On 27/02/2021 00:26, Douglas Anderson wrote:
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.
If the consumer driver is expecting the sizes to span around byte to
many bytes, then, Why not just call nvmem_cell_read() which should also
return you how many bytes it has read!
Let's solve this by allowing clients to read a "larger" value. We'll
just fill it in with 0.
That is misleading the consumer! If the consumer is expecting a u16 or
u32, cell size should be of that size!!
--srini
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@xxxxxxxxxxxx>
---
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.
