linux-kernel - [PATCH 1/2] nvmem: Handle reading cells with bit offset

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Message-Id: <20221112120447.36877-1-sven@svenpeter.dev>
Date:   Sat, 12 Nov 2022 13:04:46 +0100
From:   Sven Peter <sven@...npeter.dev>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     Sven Peter <sven@...npeter.dev>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] nvmem: Handle reading cells with bit offset > 8

Some nvmem controllers like Apple's eFuses or Nintendo's OTP have a cell
size that's larger than one byte. Consumers may however still need
access to a subset of bits that start after the first byte. Handle this
inside nvmem_shift_read_buffer_in_place().

Signed-off-by: Sven Peter <sven@...npeter.dev>
---
 drivers/nvmem/core.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 321d7d63e068..a5b8d6989f8e 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1379,15 +1379,23 @@ EXPORT_SYMBOL_GPL(nvmem_cell_put);
 static void nvmem_shift_read_buffer_in_place(struct nvmem_cell_entry *cell, void *buf)
 {
 	u8 *p, *b;
-	int i, extra, bit_offset = cell->bit_offset;
+	int i, padding, extra, bit_offset = cell->bit_offset;
+	int bytes = cell->bytes;
 
 	p = b = buf;
 	if (bit_offset) {
+		padding = bit_offset/8;
+		if (padding) {
+			memmove(buf, buf + padding, bytes - padding);
+			bit_offset -= BITS_PER_BYTE * padding;
+			bytes -= padding;
+		}
+
 		/* First shift */
 		*b++ >>= bit_offset;
 
 		/* setup rest of the bytes if any */
-		for (i = 1; i < cell->bytes; i++) {
+		for (i = 1; i < bytes; i++) {
 			/* Get bits from next byte and shift them towards msb */
 			*p |= *b << (BITS_PER_BYTE - bit_offset);
 
@@ -1400,7 +1408,7 @@ static void nvmem_shift_read_buffer_in_place(struct nvmem_cell_entry *cell, void
 	}
 
 	/* result fits in less bytes */
-	extra = cell->bytes - DIV_ROUND_UP(cell->nbits, BITS_PER_BYTE);
+	extra = bytes - DIV_ROUND_UP(cell->nbits, BITS_PER_BYTE);
 	while (--extra >= 0)
 		*p-- = 0;
 
-- 
2.25.1