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

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Date:	Wed, 5 Aug 2015 15:41:49 +0800
From:	Daniel Kurtz <djkurtz@...omium.org>
To:	Sascha Hauer <s.hauer@...gutronix.de>
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 5, 2015 at 3:36 PM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> 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?

Do you have an alternative suggestion for estimating the frequency of
a non-software controllable or measurable hardware clock?

Or can we land this series and update the frequency via some future
patch if and when that other board arrives which may have a different
frequency - and there is some reason to care what that frequency
actually is?

-Dan

>
> sascha
>
> --
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