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

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Date:   Thu, 14 Mar 2019 17:37:23 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Kevin Hilman <khilman@...libre.com>
Cc:     Jerome Brunet <jbrunet@...libre.com>, 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: [PATCH 0/2] pinctrl: meson: add g12a drive strength support

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.

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(-)

-- 
2.20.1