On Thu, 2020-11-12 at 23:26 -0800, Dmitry Torokhov wrote:
> On Thu, Nov 12, 2020 at 07:52:14PM +0200, Andy Shevchenko wrote:
> > On Thu, Nov 12, 2020 at 6:40 PM Nicolas Saenz Julienne
> > <nsaenzjulienne@xxxxxxx> wrote:
> > >
> > > When unbinding the firmware device we need to make sure it has no
> > > consumers left. Otherwise we'd leave them with a firmware handle
> > > pointing at freed memory.
> > >
> > > Keep a reference count of all consumers and introduce rpi_firmware_put()
> > > which will permit automatically decrease the reference count upon
> > > unbinding consumer drivers.
> >
> > ...
> >
> > > /**
> > > - * rpi_firmware_get - Get pointer to rpi_firmware structure.
> > > * @firmware_node: Pointer to the firmware Device Tree node.
> > > *
> > > + * The reference to rpi_firmware has to be released with rpi_firmware_put().
> > > + *
> > > * Returns NULL is the firmware device is not ready.
> > > */
> > > struct rpi_firmware *rpi_firmware_get(struct device_node *firmware_node)
> > > {
> > > struct platform_device *pdev = of_find_device_by_node(firmware_node);
> > > + struct rpi_firmware *fw;
> > >
> > > if (!pdev)
> > > return NULL;
> > >
> > > - return platform_get_drvdata(pdev);
> > > + fw = platform_get_drvdata(pdev);
> > > + if (!fw)
> > > + return NULL;
> > > +
> > > + if (!kref_get_unless_zero(&fw->consumers))
> > > + return NULL;
> >
Hi Andy, Dimitry,
> > Don't we have a more traditional way of doing this, i.e.
> > try_module_get() coupled with get_device() ?
>
> get_device() will make sure that device is there, but gives no
> assurances that device is bound to a driver, so it will not help with
> the racy access to firmware via platform_get_drvdata() call.
I also looked at using get/put_device() just as a means for refcounting (i.e.
replacing fw->consumers), but I can't make it work either. I'd need a way to
hook up into one of the struct device_ktype release() functions. AFAIK it's not
possible for private uses like this.
IIUC the way to do this would be to bypass platform device and create a special
device class/bus for RPi's firmware dependent devices (I could pretty much copy
SCMI's implementation), but I fear that's overkill.
So, for now I'll stick with the kref based implementation, I'll be happy to
change it if you find a better solution. :)
Regards,
Nicolas
Attachment:
signature.asc
Description: This is a digitally signed message part
