linux-kernel - Re: [PATCH 2/3] clk: meson: add sub EMMC clock dt-bindings IDs

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Date:   Tue, 03 Jul 2018 12:01:15 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Yixun Lan <yixun.lan@...ogic.com>,
        Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     Neil Armstrong <narmstrong@...libre.com>,
        Kevin Hilman <khilman@...libre.com>,
        Carlo Caione <carlo@...one.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Rob Herring <robh@...nel.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Liang Yang <liang.yang@...ogic.com>,
        Qiufang Dai <qiufang.dai@...ogic.com>,
        Jian Hu <jian.hu@...ogic.com>, linux-clk@...r.kernel.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 2/3] clk: meson: add sub EMMC clock dt-bindings IDs

On Tue, 2018-07-03 at 17:56 +0800, Yixun Lan wrote:
> > > Yes, It's true, the mux is parent of the div clock.
> > > 
> > > while testing for the NAND driver, I find it's kind of loose about the
> > > parent of the clock, so selecting the div (and let CCF decide freely) is
> > > actually works fine
> > > 
> > > but for the EMMC driver, especially when running at high clock, it's
> > > kind of picky about the parent of the clock, 
> > 
> > It would be nice to get an explanation about this behavior.
> > it seems that even of the rate provided by CLKID_SD_EMMC_X_CLK0 (main clock
> > controller) is correct, the eMMC cannot reliably tune with it.
> > 
> > Could you elaborate on this ?
> > 
> 
> It's during my own test in AXG platform, I found clock path
> a) fclk_div2 -> sd_emmc_c_clk0_sel -> sd_emmc_c_clk0_div ->
> sd_emmc_c_clk0 -> sd_emmc_c_mux -> sd_emmc_c_div
> 
> b) fclk_div2 -> sd_emmc_c_mux -> sd_emmc_c_div
> 
> path a) doesn't work in EMMC driver, even both clock parent of them from
> the same fclk_div2 source.
> 
>  sd_emmc_c_mux -> sd_emmc_c_div is the clock from the EMMC register base.
> I believe it's ASIC design issue

yes Yixun, I did the same test. What I meant with this question is: could you
confirm there a problem with this clock, and what it is exactly so we can adjust
the clock as necessary.

If FDIV2 entry to this clock is broken, maybe it should be removed.