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References: <20210517203724.1006254-1-martin.blumenstingl@googlemail.com> <1jwnrw1ohh.fsf@starbuckisacylon.baylibre.com>
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From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Date:   Tue, 18 May 2021 22:20:57 +0200
Message-ID: <CAFBinCCnEXNLL0rvi1XNYKb0xY1+3KMwB=xz24kPgN2H97yzOg@mail.gmail.com>
Subject: Re: [PATCH RFC v1 0/3] clk: meson: rounding for fast clocks on 32-bit SoCs
To:     Jerome Brunet <jbrunet@baylibre.com>
Cc:     mturquette@baylibre.com, sboyd@kernel.org,
        Neil Armstrong <narmstrong@baylibre.com>,
        linux-clk@vger.kernel.org, khilman@baylibre.com,
        linux-kernel@vger.kernel.org, linux-amlogic@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
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Hi Jerome,

On Tue, May 18, 2021 at 9:37 AM Jerome Brunet <jbrunet@baylibre.com> wrote:
>
>
> On Mon 17 May 2021 at 22:37, Martin Blumenstingl <martin.blumenstingl@googlemail.com> wrote:
>
> > On the 32-bit Amlogic Meson8/8b/8m2 SoCs we run into a problem with the
> > fast HDMI PLL and it's OD (post-dividers). This clock tree can run at
> > up to approx. 3GHz.
> > This however causes a problem, because these rates require BIT(31) to
> > be usable. Unfortunately this is not the case with clk_ops.round_rate
> > on 32-bit systems. BIT(31) is reserved for the sign (+ or -).
> >
> > clk_ops.determine_rate does not suffer from this limitation. It uses
> > an int to signal any errors and can then take all availble 32 bits for
> > the clock rate.
> >
> > I am sending this as RFC to start a discussion whether:
> > - this is a good way to solve it?
>
> .determine_rate() was meant to replace .round_rate() so I guess it is
> good to do it :)
ah, now things make more sense.
thanks for the background info

> > - what are the alternatives?
>
> I don't see any ATM. Even with determine_rate(), 4.29GHz limitation
> seems a bit low nowadays. In AML SoC, most PLLs should be able to reach
> 6GHz ... hopefully we won't need that on the 32bits variant ;)
according to the public datasheet the maximum PLL frequency is at around 3GHz
so I also hope that we're safe with this


Martin