linux-kernel - Re: [PATCH 0/2] pinctrl: meson: add g12a drive strength support

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Date:   Mon, 25 Mar 2019 10:44:00 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Kevin Hilman <khilman@...libre.com>,
        Gołaszewski, Bartosz 
        <bgolaszewski@...libre.com>, Kevin Hilman <khilman@...libre.com>
Cc:     devicetree@...r.kernel.org, linux-amlogic@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        Guillaume La Roque <glaroque@...libre.com>
Subject: Re: [PATCH 0/2] pinctrl: meson: add g12a drive strength support

On Thu, 2019-03-14 at 17:37 +0100, Jerome Brunet wrote:
> The purpose of this patchset is to add drive-strength support in meson pinconf
> driver. This is a new feature that was added on the g12a. It is critical for us
> to support this since many functions are failing with default pad drive-strength.
> 
> Now the slightly annoying part :(
> The value achievable by the SoC are 0.5mA, 2.5mA, 3mA and 4mA and the DT property
> 'drive-strength' is expressed in mA.
> 
> 1) Rounding down the value, we could be requesting a 0mA drive strength.
>    That would look weird.
> 2) Rounding up, we can't distinguish between 2.5mA and 3mA
> 
> To solve this issue in this in this v1, we chose to document that, on Amlogic,
> drive-strength is expressed in uA instead of mA.
> It works well and there is no impact on the other platforms but I'm not sure this
> is really OK with the DT rules ?
> 
> Linus, if this is not OK with you, here are 2 other options we are
> considering. We would be very interested to get your opinion on the matter:
> 
> 1) instead the generic 'drive-strength' property, we could add an amlogic
> specific property, 'amlogic,drive-strength'. It would be expressed in uA
> and parsed in amlogic specific code.
> I think this option is kind of overkill. Expressing drive strength in uA is
> not really amlogic specific so it does not make much sense, but it would
> work ...
> 
> 2) Add another generic property "drive-strength-uA". The change to do so
> would be minimal and could be benefit to other platforms later on.

Hi Linus,

I know it has only been 10 days and you must be busy but I was wondering if we
could get your view on the issue above ?

Since the vast majority of SoC functions need a drive strength setting, DT
patches are somehow blocked until we decide which binding to use for it.

Sorry for this early ping.
Jerome


> 
> Cheers
> Jerome
> 
> Guillaume La Roque (2):
>   dt-bindings: pinctrl: meson: Add drive-strength property
>   pinctrl: meson: add support of drive-strength
> 
>  .../bindings/pinctrl/meson,pinctrl.txt        |   3 +
>  drivers/pinctrl/meson/pinctrl-meson-g12a.c    |  36 ++--
>  drivers/pinctrl/meson/pinctrl-meson.c         | 166 +++++++++++++-----
>  drivers/pinctrl/meson/pinctrl-meson.h         |  20 ++-
>  4 files changed, 165 insertions(+), 60 deletions(-)
>