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Subject: Re: [PATCH 1/3] phy: rockchip-emmc: Allow to set drive impedance via
 DTS.
From:   Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
In-Reply-To: <CB9331AE-2FEC-4646-BD8E-BDF9F4E48F74@theobroma-systems.com>
Date:   Fri, 1 Mar 2019 19:41:03 +0100
Cc:     Doug Anderson <dianders@chromium.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        =?utf-8?Q?Heiko_St=C3=BCbner?= <heiko@sntech.de>,
        Shawn Lin <shawn.lin@rock-chips.com>,
        Kishon Vijay Abraham I <kishon@ti.com>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        Klaus Goger <klaus.goger@theobroma-systems.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Matthias Brugger <mbrugger@suse.com>,
        Emil Renner Berthing <kernel@esmil.dk>,
        Tony Xie <tony.xie@rock-chips.com>,
        Randy Li <ayaka@soulik.info>,
        Vicente Bergas <vicencb@gmail.com>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        devicetree@vger.kernel.org,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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Message-Id: <72859695-2E77-4EFF-9555-54907830849F@theobroma-systems.com>
References: <20190301153348.29870-1-christoph.muellner@theobroma-systems.com>
 <CAD=FV=UUuQYwh2DMgB5FLuiE1BwbTMTuc9rAte1J5L_JP5qk1Q@mail.gmail.com>
 <CB9331AE-2FEC-4646-BD8E-BDF9F4E48F74@theobroma-systems.com>
To:     =?utf-8?Q?Christoph_M=C3=BCllner?= 
        <christoph.muellner@theobroma-systems.com>
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> On 01.03.2019, at 19:09, Christoph Müllner <christoph.muellner@theobroma-systems.com> wrote:
> 
> Hi Doug,
> 
>> On 01.03.2019, at 17:48, Doug Anderson <dianders@chromium.org> wrote:
>> 
>> Hi,
>> 
>> On Fri, Mar 1, 2019 at 7:37 AM Christoph Muellner
>> <christoph.muellner@theobroma-systems.com> wrote:
>>> 
>>> The rockchip-emmc PHY can be configured with different
>>> drive impedance values. Currenlty a value of 50 Ohm is
>>> hard coded into the driver.
>>> 
>>> This patch introduces the DTS property 'drive-impedance-ohm'
>>> for the rockchip-emmc phy node, which uses the value from the DTS
>>> to setup the drive impedance accordingly.
>>> 
>>> Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
>>> Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
>>> ---
>>> drivers/phy/rockchip/phy-rockchip-emmc.c | 38 ++++++++++++++++++++++++++++++--
>>> 1 file changed, 36 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/drivers/phy/rockchip/phy-rockchip-emmc.c b/drivers/phy/rockchip/phy-rockchip-emmc.c
>>> index 19bf84f0bc67..5413fa73dd45 100644
>>> --- a/drivers/phy/rockchip/phy-rockchip-emmc.c
>>> +++ b/drivers/phy/rockchip/phy-rockchip-emmc.c
>>> @@ -87,6 +87,7 @@ struct rockchip_emmc_phy {
>>>       unsigned int    reg_offset;
>>>       struct regmap   *reg_base;
>>>       struct clk      *emmcclk;
>>> +       unsigned int drive_impedance;
>>> };
>>> 
>>> static int rockchip_emmc_phy_power(struct phy *phy, bool on_off)
>>> @@ -281,10 +282,10 @@ static int rockchip_emmc_phy_power_on(struct phy *phy)
>>> {
>>>       struct rockchip_emmc_phy *rk_phy = phy_get_drvdata(phy);
>>> 
>>> -       /* Drive impedance: 50 Ohm */
>>> +       /* Drive impedance: from DTS */
>>>       regmap_write(rk_phy->reg_base,
>>>                    rk_phy->reg_offset + GRF_EMMCPHY_CON6,
>>> -                    HIWORD_UPDATE(PHYCTRL_DR_50OHM,
>>> +                    HIWORD_UPDATE(rk_phy->drive_impedance,
>>>                                  PHYCTRL_DR_MASK,
>>>                                  PHYCTRL_DR_SHIFT));
>>> 
>>> @@ -314,6 +315,28 @@ static const struct phy_ops ops = {
>>>       .owner          = THIS_MODULE,
>>> };
>>> 
>>> +static u32 convert_drive_impedance_ohm(struct platform_device *pdev, u32 dr_ohm)
>>> +{
>>> +       switch (dr_ohm) {
>>> +       case 100:
>>> +               return PHYCTRL_DR_100OHM;
>>> +       case 66:
>>> +               return PHYCTRL_DR_66OHM;
>>> +       case 50:
>>> +               return PHYCTRL_DR_50OHM;
>>> +       case 40:
>>> +               return PHYCTRL_DR_40OHM;
>>> +       case 33:
>>> +               return PHYCTRL_DR_33OHM;
>>> +       }
>>> +
>>> +       dev_warn(&pdev->dev,
>>> +               "Invalid value %u for drive-impedance-ohm. "
>>> +               "Falling back to 50 Ohm.\n",
>>> +               dr_ohm);
>>> +       return PHYCTRL_DR_50OHM;
>>> +}
>>> +
>>> static int rockchip_emmc_phy_probe(struct platform_device *pdev)
>>> {
>>>       struct device *dev = &pdev->dev;
>>> @@ -322,6 +345,7 @@ static int rockchip_emmc_phy_probe(struct platform_device *pdev)
>>>       struct phy_provider *phy_provider;
>>>       struct regmap *grf;
>>>       unsigned int reg_offset;
>>> +       u32 val;
>>> 
>>>       if (!dev->parent || !dev->parent->of_node)
>>>               return -ENODEV;
>>> @@ -345,6 +369,16 @@ static int rockchip_emmc_phy_probe(struct platform_device *pdev)
>>>       rk_phy->reg_offset = reg_offset;
>>>       rk_phy->reg_base = grf;
>>> 
>>> +       if (of_property_read_u32(dev->of_node, "drive-impedance-ohm", &val)) {
>>> +               dev_info(dev,
>>> +                       "Missing drive-impedance-ohm property in node %s "
>>> +                       "Falling back to 50 Ohm.\n",
>>> +                       dev->of_node->name);
>> 
>> This is awfully noisy for something that pretty much all existing
>> boards will run.  debug level instead of info level?  Also:
> 
> Removed for v2 as suggested by Robin.
> 
>> 
>> * Don't split strings
>> across lines
>> 
>> * There's a magic % thing to get the name of an OF node.  Use that.
>> 
>> 
>>> +               rk_phy->drive_impedance = PHYCTRL_DR_50OHM;
>>> +       } else {
>>> +               rk_phy->drive_impedance = convert_drive_impedance_ohm(pdev, val);
>>> +       }
>> 
>> It's been a long time since I looked at this, but I could have sworn
>> that it was more complicated than that.  Specifically I though you
>> were supposed to query the eMMC card for what it supported and then
>> resolve that with what the host could support.
>> 
>> Assuming that this is supposed to be queried from the card (which is
>> how I remember it) then you definitely don't want it in the device
>> tree since you want to be able to stuff various different eMMC parts
>> and we should be able to figure out the impedance at runtime.
>> 
>> 
>> NOTE: IIRC the old value of 50 ohms is required to be supported by all
>> hosts and cards and is specified to be the default.
>> 
> 
> When using 50 ohms on our board in HS-400 mode (200 MHz), then
> we see communication errors. These are cause by additional components
> on the clock line, which damp the 200 Mhz signal too much.
> 
> We could do the following:
> 
> * sdhci.c provides a default implementation to set the drive impedance (in HOST_CONTROL2,
>  like it is done already now as part of sdhci_set_ios())
> * sdhci-of-arasan.c installs an alternative implementation in case of Rockchip's eMMC controller
> * the alternative implementation uses a callback to the Rockchip's eMMC phy driver

Generally, I would not make this either-or (I may have been less than clear
in my comments to the internal ticket), but rather teach the sdhci-driver to
always notify the eMMC PHY driver (if there is one) of the change.

For the RK3399’s sdhci implementation, the bits in the HOST_CONTROL2
register are ‘reserved’ and marked R/O, so the current implementation will
not work anyway and the PHY driver needs to be notified of the change in
requested drive-strength.

> * the Rockchip eMMC phy driver sets the drive impedance accordingly
> 
> But still we would need a mechanism to override this for our board.
> And exactly this override mechanism is provided by the patch series.

The PHY would then need to determine the proper operation point (or an
mapping from a table) to achieve the requested line characteristic for any
given board.  In other words: the DTS for puma would still contain an entry
to allow us to override to 33 Ohm when switching to HS-400.

> Thanks,
> Christoph
> 
>> 
>> I do see at least the summary of what I thought of this before at
>> <https://patchwork.kernel.org/patch/9086541/>
>> 
>> 
>> (Sorry if the above is wrong and feel free to correct me--I don't have
>> time at the moment to do all the full research but hopefully you can
>> dig more based on my pointers.  Feel free to call me on my BS)
>> 
>> 
>> -Doug