nvmem: core: support specifying both: cell raw data & post read lengths

Callback .read_post_process() is designed to modify raw cell content
before providing it to the consumer. So far we were dealing with
modifications that didn't affect cell size (length). In some cases
however cell content needs to be reformatted and resized.

It's required e.g. to provide properly formatted MAC address in case
it's stored in a non-binary format (e.g. using ASCII).

There were few discussions how to optimally handle that. Following
possible solutions were considered:
1. Allow .read_post_process() to realloc (resize) content buffer
2. Allow .read_post_process() to adjust (decrease) just buffer length
3. Register NVMEM cells using post-read sizes

The preferred solution was the last one. The problem is that simply
adjusting "bytes" in NVMEM providers would result in core code NOT
passing whole raw data to .read_post_process() callbacks. It means
callback functions couldn't do their job without somehow manually
reading original cell content on their own.

This patch deals with that by registering NVMEM cells with both lengths:
raw content one and post read one. It allows:
1. Core code to read whole raw cell content
2. Callbacks to return content they want
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-35-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/core.c

@@ -50,6 +50,7 @@ struct nvmem_device {
 struct nvmem_cell_entry {
 	const char		*name;
 	int			offset;
+	size_t			raw_len;
 	int			bytes;
 	int			bit_offset;
 	int			nbits;
@@ -469,6 +470,7 @@ static int nvmem_cell_info_to_nvmem_cell_entry_nodup(struct nvmem_device *nvmem,
 {
 	cell->nvmem = nvmem;
 	cell->offset = info->offset;
+	cell->raw_len = info->raw_len ?: info->bytes;
 	cell->bytes = info->bytes;
 	cell->name = info->name;
 	cell->read_post_process = info->read_post_process;
@@ -1560,7 +1562,7 @@ static int __nvmem_cell_read(struct nvmem_device *nvmem,
 {
 	int rc;
 
-	rc = nvmem_reg_read(nvmem, cell->offset, buf, cell->bytes);
+	rc = nvmem_reg_read(nvmem, cell->offset, buf, cell->raw_len);
 
 	if (rc)
 		return rc;
@@ -1571,7 +1573,7 @@ static int __nvmem_cell_read(struct nvmem_device *nvmem,
 
 	if (cell->read_post_process) {
 		rc = cell->read_post_process(cell->priv, id, index,
-					     cell->offset, buf, cell->bytes);
+					     cell->offset, buf, cell->raw_len);
 		if (rc)
 			return rc;
 	}
@@ -1594,14 +1596,15 @@ static int __nvmem_cell_read(struct nvmem_device *nvmem,
  */
 void *nvmem_cell_read(struct nvmem_cell *cell, size_t *len)
 {
-	struct nvmem_device *nvmem = cell->entry->nvmem;
+	struct nvmem_cell_entry *entry = cell->entry;
+	struct nvmem_device *nvmem = entry->nvmem;
 	u8 *buf;
 	int rc;
 
 	if (!nvmem)
 		return ERR_PTR(-EINVAL);
 
-	buf = kzalloc(cell->entry->bytes, GFP_KERNEL);
+	buf = kzalloc(max_t(size_t, entry->raw_len, entry->bytes), GFP_KERNEL);
 	if (!buf)
 		return ERR_PTR(-ENOMEM);
 

include/linux/nvmem-provider.h

@@ -51,6 +51,7 @@ struct nvmem_keepout {
  * struct nvmem_cell_info - NVMEM cell description
  * @name:	Name.
  * @offset:	Offset within the NVMEM device.
+ * @raw_len:	Length of raw data (without post processing).
  * @bytes:	Length of the cell.
  * @bit_offset:	Bit offset if cell is smaller than a byte.
  * @nbits:	Number of bits.
@@ -62,6 +63,7 @@ struct nvmem_keepout {
 struct nvmem_cell_info {
 	const char		*name;
 	unsigned int		offset;
+	size_t			raw_len;
 	unsigned int		bytes;
 	unsigned int		bit_offset;
 	unsigned int		nbits;