linux-kernel - Re: [PATCH 3/3] tty: serial: meson: add UART driver compatible with S4 SoC on-chip

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Date:   Mon, 27 Dec 2021 21:04:13 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Yu Tu <yu.tu@...ogic.com>
Cc:     linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>
Subject: Re: [PATCH 3/3] tty: serial: meson: add UART driver compatible with
 S4 SoC on-chip

Hello,

On Mon, Dec 27, 2021 at 7:56 AM Yu Tu <yu.tu@...ogic.com> wrote:
[...]
> > Does the new S4 SoC use an external 12MHz XTAL or does it use a 24MHz XTAL?
> > If there's still a 24MHz XTAL then I think this description is not
> > correct - at least based on how I understand the UART controller.
> >
> The S4 SoC uses 12MHz(UART_EE_A_REG5[27]=0x1,the bit is set in romcode).
> This register description is the same as the G12A and G12B you know.
Thank you for this explanation!
So the problem is that we're not touching bit 26 and bit 27 - and with
the updated romcode you would not get any serial output since the
divider is calculated off the wrong clock.

I agree with Jerome that we shouldn't put a flag in device-tree.

Also I did some experimenting with Jerome's idea to implement the
clocks using CCF (common clock framework), see the attached patches.
It was a bit tricky because some initial clean-ups were needed in the
serial driver.
Note: I have only briefly tested this on a 32-bit Meson8m2 SoC, see my
attached patches and the clk_summary debugfs output.
In fact, I expect that there are some issues with at least one of the
patches as the whole bit 26 and bit 27 code is untested.

Do you see any problems with this patch?
Could you try to implement CCF support with the idea from the attached
patches (you don't need to re-use them, I just wrote them to make it
clearer in our discussion what we're talking about).

Best regards,
Martin

View attachment "clk-debug-output.txt" of type "text/plain" (1702 bytes)

View attachment "0001-tty-serial-meson-Drop-the-legacy-compatible-strings-.patch" of type "text/x-patch" (2747 bytes)

View attachment "0002-tty-serial-meson-Request-the-register-region-in-meso.patch" of type "text/x-patch" (2230 bytes)

View attachment "0003-tty-serial-meson-UART-clk-test-TODO.patch" of type "text/x-patch" (13748 bytes)