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

[Doug Anderson]

Hi,

On Fri, Mar 5, 2021 at 2:27 AM Srinivas Kandagatla
<srinivas.kandagatla@xxxxxxxxxx> wrote:
>
>
>
> 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

I guess in my mind that's outside of the scope of what the consumer
should need to know.  The consumer wants a number and they know it's
stored in nvmem.  They shouldn't need to consider the bit packing
within nvmem.  Imagine that have a structure definition:

struct example {
  int num1:6;
  int num2:6;
  int num3:6;
  int num4:6;
};
struct example e;

What I think you're saying is that you should need a different syntax
for accessing "e.num1" and "e.num4" (because they happen not to span
bytes) compared to accessing "e.num2" and "e.num3". As it is, C
abstracts this out and allows you not to care. You can just do:

e.num1 + e.num2 + e.num3 + e.num4

...and it works fine even though some of those span bytes and some
don't.  I want the same thing.


> , then, Why not just call nvmem_cell_read() which should also
> return you how many bytes it has read!

See my response to patch #1. This requires open-coding a small but
still non-trivial bit of code for all consumers. It should be in the
core.


> > 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!!

If you think it's confusing to change the behavior of the existing
functions, would you be opposed to me adding a new function like
nvmem_cell_read_le_u32_or_smaller() (or provide me a better name) that
would be flexible like this?

-Doug