linux-kernel - Re: [PATCH v11 net-next 1/9] mfd: ocelot: add helper to get regmap from a resource

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Date:   Tue, 28 Jun 2022 15:33:21 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Colin Foster <colin.foster@...advantage.com>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        Steen Hegelund <Steen.Hegelund@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Wolfram Sang <wsa@...nel.org>,
        Terry Bowman <terry.bowman@....com>
Subject: Re: [PATCH v11 net-next 1/9] mfd: ocelot: add helper to get regmap
 from a resource

On Tue, Jun 28, 2022 at 02:50:36PM +0200, Andy Shevchenko wrote:
> On Tue, Jun 28, 2022 at 10:17 AM Colin Foster
> <colin.foster@...advantage.com> wrote:
> >
> > Several ocelot-related modules are designed for MMIO / regmaps. As such,
> > they often use a combination of devm_platform_get_and_ioremap_resource and
> > devm_regmap_init_mmio.
> >
> > Operating in an MFD might be different, in that it could be memory mapped,
> > or it could be SPI, I2C... In these cases a fallback to use IORESOURCE_REG
> > instead of IORESOURCE_MEM becomes necessary.
> >
> > When this happens, there's redundant logic that needs to be implemented in
> > every driver. In order to avoid this redundancy, utilize a single function
> > that, if the MFD scenario is enabled, will perform this fallback logic.
> 
> > +       regs = devm_platform_get_and_ioremap_resource(pdev, index, &res);
> > +
> > +       if (!res)
> > +               return ERR_PTR(-ENOENT);
> 
> This needs a comment why the original error code from devm_ call above
> is not good here.

I think what is really needed is an _optional() variant of
ocelot_platform_init_regmap_from_resource(), which just returns NULL on
missing resource and doesn't ioremap anything. It can be easily open
coded, i.e. instead of creating devm_platform_get_and_ioremap_resource_optional(),
we could just call platform_get_resource() and devm_ioremap_resource()
individually.