mtd: spi-nor: write support for minor aligned partitions

Do not prevent writing to mtd partitions where a partition boundary sits
on a minor erasesize boundary.
This addresses a FIXME that has been present since the start of the
linux git history:
/* 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 */

Allow a uniform erase region spi-nor device to be configured
to use the non-uniform erase regions code path for an erase with:
CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE=y

On supporting hardware (SECT_4K: majority of current SPI-NOR device)
provide the facility for an erase to use the least number
of SPI-NOR operations, as well as access to 4K erase without
requiring CONFIG_MTD_SPI_NOR_USE_4K_SECTORS

Introduce erasesize_minor to the mtd struct,
the smallest erasesize supported by the device

On existing devices, this is useful where write support is wanted
for data on a 4K partition, such as some u-boot-env partitions,
or RouterBoot soft_config, while still netting the performance
benefits of using 64K sectors

Performance:
time mtd erase firmware
OpenWrt 5.10 ramips MT7621 w25q128jv 0xfc0000 partition length

Without this patch
MTD_SPI_NOR_USE_4K_SECTORS=y	|n
real    2m 11.66s		|0m 50.86s
user    0m 0.00s		|0m 0.00s
sys     1m 56.20s		|0m 50.80s

With this patch
MTD_SPI_NOR_USE_VARIABLE_ERASE=n|y		|4K_SECTORS=y
real    0m 51.68s		|0m 50.85s	|2m 12.89s
user    0m 0.00s		|0m 0.00s	|0m 0.01s
sys     0m 46.94s		|0m 50.38s	|2m 12.46s

Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
Signed-off-by: Thibaut VARÈNE <hacks+kernel@slashdirt.org>

drivers/mtd/mtdcore.c

@@ -168,6 +168,15 @@ static ssize_t mtd_erasesize_show(struct device *dev,
 }
 MTD_DEVICE_ATTR_RO(erasesize);
 
+static ssize_t mtd_erasesize_minor_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct mtd_info *mtd = dev_get_drvdata(dev);
+
+	return sysfs_emit(buf, "%lu\n", (unsigned long)mtd->erasesize_minor);
+}
+MTD_DEVICE_ATTR_RO(erasesize_minor);
+
 static ssize_t mtd_writesize_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
@@ -313,6 +322,7 @@ static struct attribute *mtd_attrs[] = {
 	&dev_attr_flags.attr,
 	&dev_attr_size.attr,
 	&dev_attr_erasesize.attr,
+	&dev_attr_erasesize_minor.attr,
 	&dev_attr_writesize.attr,
 	&dev_attr_subpagesize.attr,
 	&dev_attr_oobsize.attr,

drivers/mtd/mtdpart.c

@@ -39,6 +39,7 @@ static struct mtd_info *allocate_partition(struct mtd_info *parent,
 	struct mtd_info *master = mtd_get_master(parent);
 	int wr_alignment = (parent->flags & MTD_NO_ERASE) ?
 			   master->writesize : master->erasesize;
+	int wr_alignment_minor = 0;
 	u64 parent_size = mtd_is_partition(parent) ?
 			  parent->part.size : parent->size;
 	struct mtd_info *child;
@@ -163,33 +164,47 @@ static struct mtd_info *allocate_partition(struct mtd_info *parent,
 	} else {
 		/* Single erase size */
 		child->erasesize = master->erasesize;
+		child->erasesize_minor = master->erasesize_minor;
 	}
 
 	/*
 	 * Child erasesize might differ from the parent one if the parent
 	 * exposes several regions with different erasesize. Adjust
 	 * wr_alignment accordingly.
 	 */
-	if (!(child->flags & MTD_NO_ERASE))
+	if (!(child->flags & MTD_NO_ERASE)) {
 		wr_alignment = child->erasesize;
+		wr_alignment_minor = child->erasesize_minor;
+	}
 
 	tmp = mtd_get_master_ofs(child, 0);
 	remainder = do_div(tmp, wr_alignment);
 	if ((child->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 */
-		child->flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't start on an erase/write block boundary -- force read-only\n",
-			part->name);
+		if (wr_alignment_minor) {
+			/* rely on minor being a factor of major erasesize */
+			tmp = remainder;
+			remainder = do_div(tmp, wr_alignment_minor);
+		}
+		if (remainder) {
+			child->flags &= ~MTD_WRITEABLE;
+			printk(KERN_WARNING"mtd: partition \"%s\" doesn't start on an erase/write block boundary -- force read-only\n",
+				part->name);
+		}
 	}
 
 	tmp = mtd_get_master_ofs(child, 0) + child->part.size;
 	remainder = do_div(tmp, wr_alignment);
 	if ((child->flags & MTD_WRITEABLE) && remainder) {
-		child->flags &= ~MTD_WRITEABLE;
-		printk(KERN_WARNING"mtd: partition \"%s\" doesn't end on an erase/write block -- force read-only\n",
-			part->name);
+		if (wr_alignment_minor) {
+			tmp = remainder;
+			remainder = do_div(tmp, wr_alignment_minor);
+		}
+
+		if (remainder) {
+			child->flags &= ~MTD_WRITEABLE;
+			printk(KERN_WARNING"mtd: partition \"%s\" doesn't end on an erase/write block -- force read-only\n",
+				part->name);
+		}
 	}
 
 	child->size = child->part.size;

drivers/mtd/spi-nor/Kconfig

@@ -10,6 +10,16 @@ menuconfig MTD_SPI_NOR
 
 if MTD_SPI_NOR
 
+config MTD_SPI_NOR_USE_VARIABLE_ERASE
+	bool "Disable uniform_erase to allow use of all hardware supported erasesizes"
+	depends on !MTD_SPI_NOR_USE_4K_SECTORS
+	default n
+	help
+	  Allow mixed use of all hardware supported erasesizes,
+	  by forcing spi_nor to use the multiple eraseregions code path.
+	  For example: A 68K erase will use one 64K erase, and one 4K erase
+	  on supporting hardware.
+
 config MTD_SPI_NOR_USE_4K_SECTORS
 	bool "Use small 4096 B erase sectors"
 	default y

drivers/mtd/spi-nor/core.c

@@ -1117,6 +1117,8 @@ static u8 spi_nor_convert_3to4_erase(u8 opcode)
 
 static bool spi_nor_has_uniform_erase(const struct spi_nor *nor)
 {
+	if (IS_ENABLED(CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE))
+		return false;
 	return !!nor->params->erase_map.uniform_erase_type;
 }
 
@@ -2548,6 +2550,7 @@ static int spi_nor_select_erase(struct spi_nor *nor)
 {
 	struct spi_nor_erase_map *map = &nor->params->erase_map;
 	const struct spi_nor_erase_type *erase = NULL;
+	const struct spi_nor_erase_type *erase_minor = NULL;
 	struct mtd_info *mtd = &nor->mtd;
 	u32 wanted_size = nor->info->sector_size;
 	int i;
@@ -2580,15 +2583,19 @@ static int spi_nor_select_erase(struct spi_nor *nor)
 	 */
 	for (i = SNOR_ERASE_TYPE_MAX - 1; i >= 0; i--) {
 		if (map->erase_type[i].size) {
-			erase = &map->erase_type[i];
-			break;
+			if (!erase)
+				erase = &map->erase_type[i];
+			erase_minor = &map->erase_type[i];
 		}
 	}
 
 	if (!erase)
 		return -EINVAL;
 
 	mtd->erasesize = erase->size;
+	if (IS_ENABLED(CONFIG_MTD_SPI_NOR_USE_VARIABLE_ERASE) &&
+			erase_minor && erase_minor->size < erase->size)
+		mtd->erasesize_minor = erase_minor->size;
 	return 0;
 }
 

include/linux/mtd/mtd.h

@@ -245,6 +245,8 @@ struct mtd_info {
 	 * information below if they desire
 	 */
 	uint32_t erasesize;
+	/* "Minor" (smallest) erase size supported by the whole device */
+	uint32_t erasesize_minor;
 	/* Minimal writable flash unit size. In case of NOR flash it is 1 (even
 	 * though individual bits can be cleared), in case of NAND flash it is
 	 * one NAND page (or half, or one-fourths of it), in case of ECC-ed NOR