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From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Date:   Thu, 27 Sep 2018 11:12:32 +0200
Message-ID: <CAFBinCBLfoqnLK_hu6fufO4MXKKoRDPDuJgLasCNYWeuL6tCKw@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] mtd: rawnand: meson: add support for Amlogic NAND
 flash controller
To:     liang.yang@amlogic.com
Cc:     jianxin.pan@amlogic.com, boris.brezillon@bootlin.com,
        linux-mtd@lists.infradead.org, yixun.lan@amlogic.com,
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        khilman@baylibre.com, robh@kernel.org, jian.hu@amlogic.com,
        hanjie.lin@amlogic.com, victor.wan@amlogic.com,
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Hello Liang,

On Thu, Sep 27, 2018 at 10:19 AM Liang Yang <liang.yang@amlogic.com> wrote:
>
> Hello Martin,
>
> On 9/22/2018 11:32 PM, Martin Blumenstingl wrote:
> > Hello,
> >
> > On Thu, Sep 20, 2018 at 10:51 AM Jianxin Pan <jianxin.pan@amlogic.com> wrote:
> > [snip]
> >> +static int meson_nfc_clk_init(struct meson_nfc *nfc)
> >> +{
> >> +       int ret;
> >> +
> >> +       /* request core clock */
> >> +       nfc->core_clk = devm_clk_get(nfc->dev, "core");
> >> +       if (IS_ERR(nfc->core_clk)) {
> >> +               dev_err(nfc->dev, "failed to get core clk\n");
> >> +               return PTR_ERR(nfc->core_clk);
> >> +       }
> >> +
> >> +       nfc->device_clk = devm_clk_get(nfc->dev, "device");
> >> +       if (IS_ERR(nfc->device_clk)) {
> >> +               dev_err(nfc->dev, "failed to get device clk\n");
> >> +               return PTR_ERR(nfc->device_clk);
> >> +       }
> >> +
> >> +       nfc->phase_tx = devm_clk_get(nfc->dev, "tx");
> >> +       if (IS_ERR(nfc->phase_tx)) {
> >> +               dev_err(nfc->dev, "failed to get tx clk\n");
> >> +               return PTR_ERR(nfc->phase_tx);
> >> +       }
> >> +
> >> +       nfc->phase_rx = devm_clk_get(nfc->dev, "rx");
> >> +       if (IS_ERR(nfc->phase_rx)) {
> >> +               dev_err(nfc->dev, "failed to get rx clk\n");
> >> +               return PTR_ERR(nfc->phase_rx);
> >> +       }
> > neither the "rx" nor the "tx" clock are documented in the dt-bindings patch
> >
>
> ok, i will add them later.
>
> >> +       /* init SD_EMMC_CLOCK to sane defaults w/min clock rate */
> >> +       regmap_update_bits(nfc->reg_clk, 0,
> >> +                          CLK_SELECT_NAND | CLK_ALWAYS_ON | CLK_DIV_MASK,
> >> +                          CLK_SELECT_NAND | CLK_ALWAYS_ON | CLK_DIV_MASK);
> > clk_set_rate also works for clocks that are not enabled yet (except if
> > they have the flag CLK_SET_RATE_UNGATE)
> > this should help you to remove CLK_DIV_MASK here
> >
>
> if not set clk_div_mask here, the value 0x00 of divider means nand clock
> off, even read/write nand register is forbidden.
ah, now I see the pattern here.
Jerome has written a "sclk-div" driver (which is currently only used
for the audio clocks on AXG). based on reading the code it seems that
switching the driver of the divider clock to sclk-div would allow you
to remove setting CLK_DIV_MASK here:
- the  "hi" parameter in struct meson_sclk_div_data is optional ->
then the sclk-div clock won't try to change the duty cycle
- sclk_div_init reads the divider at initialization time - if it's 0
it takes the maximum possible divider value
- sclk_div_enable (which you're going to call anyways, through
clk_prepare_enable) will then set the divider to the greatest possible
value

> > is CLK_SELECT_NAND a bit that switches the clock output from the sdmmc
> > controller to the NAND controller?
> > if so: can this be modeled as a mux clock?
> >
>
> it seems to like a gate. 1: nand is selected; 0: emmc is selected.
> Is it suitable for making it as a mux clock?
my understanding of a gate is:
- register value X = OFF, value Y = ON

but in your case it's:
- 0 = eMMC is ON but NAND is OFF
- 1 = eMMC is OFF but NAND is ON
(so both values mean "on", just for different contexts)

I believe you need to set this value for eMMC as well:
what if the bootloader (or hardware defaults, etc.) incorrectly sets
the value to 1 but the Linux .dts is configured to use eMMC?

> > the public S905 datasheet doesn't mention CLK_ALWAYS_ON at bit 28 but
> > uses bit 24 instead. the description from the datasheet:
> > Cfg_always_on:
> > 1: Keep clock always on
> > 0: Clock on/off controlled by activities.
> >      Any APB3 access or descriptor execution will turn clock on.
> > Recommended value: 0
> >
> > can you please explain what CLK_ALWAYS_ON does and why it has to be 1?
> >
>
> em , it is the same as bit 24 in S905 datasheet, only moves to bit 28.
> it means keeping internal clock on whether nand wroks or not.
> it doesn't have to be 1; i have tried 0 successfully on AXG platform.
my preference would be to use the recommended value from the datasheet
unless there's a good argument why it has to be different


Regards
Martin