linux-kernel - Re: [PATCH v1 RFC 1/3] dt-bindings: serial: amlogic,meson-uart: Add compatible string for G12A

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Date:   Thu, 23 Feb 2023 21:14:14 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     linux-amlogic@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        jirislaby@...nel.org, neil.armstrong@...aro.org,
        krzysztof.kozlowski+dt@...aro.org, robh+dt@...nel.org,
        gregkh@...uxfoundation.org,
        Christian Hewitt <christianshewitt@...il.com>
Subject: Re: [PATCH v1 RFC 1/3] dt-bindings: serial: amlogic,meson-uart: Add
 compatible string for G12A

Hello Krzysztof,

On Thu, Feb 23, 2023 at 10:12 AM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 22/02/2023 22:04, Martin Blumenstingl wrote:
> > Amlogic G12A SoCs gained a new "divide XTAL by 2" bit. Everything else
> > (we know about) is identical to the UART IP on GX (GXBB/GXL/GXM) SoCs.
> > Add a new compatible string for this SoC so this new bit can be managed
> > accordingly.
> >
> > Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> > ---
> > Why am I sending this as RFC? The last change in this series means
> > that the resulting .dtbs are not compatible with old kernels anymore.
> > My understanding is that this is fine and only the opposite case (using
> > old .dtbs on new kernels) has to be supported (which is still the case
> > with this series). I'd like to get some confirmation for this.
>
> The other way around is also nice to have, because DTS is used in other
> projects. You fixed here Linux kernel, but what about all other
> out-of-tree kernels, BSDs, firmwares and bootloaders? Did you fix them
> as well?
Indeed, u-boot is of concern here (as mainline u-boot does have Meson
G12A SoC support).

> The question is whether the devices can be made compatible thus keeping
> DTS working on older kernel. This commit suggests they are. Your DTS
> commit is written in different tone - something was broken and is being
> fixed.
If we keep "amlogic,meson-gx-uart" as fallback compatible string then
old kernels (or other .dtb consumers - like u-boot) would still work.
Without the new "amlogic,meson-g12a-uart" compatible string we're
unable to make use of a newly added clock divider within the UART IP
block which allows baud rates such as 1500000 to work without (a lot
of) jitter. Old kernels - with the new .dtb - would still be able to
use serial (thanks to the "amlogic,meson-gx-uart" fallback compatible
string) albeit with limited divider support (so not all baud rates can
be used).
Is this a valid plan?


Best regards,
Martin