linux-kernel - Re: [PATCH 1/3] nvmem: core: Add functions to make number reading easy

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Message-ID: <CAD=FV=WU1-5gtgBz1SNdS7Vkf8QLivrVdH4O2p1Vn-X57g3-dw@mail.gmail.com>
Date:   Wed, 10 Mar 2021 07:50:02 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Rob Clark <robdclark@...il.com>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Akhil P Oommen <akhilpo@...eaurora.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] nvmem: core: Add functions to make number reading easy

Hi,

On Wed, Mar 10, 2021 at 2:37 AM Srinivas Kandagatla
<srinivas.kandagatla@...aro.org> wrote:
>
>
>
> On 06/03/2021 00:26, Douglas Anderson wrote:
> > Sometimes the clients of nvmem just want to get a number out of
> > nvmem. They don't want to think about exactly how many bytes the nvmem
> > cell took up. They just want the number. Let's make it easy.
> >
> > In general this concept is useful because nvmem space is precious and
> > usually the fewest bits are allocated that will hold a given value on
> > a given system. However, even though small numbers might be fine on
> > one system that doesn't mean that logically the number couldn't be
> > bigger. Imagine nvmem containing a max frequency for a component. On
> > one system perhaps that fits in 16 bits. On another system it might
> > fit in 32 bits. The code reading this number doesn't care--it just
> > wants the number.
> >
> > We'll provide two functions: nvmem_cell_read_variable_le_u32() and
> > nvmem_cell_read_variable_le_u64().
> >
> > Comparing these to the existing functions like nvmem_cell_read_u32():
> > * These new functions have no problems if the value was stored in
> >    nvmem in fewer bytes. It's OK to use these function as long as the
> >    value stored will fit in 32-bits (or 64-bits).
> > * These functions avoid problems that the earlier APIs had with bit
> >    offsets. For instance, you can't use nvmem_cell_read_u32() to read a
> >    value has nbits=32 and bit_offset=4 because the nvmem cell must be
> >    at least 5 bytes big to hold this value. The new API accounts for
> >    this and works fine.
> > * These functions make it very explicit that they assume that the
> >    number was stored in little endian format. The old functions made
> >    this assumption whenever bit_offset was non-zero (see
> >    nvmem_shift_read_buffer_in_place()) but didn't whenever the
> >    bit_offset was zero.
> >
> > NOTE: it's assumed that we don't need an 8-bit or 16-bit version of
> > this function. The 32-bit version of the function can be used to read
> > 8-bit or 16-bit data.
> >
> > At the moment, I'm only adding the "unsigned" versions of these
> > functions, but if it ends up being useful someone could add a "signed"
> > version that did 2's complement sign extension.
> >
> > At the moment, I'm only adding the "little endian" versions of these
> > functions. Adding the "big endian" version would require adding "big
> > endian" support to nvmem_shift_read_buffer_in_place().
> >
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> > This is a logical follow-up to:
> >    https://lore.kernel.org/r/20210227002603.3260599-1-dianders@chromium.org/
> > ...but since it doesn't really share any of the same patches I'm not
> > marking it as a v2.
> >
> >   drivers/nvmem/core.c           | 95 ++++++++++++++++++++++++++++++++++
> >   include/linux/nvmem-consumer.h |  4 ++
> >   2 files changed, 99 insertions(+)
> >
>
> This patch as it is LGTM.
>
> If you plan to take this via other trees, here is
>
> Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>

Thanks! I think none of this is terribly urgent, though. Unless
someone has a different opinion, my thought would be:

* This patch lands in your tree for 5.13.

* I'll snooze the email for 2 months and poke patch #2 and #3 once
5.13-rc1 is out.

Does that sound OK to you?

-Doug