linux-kernel - Re: [PATCH v6 1/2] media: rc: meson-ir-tx: document device tree bindings

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Date:   Sat, 17 Jul 2021 21:57:37 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Viktor Prutyanov <viktor.prutyanov@...stech.edu>
Cc:     sean@...s.org, mchehab@...nel.org, robh+dt@...nel.org,
        khilman@...libre.com, Neil Armstrong <narmstrong@...libre.com>,
        jbrunet@...libre.com, linux-media <linux-media@...r.kernel.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, rockosov@...il.com
Subject: Re: [PATCH v6 1/2] media: rc: meson-ir-tx: document device tree bindings

Hi Viktor,

On Fri, Jul 16, 2021 at 4:45 PM Viktor Prutyanov
<viktor.prutyanov@...stech.edu> wrote:
[...]
> +  amlogic,fifo-threshold:
> +    description: TX FIFO threshold
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    minimum: 0
> +    maximum: 127
I tried to make sense of this property but I don't understand it yet
(even after reading the second patch in this series).
A "FIFO" (in my own words) is some physical property of the IR
transmitter in these Amlogic SoCs.
So for one specific SoC there can only be one FIFO size, not a range (0..127).

What about a value of 0: my understanding is that this means that
there's no FIFO. Will this hardware still work in that case?

>From reading the driver code it seems to me that the FIFO size is 128.
The driver can use fewer FIFO entries if it wants, but this must not
affect the dt-bindings (as these describe the hardware - they don't
"configure" the driver).
If you look at arch/arm64/boot/dts/amlogic/meson-g12.dtsi you'll find
toddr_a, toddr_b and toddr_c there:
All three of them use identical circuitry internally, except that
toddr_a has a bigger FIFO size than the other two. Using a FIFO size
of 256 for toddr_a is not correct as it's an improper description of
the toddr_a hardware (that said, the driver can still decide that it
only wants to use 256 FIFO entries).


Best regards,
Martin