd/mtd.h |  4 +++
 2 files changed, 85 insertions(+), 9 deletions(-)

--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -37,6 +37,8 @@
 #include "mtdcore.h"
 #include "mtdsplit/mtdsplit.h"
 
+#define MTD_ERASE_PARTIAL	0x8000 /* partition only covers parts of an erase block */
+
 /* Our partition linked list */
 static LIST_HEAD(mtd_partitions);
 static DEFINE_MUTEX(mtd_partitions_mutex);
@@ -221,6 +223,53 @@ static int part_erase(struct mtd_info *m
 {
 	struct mtd_part *part = mtd_to_part(mtd);
 	int ret;
+	size_t wrlen = 0;
+	u8 *erase_buf = NULL;
+	u32 erase_buf_ofs = 0;
+	bool partial_start = false;
+
+	if (mtd->flags & MTD_ERASE_PARTIAL) {
+		size_t readlen = 0;
+		u64 mtd_ofs;
+
+		erase_buf = kmalloc(part->parent->erasesize, GFP_ATOMIC);
+		if (!erase_buf)
+			return -ENOMEM;
+
+		mtd_ofs = part->offset + instr->addr;
+		erase_buf_ofs = do_div(mtd_ofs, part->parent->erasesize);
+
+		if (erase_buf_ofs > 0) {
+			instr->addr -= erase_buf_ofs;
+			ret = mtd_read(part->parent,
+				instr->addr + part->offset,
+				part->parent->erasesize,
+				&readlen, erase_buf);
+
+			instr->len += erase_buf_ofs;
+			partial_start = true;
+		} else {
+			mtd_ofs = part->offset + part->mtd.size;
+			erase_buf_ofs = part->parent->erasesize -
+				do_div(mtd_ofs, part->parent->erasesize);
+
+			if (erase_buf_ofs > 0) {
+				instr->len += erase_buf_ofs;
+				ret = mtd_read(part->parent,
+					part->offset + instr->addr +
+					instr->len - part->parent->erasesize,
+					part->parent->erasesize, &readlen,
+					erase_buf);
+			} else {
+				ret = 0;
+			}
+		}
+		if (ret < 0) {
+			kfree(erase_buf);
+			return ret;
+		}
+
+	}
 
 	instr->addr += part->offset;
 	ret = part->parent->_erase(part->parent, instr);
@@ -228,6 +277,24 @@ static int part_erase(struct mtd_info *m
 		instr->fail_addr -= part->offset;
 	instr->addr -= part->offset;
 
+	if (mtd->flags & MTD_ERASE_PARTIAL) {
+		if (partial_start) {
+			part->parent->_write(part->parent,
+				instr->addr, erase_buf_ofs,
+				&wrlen, erase_buf);
+			instr->addr += erase_buf_ofs;
+		} else {
+			instr->len -= erase_buf_ofs;
+			part->parent->_write(part->parent,
+				instr->addr + instr->len,
+				erase_buf_ofs, &wrlen,
+				erase_buf +
+				part->parent->erasesize -
+				erase_buf_ofs);
+		}
+		kfree(erase_buf);
+	}
+
 	return ret;
 }
 
@@ -536,19 +603,22 @@ static struct mtd_part *allocate_partiti
 	remainder = do_div(tmp, wr_alignment);
 	if ((slave->mtd.flags & MTD_WRITEABLE) && remainder) {
 		/* Doesn't start on a boundary of major erase size */
-		/* FIXME: Let it be writable if it is on a boundary of
-		 * _minor_ erase size though */
-		slave->mtd.flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't start on an erase/write block boundary -- force read-only\n",
-			part->name);
+		slave->mtd.flags |= MTD_ERASE_PARTIAL;
+		if (((u32)slave->mtd.size) > parent->erasesize)
+			slave->mtd.flags &= ~MTD_WRITEABLE;
+		else
+			slave->mtd.erasesize = slave->mtd.size;
 	}
 
-	tmp = part_absolute_offset(parent) + slave->mtd.size;
+	tmp = part_absolute_offset(parent) + slave->offset + slave->mtd.size;
 	remainder = do_div(tmp, wr_alignment);
 	if ((slave->mtd.flags & MTD_WRITEABLE) && remainder) {
-		slave->mtd.flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't end on an erase/write block -- force read-only\n",
-			part->name);
+		slave->mtd.flags |= MTD_ERASE_PARTIAL;
+
+		if ((u32)slave->mtd.size > parent->erasesize)
+			slave->mtd.flags &= ~MTD_WRITEABLE;
+		else
+			slave->mtd.erasesize = slave->mtd.size;
 	}
 
 	mtd_set_ooblayout(&slave->mtd, &part_ooblayout_ops);