linux-kernel - Re: [PATCH v3 01/11] firmware: raspberrypi: Introduce devm_rpi_firmware

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Date:   Tue, 10 Nov 2020 14:38:52 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com, linux-pwm@...r.kernel.org,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        linux-devicetree <devicetree@...r.kernel.org>, wahrenst@....net,
        Linux Input <linux-input@...r.kernel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org, Philipp Zabel <p.zabel@...gutronix.de>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-rpi-kernel@...ts.infradead.org,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH v3 01/11] firmware: raspberrypi: Introduce
 devm_rpi_firmware_get()

Hi Bartosz, thanks for the feedback.

On Thu, 2020-11-05 at 10:42 +0100, Bartosz Golaszewski wrote:
> On Thu, Nov 5, 2020 at 10:28 AM Nicolas Saenz Julienne
> <nsaenzjulienne@...e.de> wrote:
> > Hi Bartosz, thanks for the review.
> > 
> > On Thu, 2020-11-05 at 10:13 +0100, Bartosz Golaszewski wrote:
> > > > +/**
> > > > + * devm_rpi_firmware_get - Get pointer to rpi_firmware structure.
> > > > + * @firmware_node:    Pointer to the firmware Device Tree node.
> > > > + *
> > > > + * Returns NULL is the firmware device is not ready.
> > > > + */
> > > > +struct rpi_firmware *devm_rpi_firmware_get(struct device *dev,
> > > > +                                          struct device_node *firmware_node)
> > > > +{
> > > > +       struct platform_device *pdev = of_find_device_by_node(firmware_node);
> > > > +       struct rpi_firmware *fw;
> > > > +
> > > > +       if (!pdev)
> > > > +               return NULL;
> > > > +
> > > > +       fw = platform_get_drvdata(pdev);
> > > > +       if (!fw)
> > > > +               return NULL;
> > > > +
> > > > +       if (!refcount_inc_not_zero(&fw->consumers))
> > > > +               return NULL;
> > > > +
> > > > +       if (devm_add_action_or_reset(dev, rpi_firmware_put, fw))
> > > > +               return NULL;
> > > > +
> > > > +       return fw;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(devm_rpi_firmware_get);
> > > 
> > > Usually I'd expect the devres variant to simply call
> > > rpi_firmware_get() and then schedule a release callback which would
> > > call whatever function is the release counterpart for it currently.
> > > Devres actions are for drivers which want to schedule some more
> > > unusual tasks at driver detach. Any reason for designing it this way?
> > 
> > Yes, see patch #8 where I get rid of rpi_firmware_get() altogether after
> > converting all users to devres. Since there is no use for the vanilla version
> > of the function anymore, I figured it'd be better to merge everything into
> > devm_rpi_firmware_get(). That said it's not something I have strong feelings
> > about.
> > 
> 
> I see. So the previous version didn't really have any reference
> counting and it leaked the reference returned by
> of_find_device_by_node(), got it. Could you just clarify for me the
> logic behind the wait_queue in rpi_firmware_remove()? If the firmware
> driver gets detached and remove() stops on the wait_queue - it will be
> stuck until the last user releases the firmware. I'm not sure this is
> correct.

Yes, that's what I meant to implement.

> I'd prefer to see a kref with a release callback and remove
> would simply decrease the kref and return. Each user would do the same
> and then after the last user is detached the firmware would be
> destroyed.

Sounds good to me. I'll update it.

> Don't we really have some centralized firmware subsystem that would handle this?

Sadly no, this is an RPi specific thing, it doesn't live behind a standard like
other firmware based protocols (for ex. SCMI), and evolves as the needs arise.

Regards,
Nicolas


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