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Date:   Sat, 9 Dec 2023 00:10:13 +0000
From:   Daniel Golle <daniel@makrotopia.org>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Sabrina Dubroca <sd@queasysnail.net>,
        Jianhui Zhao <zhaojh329@gmail.com>,
        Chen-Yu Tsai <wenst@chromium.org>,
        "Garmin.Chang" <Garmin.Chang@mediatek.com>,
        Sam Shih <sam.shih@mediatek.com>,
        Frank Wunderlich <frank-w@public-files.de>,
        Dan Carpenter <dan.carpenter@linaro.org>,
        James Liao <jamesjj.liao@mediatek.com>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, netdev@vger.kernel.org
Subject: Re: [PATCH v3 3/4] clk: mediatek: Add pcw_chg_shift control
Message-ID: <ZXOwZdyAJoyLRbk5@makrotopia.org>
References: <23bc89d407e7797e97b703fa939b43bfe79296ce.1701823757.git.daniel@makrotopia.org>
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 <0ebce75d-0074-4128-b35e-e86ee3ee546b@collabora.com>
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Hi Angelo,

thank you for taking the time to review and for the helpful comments.

On Wed, Dec 06, 2023 at 11:38:36AM +0100, AngeloGioacchino Del Regno wrote:
> Il 06/12/23 01:57, Daniel Golle ha scritto:
> > From: Sam Shih <sam.shih@mediatek.com>
> > 
> > Introduce pcw_chg_shfit control to optionally use that instead of the
> > hardcoded PCW_CHG_MASK macro.
> > This will needed for clocks on the MT7988 SoC.
> > 
> > Signed-off-by: Sam Shih <sam.shih@mediatek.com>
> > Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> > ---
> > v3: use git --from ...
> > v2: no changes
> > 
> >   drivers/clk/mediatek/clk-pll.c | 5 ++++-
> >   drivers/clk/mediatek/clk-pll.h | 1 +
> >   2 files changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/clk/mediatek/clk-pll.c b/drivers/clk/mediatek/clk-pll.c
> > index 513ab6b1b3229..9f08bc5d2a8a2 100644
> > --- a/drivers/clk/mediatek/clk-pll.c
> > +++ b/drivers/clk/mediatek/clk-pll.c
> > @@ -114,7 +114,10 @@ static void mtk_pll_set_rate_regs(struct mtk_clk_pll *pll, u32 pcw,
> >   			pll->data->pcw_shift);
> >   	val |= pcw << pll->data->pcw_shift;
> >   	writel(val, pll->pcw_addr);
> > -	chg = readl(pll->pcw_chg_addr) | PCW_CHG_MASK;
> > +	if (pll->data->pcw_chg_shift)
> > +		chg = readl(pll->pcw_chg_addr) | BIT(pll->data->pcw_chg_shift);
> > +	else
> > +		chg = readl(pll->pcw_chg_addr) | PCW_CHG_MASK;
> >   	writel(chg, pll->pcw_chg_addr);
> >   	if (pll->tuner_addr)
> >   		writel(val + 1, pll->tuner_addr);
> > diff --git a/drivers/clk/mediatek/clk-pll.h b/drivers/clk/mediatek/clk-pll.h
> > index f17278ff15d78..d28d317e84377 100644
> > --- a/drivers/clk/mediatek/clk-pll.h
> > +++ b/drivers/clk/mediatek/clk-pll.h
> > @@ -44,6 +44,7 @@ struct mtk_pll_data {
> >   	u32 pcw_reg;
> >   	int pcw_shift;
> >   	u32 pcw_chg_reg;
> > +	int pcw_chg_shift;
> >   	const struct mtk_pll_div_table *div_table;
> >   	const char *parent_name;
> >   	u32 en_reg;
> 
> Hmm... no, this is not the best at all and can be improved.
> 
> Okay, so, the situation here is that one or some PLL(s) on MT7988 have a different
> PCW_CHG_MASK as far as I understand.

Correct. *All* clocks of MT7988 have a different PCW_CHG_MASK, BIT(2)
instead of BIT(31).

> 
> Situation here is:
>  - Each PLL must be registered to clk-pll
>  - Each driver declaring a PLL does exactly so
>    - There's a function to register the PLL
> 
> You definitely don't want to add a conditional in pll_set_rate(): even though
> this is technically not a performance path on the current SoCs (and will probably
> never be), it's simply useless to have this (very small) overhead there.
> 
> The solution is to:
>  - Change that pcw_chg_shift to an unsigned short int type (or u8, your call):
>    you don't need 32 bits for this number, as the expected range of this member
>    is [0-31], and this can be expressed in just 4 bits (u8 is the smallest though)

Ack will use u8 instead, despite the struct not being packed, so I
wonder if it actually makes a difference.

>  - Add that to function mtk_clk_register_pll_ops()
>  - Change mtk_pll_set_rate_regs() to always do
>    chg = readl(pll->pcw_chg_addr) | BIT(pll->data->pcw_chg_shift);

As mtk_pll_data is a read-only member of the mtk_pll struct, we can't
set pcw_chg_shift to 31 in mtk_clk_register_pll_ops() in case it
is set to 0.
The only (much more intrusive change) would be to explicitely declare
.pcw_chg_shift = 31 in all current drivers setting .pcs_chg_reg != 0.
Should I do that instead?