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Date:   Sun, 3 Jan 2021 14:48:29 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Philip Chen <philipchen@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dmitry Torokhov <dtor@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Rajat Jain <rajatja@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Guenter Roeck <groeck@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Simon Glass <sjg@...omium.org>, devicetree@...r.kernel.org,
        linux-input@...r.kernel.org
Subject: Re: [PATCH 1/3] dt-bindings: input: cros-ec-keyb: Add a new property

Hi Philip,

On Sat, Jan 02, 2021 at 10:11:21PM -0800, Philip Chen wrote:
> Hi Dmitry,
> 
> I have one more question below.
> Could you take a look?
> 
> On Sat, Jan 2, 2021 at 8:53 PM Philip Chen <philipchen@...omium.org> wrote:
> >
> > Hi Dmitry,
> >
> > I see.
> > I'll update these patch sets shortly based on your suggestion.
> > Thanks.
> >
> > On Sat, Jan 2, 2021 at 1:04 PM Dmitry Torokhov
> > <dmitry.torokhov@...il.com> wrote:
> > >
> > > On Sat, Jan 02, 2021 at 11:39:34AM -0800, Philip Chen wrote:
> > > > Hi Dmitry,
> > > >
> > > > Thanks for reviewing my patch over the holiday season.
> > > > Please check my CIL.
> > > >
> > > > On Mon, Dec 28, 2020 at 10:18 PM Dmitry Torokhov
> > > > <dmitry.torokhov@...il.com> wrote:
> > > > >
> > > > > Hi Philip,
> > > > >
> > > > > On Mon, Dec 21, 2020 at 05:47:57PM -0800, Philip Chen wrote:
> > > > > > This patch adds a new property `google,custom-keyb-top-row` to the
> > > > > > device tree for the custom keyboard top row design.
> > > > >
> > > > > Why don't we use the property we have for the same purpose in atkbd.c?
> > > > > I.e. function-row-physmap?
> > > > >
> > > > Because this property serves a different purpose than function-row-physmap.
> > > > `function-row-physmap` basically links the scancode to the physical
> > > > position in the top row.
> > > > `google,custom-keyb-top-row` aims at specifying the board-specific
> > > > keyboard top row design associated with the action codes.
> > > >
> > > > In x86 path, the board-specific keyboard top row design associated
> > > > with the action codes is exposed from coreboot to kernel through
> > > > "linux,keymap" acpi table.
> > > > When coreboot generates this acpi table, it asks EC to provide this
> > > > information, since we add the board-specific top-row-design in EC
> > > > codebase.
> > > > (E.g. https://chromium.googlesource.com/chromiumos/platform/ec/+/refs/heads/main/board/jinlon/board.c#396)
> > > >
> > > > In ARM, we don't plan to involve EC in the vivaldi support stack.
> > > > So `google,custom-keyb-top-row` DT property is our replacement for the
> > > > board-specific top-row-design in x86 EC codebase.
> > >
> > > I disagree with this decision. We already have "linux,keymap" property
> > > that is supposed to hold accurate keymap for the device in question,
> > > there should be no need to introduce yet another property to adjust the
> > > keymap to reflect the reality. If a device uses "non classic" ChromeOS
> > > top row it should not be using the default keymap from
> > > arch/arm/boot/dts/cros-ec-keyboard.dtsi but supply its own. You can
> > > consider splitting the keymap into generic lower portion and the top row
> > > and moving them into an .h file so they can be easily reused.
> > >
> > > >
> > > > > Also, instead of specifying keycodes in this array we should use
> > > > > combination of row and column identifying keys, like this:
> > > > >
> > > > >         function-row-physmap = <
> > > > >                 MATRIX_KEY(0x00, 0x02, KEY_F1),
> > > > >                 MATRIX_KEY(0x03, 0x02, KEY_F2),
> > > > >                 ...
> > > > >         >;
> > > >
> > > > This mapping between row/column to function keycode is fixed for all
> > > > Chrome OS devices.
> > >
> > > *for now* The mapping for the rest of the keyboard has also stayed
> > > static, but we still did not hardcode this information in the driver but
> > > rather used DT property to pass it into the kernel.
> > >
> > > > So we don't really need to host this information in DT.
> > > > Instead, I plan to hardcode this information in cros_ec_keyb.c.
> > > > (Please see the array "top_row_key_pos[]" in my next patch: "[2/3]
> > > > Input: cros_ec_keyb - Support custom top-row keys".)
> > > >
> > > > The only thing that could make the function-row-physmap file different
> > > > among boards is the number of top row keys.
> Given the reason above, can we just add `num-top-row-keys` property
> instead of the whole `function-row-physmap`?
> I think this is the only thing cros_ec_keyb needs to know to generate
> the board-specific function-row-physmap file for the userspace.

This would mean that we need to hard-code the knowledge of the scan
matrix in the driver and will not allow us to "skip" any keys in the top
row. Given that we did not hard-code the keymap I do not see why we
would want to do it differently with the top row. function-row-physmap
provides greatest flexibility and I do not see any downsides.

Thanks.

-- 
Dmitry

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