linux-kernel - Re: [PATCH v6 7/9] clk: mediatek: Add subsystem clocks of MT8173

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Message-ID: <20150805073645.GB18700@pengutronix.de>
Date:	Wed, 5 Aug 2015 09:36:45 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Daniel Kurtz <djkurtz@...omium.org>
Cc:	James Liao <jamesjj.liao@...iatek.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Heiko Stubner <heiko@...ech.de>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ricky Liang <jcliang@...omium.org>,
	Rob Herring <robh+dt@...nel.org>,
	linux-mediatek@...ts.infradead.org,
	Sascha Hauer <kernel@...gutronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 7/9] clk: mediatek: Add subsystem clocks of MT8173

On Wed, Aug 05, 2015 at 03:26:29PM +0800, Daniel Kurtz wrote:
> On Wed, Aug 5, 2015 at 2:46 PM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> > On Tue, Aug 04, 2015 at 04:16:56PM +0800, James Liao wrote:
> >> Most multimedia subsystem clocks will be accessed by multiple
> >> drivers, so it's a better way to manage these clocks in CCF.
> >> This patch adds clock support for MM, IMG, VDEC, VENC and VENC_LT
> >> subsystems.
> >>
> >> Signed-off-by: James Liao <jamesjj.liao@...iatek.com>
> >> ---
> >>  drivers/clk/mediatek/clk-mt8173.c      | 267 +++++++++++++++++++++++++++++++++
> >>  include/dt-bindings/clock/mt8173-clk.h |  97 +++++++++++-
> >>  2 files changed, 361 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/clk/mediatek/clk-mt8173.c b/drivers/clk/mediatek/clk-mt8173.c
> >> index f37ace6..05335e5 100644
> >> --- a/drivers/clk/mediatek/clk-mt8173.c
> >> +++ b/drivers/clk/mediatek/clk-mt8173.c
> >> @@ -25,6 +25,10 @@ static DEFINE_SPINLOCK(mt8173_clk_lock);
> >>  static const struct mtk_fixed_clk fixed_clks[] __initconst = {
> >>       FIXED_CLK(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", "clk26m", 400 * MHZ),
> >>       FIXED_CLK(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", "clk26m", 125 * MHZ),
> >> +     FIXED_CLK(CLK_TOP_DSI0_DIG, "dsi0_dig", "clk26m", 130 * MHZ),
> >> +     FIXED_CLK(CLK_TOP_DSI1_DIG, "dsi1_dig", "clk26m", 130 * MHZ),
> >> +     FIXED_CLK(CLK_TOP_LVDS_PXL, "lvds_pxl", "lvdspll", 148.5 * MHZ),
> >> +     FIXED_CLK(CLK_TOP_LVDS_CTS, "lvds_cts", "lvdspll", 51.975 * MHZ),
> >
> > I would expect 51975 * KHZ here to avoid fractional numbers. Probably
> > gcc calculates that during compile time so this will work as expected,
> > still I'm not sure this is good style to use fractional numbers here.
> 
> I thought this looked a bit strange too, but for what its worth, these
> two evaluate correctly:
> 
> localhost ~ # cat /sys/kernel/debug/clk/clk_summary  | grep lvds
>     lvdspll                               0            0   149999878
>        0 0
>        lvds_pxl                           0            0   148500000
>        0 0
>        lvds_cts                           0            0    51975000
>        0 0
> 
> >
> > Anyway, on my system lvdspll is running at 150MHz. Are you sure there is
> > a clock derived from this running at 148.5MHz? Is it really correct to
> > use a fixed clock here or should it rather be lvdspll directly?
> 
> I agree it does look strange to have a 51.975 MHz and 148.5 MHz clocks
> with a 150 MHz PLL as their parent...  However, I'm not sure how much
> this matters?  I think the idea here was that these frequencies are
> best effort "nominal" clock values provided by Mediatek "designers".
> The important point is that for the hardware to generate these either
> of these clocks, lvdspll must be enabled.

This assumes that the lvdspll always runs at frequency the designers
thought that might be a good value. Should we really provide wrong clock
values when on some board for whatever reason the lvdspll is configured
for a different frequency?

sascha

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