linux-kernel - Re: [PATCH 16/24] rtc: pm8xxx: add support for nvmem offset

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Message-ID: <Y9PugpnzsqZpzSGE@hovoldconsulting.com>
Date:   Fri, 27 Jan 2023 16:32:18 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     Johan Hovold <johan+linaro@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        linux-arm-msm@...r.kernel.org, linux-rtc@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/24] rtc: pm8xxx: add support for nvmem offset

On Fri, Jan 27, 2023 at 02:13:21PM +0000, Srinivas Kandagatla wrote:
> On 26/01/2023 14:20, Johan Hovold wrote:
> > On many Qualcomm platforms the PMIC RTC control and time registers are
> > read-only so that the RTC time can not be updated. Instead an offset
> > needs be stored in some machine-specific non-volatile memory, which the
> > driver can take into account.
> > 
> > Add support for storing a 32-bit offset from the Epoch in an nvmem cell
> > so that the RTC time can be set on such platforms.
> > 
> > Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> > ---
> >   drivers/rtc/rtc-pm8xxx.c | 134 +++++++++++++++++++++++++++++++++++----
> >   1 file changed, 123 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/rtc/rtc-pm8xxx.c b/drivers/rtc/rtc-pm8xxx.c
> > index 922aef0f0241..09816b9f6282 100644
> > --- a/drivers/rtc/rtc-pm8xxx.c
> > +++ b/drivers/rtc/rtc-pm8xxx.c
> > @@ -3,6 +3,7 @@
> >    */
> > +static int pm8xxx_rtc_read_nvmem_offset(struct pm8xxx_rtc *rtc_dd)
> > +{
> > +	size_t len;
> > +	void *buf;
> > +	int rc;
> > +
> > +	buf = nvmem_cell_read(rtc_dd->nvmem_cell, &len);
> > +	if (IS_ERR(buf)) {
> > +		rc = PTR_ERR(buf);
> > +		dev_err(rtc_dd->dev, "failed to read nvmem offset: %d\n", rc);
> > +		return rc;
> > +	}
> > +
> > +	if (len != sizeof(u32)) {
> > +		dev_err(rtc_dd->dev, "unexpected nvmem cell size %zu\n", len);
> > +		kfree(buf);
> > +		return -EINVAL;
> > +	}

> how about us nvmem_cell_read_u32()

I considered that, but did not like the asymmetry of the interface.

Specifically, nvmem_cell_read_u32() would go out and look up the nvmem
cell again even though I already have and need a reference for
nvmem_cell_write().

nvmem_cell_read_u32() seems to be a better fit as a convenience wrapper
for drivers that only need to do a single read at probe.

Johan