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Subject[PATCH 4/4] nvmem: core: Support reading cells with >= 8 bit offsets
Date
For NVMEM devices with .stride > 1, some cell values may not be aligned
to the device's stride. In this case, it is necessary to use bit_offset
to access the cell. For example, to access the third byte of an NVMEM
device with .stride == 4, we need "bits = <16 8>;" in the devicetree.

Implement this on the read side. The write side implementation would be
more complicated, and it is not necessary for read-only NVMEM devices.
For now, reject writes for these cells to avoid any incorrect behavior.

Signed-off-by: Samuel Holland <samuel@sholland.org>
---

drivers/nvmem/core.c | 43 ++++++++++++++++++++++++-------------------
1 file changed, 24 insertions(+), 19 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 1e3c754efd0d..309beba8c9f0 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1373,63 +1373,67 @@ void nvmem_cell_put(struct nvmem_cell *cell)
}
EXPORT_SYMBOL_GPL(nvmem_cell_put);

-static void nvmem_shift_read_buffer_in_place(struct nvmem_cell_entry *cell, void *buf)
+static int nvmem_shift_read_buffer_in_place(struct nvmem_cell_entry *cell, void *buf)
{
+ int bit_offset = cell->bit_offset, bytes, i;
u8 *p, *b;
- int i, extra, bit_offset = cell->bit_offset;

p = b = buf;
if (bit_offset) {
+ int byte_offset = bit_offset / BITS_PER_BYTE;
+
+ b += byte_offset;
+ bit_offset %= BITS_PER_BYTE;
+ bytes = cell->bytes - byte_offset;
+
/* First shift */
- *b++ >>= bit_offset;
+ *p = *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);
-
- p = b;
- *b++ >>= bit_offset;
+ *p++ |= *b << (BITS_PER_BYTE - bit_offset);
+ *p = *b++ >> bit_offset;
}
- } else {
- /* point to the msb */
- p += cell->bytes - 1;
}

/* result fits in less bytes */
- extra = cell->bytes - DIV_ROUND_UP(cell->nbits, BITS_PER_BYTE);
- while (--extra >= 0)
- *p-- = 0;
+ bytes = DIV_ROUND_UP(cell->nbits, BITS_PER_BYTE);
+ p = buf + bytes;
+ memset(p, 0, cell->bytes - bytes);

/* clear msb bits if any leftover in the last byte */
if (cell->nbits % BITS_PER_BYTE)
- *p &= GENMASK((cell->nbits % BITS_PER_BYTE) - 1, 0);
+ p[-1] &= GENMASK((cell->nbits % BITS_PER_BYTE) - 1, 0);
+
+ return bytes;
}

static int __nvmem_cell_read(struct nvmem_device *nvmem,
struct nvmem_cell_entry *cell,
void *buf, size_t *len, const char *id)
{
+ int bytes = cell->bytes;
int rc;

- rc = nvmem_reg_read(nvmem, cell->offset, buf, cell->bytes);
+ rc = nvmem_reg_read(nvmem, cell->offset, buf, bytes);

if (rc)
return rc;

/* shift bits in-place */
if (cell->bit_offset || cell->nbits)
- nvmem_shift_read_buffer_in_place(cell, buf);
+ bytes = nvmem_shift_read_buffer_in_place(cell, buf);

if (nvmem->cell_post_process) {
rc = nvmem->cell_post_process(nvmem->priv, id,
- cell->offset, buf, cell->bytes);
+ cell->offset, buf, bytes);
if (rc)
return rc;
}

if (len)
- *len = cell->bytes;
+ *len = bytes;

return 0;
}
@@ -1526,6 +1530,7 @@ static int __nvmem_cell_entry_write(struct nvmem_cell_entry *cell, void *buf, si
int rc;

if (!nvmem || nvmem->read_only ||
+ cell->bit_offset >= BITS_PER_BYTE ||
(cell->bit_offset == 0 && len != cell->bytes))
return -EINVAL;

--
2.35.1
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