linux-kernel - Re: [PATCH] clk: renesas: r9a06g032: Fix UART34567 clock rate

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Message-ID: <CAMuHMdWCjHEZsHHq4+tEK2o-Qj3OuGpFZBFiRxHgjRqMJT0PPw@mail.gmail.com>
Date:   Mon, 10 Sep 2018 15:11:15 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Phil Edworthy <phil.edworthy@...esas.com>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: renesas: r9a06g032: Fix UART34567 clock rate

Hi Phil,

On Fri, Aug 31, 2018 at 1:26 PM Phil Edworthy <phil.edworthy@...esas.com> wrote:
> The clock for UARTs 0 through 2 is UART012, the clock for UARTs 3 through
> 7 is UART34567.
> For UART012, we stop the clock driver from changing the clock rate. This
> is because the Synopsys UART driver simply sets the reference clock to 16x
> the baud rate, but doesn't check if the actual rate is within the required
> tolerance. The RZ/N1 clock divider can't provide this (we have to rely on
> the UART's internal divider to set the correct clock rate), so you end up
> with a clock rate that is way off what you wanted.
>
> In addition, since the clock is shared between multiple UARTs, you don't
> want the driver trying to change the clock rate as it may affect the other
> UARTs (which may not have been configured yet, so you don't know what baud
> rate they will use). Normally, the clock rate is set early on before Linux
> to some very high rate that supports all of the clock rates you want.
>
> This change stops the UART34567 clock rate from changing for the same
> reasons.
>
> Signed-off-by: Phil Edworthy <phil.edworthy@...esas.com>

Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
i.e. will queue in clk-renesas-for-v4.20.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds