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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/nvmem-provider.h b/include/linux/nvmem-provider.h
index 0cf9f9490514..8ffb42ba0f62 100644
--- a/include/linux/nvmem-provider.h
+++ b/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;