linux-kernel - Re: [PATCH 1/5] misc: eeprom: at24: Initialise AT24 NVMEM ID field

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Message-ID: <CAMpxmJUAhJmGA_3nKs51hF=08vY19U5Rpfz30DLgOOiTMuxjzA@mail.gmail.com>
Date:   Fri, 11 Sep 2020 09:45:43 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-devicetree <devicetree@...r.kernel.org>,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH 1/5] misc: eeprom: at24: Initialise AT24 NVMEM ID field

On Thu, Sep 10, 2020 at 8:19 PM Jon Hunter <jonathanh@...dia.com> wrote:
>
>
> On 10/09/2020 19:15, Jon Hunter wrote:
> >
> > On 10/09/2020 16:35, Bartosz Golaszewski wrote:
> >> On Thu, Sep 10, 2020 at 3:43 PM Jon Hunter <jonathanh@...dia.com> wrote:
> >>>
> >>> The AT24 EEPROM driver does not initialise the 'id' field of the
> >>> nvmem_config structure and because the entire structure is not
> >>> initialised, it ends up with a random value. This causes the NVMEM
> >>> driver to append the device 'devid' value to name of the NVMEM
> >>> device. Although this is not a problem per-se, for I2C devices such as
> >>> the AT24, that already have a device unique name, there does not seem
> >>> much value in appending an additional 0 to the I2C name. For example,
> >>> appending a 0 to an I2C device name such as 1-0050 does not seem
> >>> necessary and maybe even a bit confusing. Therefore, fix this by
> >>> setting the NVMEM config.id to NVMEM_DEVID_NONE for AT24 EEPROMs.
> >>>
> >>> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
> >>> ---
> >>>  drivers/misc/eeprom/at24.c | 1 +
> >>>  1 file changed, 1 insertion(+)
> >>>
> >>> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
> >>> index e9df1ca251df..3f7a3bb6a36c 100644
> >>> --- a/drivers/misc/eeprom/at24.c
> >>> +++ b/drivers/misc/eeprom/at24.c
> >>> @@ -715,6 +715,7 @@ static int at24_probe(struct i2c_client *client)
> >>>
> >>>         nvmem_config.name = dev_name(dev);
> >>>         nvmem_config.dev = dev;
> >>> +       nvmem_config.id = NVMEM_DEVID_NONE;
> >>>         nvmem_config.read_only = !writable;
> >>>         nvmem_config.root_only = !(flags & AT24_FLAG_IRUGO);
> >>>         nvmem_config.owner = THIS_MODULE;
> >>> --
> >>> 2.25.1
> >>>
> >>
> >> This patch is correct and thanks for catching it. I vaguely recall
> >> wondering at some point why the appended 0 in the nvmem name for at24.
> >> Unfortunately this change would affect how the device is visible in
> >> user-space in /sys/bus/nvmem/devices/ and this could break existing
> >> users. Also: there are many in-kernel users that would need to be
> >> updated. I'm afraid we'll need some sort of backward compatibility.
> >
> >
> > Thanks, yes that is a problem. I guess for now we could explicitly init
> > to NVMEM_DEVID_AUTO or maybe just 0 so that it defaults to the same path
> > in the NVMEM driver. However, I am not sure how we can make allow some
> > devices to use NVMEM_DEVID_NONE and others use something else. This is
> > not really something that we can describe in DT because it has nothing
> > to do with h/w.
>
>
> Unless we make the configuration of the 'id' dependent on the 'label'
> property so something like ...
>
>         if (device_property_present(dev, "label")) {
>                 nvmem_config.id = NVMEM_DEVID_NONE;
>                 err = device_property_read_string(dev, "label",
>                                                   &nvmem_config.name);
>                 if (err)
>                         return err;
>         } else {
>                 nvmem_config.id = NVMEM_DEVID_AUTO;
>                 nvmem_config.name = dev_name(dev);
>         }
>
> Cheers
> Jon
>
> --
> nvpublic

Yes, this looks like the best compromise we can get for now. Please
make sure to document why we do this in the code.

Bartosz