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

[Stephan Gerhold]

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.

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. :)

Stephan