[PATCH 2/3] nvmem: core: Allow nvmem_cell_read_u16/32/64 to read smaller cells

[Douglas Anderson <dianders@chromium.org>]

The current way that cell "length" is specified for nvmem cells is a
little fuzzy. For instance, let's look at the gpu speed bin currently
in sc7180.dtsi:

  gpu_speed_bin: gpu_speed_bin@1d2 {
    reg = <0x1d2 0x2>;
    bits = <5 8>;
  };

This is an 8-bit value (as specified by the "bits" field). However,
it has a "length" of 2 (bytes), presumably because the value spans
across two bytes.

When querying this value right now, it's hard for a client to know if
they should be calling nvmem_cell_read_u16() or nvmem_cell_read_u8().
Today they must call nvmem_cell_read_u16() because the "length" of the
cell was 2 (bytes). However, if a later SoC ever came around and
didn't span across 2 bytes it would be unclear.  If a later Soc
specified, for instance:

  gpu_speed_bin: gpu_speed_bin@100 {
    reg = <0x100 0x1>;
    bits = <0 8>;
  };

...then the caller would need to change to try calling
nvmem_cell_read_u8() because the u16 version would fail.

Let's solve this by allowing clients to read a "larger" value. We'll
just fill it in with 0. If a client truly wants to know exactly how
big the cell was they can fall back to calling nvmem_cell_read()
directly.

NOTE: current implementation assumes that cells are little endian when
up-casting the size, but that's already pretty implicit in the way
nvmem works now anyway. See nvmem_shift_read_buffer_in_place(). Let's
document this but not do any auto-conversions just in case there was
an instance where someone was using this API to read big endian data
on a big endian machine and it happened to be working because there
was no bit offset.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
I will freely admit that I always end up thinking in circles and
getting dizzy when I think too much about endian considerations. If
anyone has better intuition than I do and see that I've goofed this up
then please yell.

 drivers/nvmem/core.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index a5ab1e0c74cf..8602390bb124 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1534,12 +1534,14 @@ static int nvmem_cell_read_common(struct device *dev, const char *cell_id,
 		nvmem_cell_put(cell);
 		return PTR_ERR(buf);
 	}
-	if (len != count) {
+	if (len > count) {
 		kfree(buf);
 		nvmem_cell_put(cell);
 		return -EINVAL;
+	} else if (len != count) {
+		memset(val + len, 0, count - len);
 	}
-	memcpy(val, buf, count);
+	memcpy(val, buf, len);
 	kfree(buf);
 	nvmem_cell_put(cell);
 
@@ -1564,6 +1566,11 @@ EXPORT_SYMBOL_GPL(nvmem_cell_read_u8);
 /**
  * nvmem_cell_read_u16() - Read a cell value as a u16
  *
+ * This function can be used to read any cell value 16-bits or less.  If
+ * this function needs to upcast (or if the cell was stored in nvmem with
+ * a bit offset) it will assume that the cell is little endian.  Clients
+ * should generally call le16_to_cpu() on the returned value.
+ *
  * @dev: Device that requests the nvmem cell.
  * @cell_id: Name of nvmem cell to read.
  * @val: pointer to output value.
@@ -1579,6 +1586,11 @@ EXPORT_SYMBOL_GPL(nvmem_cell_read_u16);
 /**
  * nvmem_cell_read_u32() - Read a cell value as a u32
  *
+ * This function can be used to read any cell value 32-bits or less.  If
+ * this function needs to upcast (or if the cell was stored in nvmem with
+ * a bit offset) it will assume that the cell is little endian.  Clients
+ * should generally call le32_to_cpu() on the returned value.
+ *
  * @dev: Device that requests the nvmem cell.
  * @cell_id: Name of nvmem cell to read.
  * @val: pointer to output value.
@@ -1594,6 +1606,11 @@ EXPORT_SYMBOL_GPL(nvmem_cell_read_u32);
 /**
  * nvmem_cell_read_u64() - Read a cell value as a u64
  *
+ * This function can be used to read any cell value 64-bits or less.  If
+ * this function needs to upcast (or if the cell was stored in nvmem with
+ * a bit offset) it will assume that the cell is little endian.  Clients
+ * should generally call le64_to_cpu() on the returned value.
+ *
  * @dev: Device that requests the nvmem cell.
  * @cell_id: Name of nvmem cell to read.
  * @val: pointer to output value.
-- 
2.30.1.766.gb4fecdf3b7-goog