Re: [linux-nfc] Re: [PATCH 2/2] nfc: s3fwrn5: i2c: Enable optional clock from device tree

[Bongsu Jeon <bongsu.jeon2@gmail.com>]

On Thu, May 20, 2021 at 12:58 AM Krzysztof Kozlowski
<krzysztof.kozlowski@canonical.com> wrote:
>
> On 19/05/2021 04:07, Stephan Gerhold wrote:
> > On Tue, May 18, 2021 at 11:25:55AM -0400, Krzysztof Kozlowski wrote:
> >> On 18/05/2021 11:00, Stephan Gerhold wrote:
> >>> On Tue, May 18, 2021 at 10:30:43AM -0400, Krzysztof Kozlowski wrote:
> >>>> On 18/05/2021 09:39, Stephan Gerhold wrote:
> >>>>> s3fwrn5 has a NFC_CLK_REQ output GPIO, which is asserted whenever
> >>>>> the clock is needed for the current operation. This GPIO can be either
> >>>>> connected directly to the clock provider, or must be monitored by
> >>>>> this driver.
> >>>>>
> >>>>> As an example for the first case, on many Qualcomm devices the
> >>>>> NFC clock is provided by the main PMIC. The clock can be either
> >>>>> permanently enabled (clocks = <&rpmcc RPM_SMD_BB_CLK2>) or enabled
> >>>>> only when requested through a special input pin on the PMIC
> >>>>> (clocks = <&rpmcc RPM_SMD_BB_CLK2_PIN>).
> >>>>>
> >>>>> On the Samsung Galaxy A3/A5 (2015, Qualcomm MSM8916) this mechanism
> >>>>> is used with S3FWRN5's NFC_CLK_REQ output GPIO to enable the clock
> >>>>> only when necessary. However, to make that work the s3fwrn5 driver
> >>>>> must keep the RPM_SMD_BB_CLK2_PIN clock enabled.
> >>>>
> >>>> This contradicts the code. You wrote that pin should be kept enabled
> >>>> (somehow... by driver? by it's firmware?) but your code requests the
> >>>> clock from provider.
> >>>>
> >>>
> >>> Yeah, I see how that's a bit confusing. Let me try to explain it a bit
> >>> better. So the Samsung Galaxy A5 (2015) has a "S3FWRN5XS1-YF30", some
> >>> variant of S3FWRN5 I guess. That S3FWRN5 has a "XI" and "XO" pin in the
> >>> schematics. "XO" seems to be floating, but "XI" goes to "BB_CLK2"
> >>> on PM8916 (the main PMIC).
> >>>
> >>> Then, there is "GPIO2/NFC_CLK_REQ" on the S3FWRN5. This goes to
> >>> GPIO_2_NFC_CLK_REQ on PM8916. (Note: I'm talking about two different
> >>> GPIO2 here, one on S3FWRN5 and one on PM8916, they just happen to have
> >>> the same number...)
> >>>
> >>> So in other words, S3FWRN5 gets some clock from BB_CLK2 on PM8916,
> >>> and can tell PM8916 that it needs the clock via GPIO2/NFC_CLK_REQ.
> >>>
> >>> Now the confusing part is that the rpmcc/clk-smd-rpm driver has two
> >>> clocks that represent BB_CLK2 (see include/dt-bindings/clock/qcom,rpmcc.h):
> >>>
> >>>   - RPM_SMD_BB_CLK2
> >>>   - RPM_SMD_BB_CLK2_PIN
> >>>
> >>> (There are also *_CLK2_A variants but they are even more confusing
> >>>  and not needed here...)
> >>>
> >>> Those end up in different register settings in PM8916. There is one bit
> >>> to permanently enable BB_CLK2 (= RPM_SMD_BB_CLK2), and one bit to enable
> >>> BB_CLK2 based on the status of GPIO_2_NFC_CLK_REQ on PM8916
> >>> (= RPM_SMD_BB_CLK2_PIN).
> >>>
> >>> So there is indeed some kind of "AND" inside PM8916 (the register bit
> >>> and "NFC_CLK_REQ" input pin). To make that "AND" work I need to make
> >>> some driver (here: the s3fwrn5 driver) enable the clock so the register
> >>> bit in PM8916 gets set.
> >>
> >> Thanks for the explanation, it sounds good. The GPIO2 (or how you call
> >> it NFC_CLK_REQ) on S3FWRN5 looks like non-configurable from Linux point
> >> of view. Probably the device firmware plays with it always or at least
> >> handles it in an unknown way for us.
> >>
> >
> > FWIW, I was looking at some more s3fwrn5 code yesterday and came
> > across this (in s3fwrn5_nci_rf_configure()):
> >
> >       /* Set default clock configuration for external crystal */
> >       fw_cfg.clk_type = 0x01;
> >       fw_cfg.clk_speed = 0xff;
> >       fw_cfg.clk_req = 0xff;
> >       ret = nci_prop_cmd(info->ndev, NCI_PROP_FW_CFG,
> >               sizeof(fw_cfg), (__u8 *)&fw_cfg);
> >       if (ret < 0)
> >               goto out;
> >
> > It does look quite suspiciously like that configures how s3fwrn5 expects
> > the clock and possibly (fw_cfg.clk_req?) how GPIO2 behaves. But it's not
> > particularly useful without some documentation for the magic numbers.
>
> Right, without documentation of FW protocol there is not much we can
> deduct here. There is no proof even that the comment matches actual code.
>
> Dear Bongsu,
> Maybe you could share some details about clock selection?

These configuration values depend on the HW circuit for NFC.

There are  two types of fw_cfg.clk_type for N5.
0x01 : external XTAL ( don't need to control the clock because XTAL
always supplies
the NFC clock automatically.)
0x00 : PLL clock (need to control clock. )

There are three types of fw_cfg.clk_speed for N5.
0xFF : for external XTAL
0x00 : 24M for PLL.
0x01 : 19.12M for PLL.

There are two types of fw_cfg.clk_req for N5.
0xFF: NFC firmware controls CLK Req when NFC needs the external clock.
0xF0: NFC firmware doesn't control CLK Req.

>
> >
> > Personally, I just skip all firmware/RF configuration (which works thanks
> > to commit 4fb7b98c7be3 ("nfc: s3fwrn5: skip the NFC bootloader mode")).
> > That way, S3FWRN5 just continues using the proper configuration
> > that was loaded by the vendor drivers at some point. :)
>
> But isn't that configuration lost after power off?
>

If you skip all firmware/RF configuration, you can use  the preserved
firmware and
RF configuration on the chip.

>
> Best regards,
> Krzysztof