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

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Date:   Fri, 24 Dec 2021 18:25:18 +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 Tue, Dec 21, 2021 at 8:17 AM Yu Tu <yu.tu@...ogic.com> wrote:
>
> The S4 SoC on-chip UART uses a 12M clock as the clock source for
> calculating the baud rate of the UART. But previously, chips used 24M or
> other clock sources. So add this change. The specific clock source is
> determined by chip design.
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.

SoCs up to GXL and GXM had an internal divide-by-3 (clock divider) in
the UART controller IP and an external 24MHz XTAL.
This was not configurable, so the clock for all baud-rates had to be
derived from an 8MHz (24MHz divided by 3) clock.

With the A311D (G12B, which is still using an external 24MHz XTAL) SoC
the UART controller gained two new bits - with configurable dividers -
according to the public datasheets:
UART_EE_A_REG5[26]:
- 0x0: divide the input clock by 3 (meaning: this internally works
with an 8MHz clock)
- 0x1: use the input clock directly without further division (meaning:
this internally work with an 24MHz clock)
UART_EE_A_REG5[27]:
- 0x0: use the clock as configured in UART_EE_A_REG5[26]
- 0x1: divide the input clock by 2 (meaning: this internally works
with an 12MHz clock)

While writing this email I did some investigation and found that
UART_EE_A_REG5[26] is used in the vendor kernel even for GXL and GXM
SoCs.
So this probably has been introduced with the GXL generation (and thus
is missing on GXBB and earlier SoCs).
Also UART_EE_A_REG5[27] seems to have been introduced with the G12A
generation of SoCs (not surprising since G12A and G12B peripherals are
very similar).

Does the UART controller not work with divide-by-3 (as we have it
today) or are these configurable dividers to reduce jitter?

Best regards,
Martin