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

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Date:   Tue, 18 May 2021 10:30:43 -0400
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Stephan Gerhold <stephan@...hold.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>, linux-nfc@...ts.01.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bongsu Jeon <bongsu.jeon@...sung.com>,
        ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [linux-nfc] [PATCH 2/2] nfc: s3fwrn5: i2c: Enable optional clock
 from device tree


Hi,

Thanks for the patch.

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.

> 
> This commit adds support for this by requesting an optional clock

Don't write "This commit".
https://elixir.bootlin.com/linux/latest/source/Documentation/process/submitting-patches.rst#L89

> and keeping it permanently enabled. Note that the actual (physical)
> clock won't be permanently enabled since this will depend on the
> output of NFC_CLK_REQ from S3FWRN5.

What pin is that "NFC_CLK_REQ"? I cannot find such name. Is it GPIO2?
What clock are you talking here? The one going to the modem part?

I also don't see here how this clock is going to be automatically
on-off... driver does not perform such. Unless you speak about your
particular HW configuration where the GPIO is somehow connected with AND
(but then it is not relevant to the code).

> 
> In the future (when needed by some other device) this could be
> extended to work for the second case (monitoring the NFC_CLK_REQ
> GPIO and enabling the clock from the kernel when needed).
> 
> Signed-off-by: Stephan Gerhold <stephan@...hold.net>
> ---
>  drivers/nfc/s3fwrn5/i2c.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 


Best regards,
Krzysztof