linux-kernel - Re: [PATCH v2 1/2] driver core: Move the "removable" attribute from USB to core

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Date:   Tue, 11 May 2021 18:20:55 -0700
From:   Rajat Jain <rajatxjain@...il.com>
To:     Krzysztof Wilczyński <kw@...ux.com>
Cc:     Rajat Jain <rajatja@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>, linux-usb@...r.kernel.org,
        Bjorn Helgaas <helgaas@...nel.org>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Dmitry Torokhov <dtor@...gle.com>
Subject: Re: [PATCH v2 1/2] driver core: Move the "removable" attribute from
 USB to core

Hi Krzysztof,

Thanks a lot for your comments. Please see inline.

On Tue, May 11, 2021 at 6:00 PM Krzysztof Wilczyński <kw@...ux.com> wrote:
>
> Hi Rajat,
>
> I have few questions below, but to add in advance, I might be confusing
> the role that "type->supports_removable" and "dev->removable" plays
> here, and if so then I apologise.
>
> [...]
> > @@ -2504,8 +2523,16 @@ static int device_add_attrs(struct device *dev)
> >                       goto err_remove_dev_online;
> >       }
> >
> > +     if (type && type->supports_removable) {
> > +             error = device_create_file(dev, &dev_attr_removable);
> > +             if (error)
> > +                     goto err_remove_dev_waiting_for_supplier;
> > +     }
> > +
> >       return 0;
>
> Would a check for "dev->removable == DEVICE_REMOVABLE" here be more
> appropriate?
>
> Unless you wanted to add sysfs objects when the device hints that it has
> a notion of being removable even though it might be set to "unknown" or
> "fixed" (if that state is at all possible then), and in which case using
> the dev_is_removable() helper would also not be an option since it does
> a more complex check internally.
>
> Technically, you could always add this sysfs object (similarly to what
> USB core did) as it would then show the correct state depending on
> "dev->removable".
>
> Also, I suppose, it's not possible for a device to have
> "supports_removable" set to true, but "removable" would be different
> than "DEVICE_REMOVABLE", correct?

No, that is not true.

device_type->supports_removable=1 indicates that the bus / subsystem
is capable of differentiating between removable and fixed devices.
It's essentially describing a capability of the bus / subsystem. This
flag needs to be true for a subsystem for any it's devices'
dev->removable field to be considered meaningful.

OTOH, the dev->removable => indicates the location of the device IF
device_type->supports location is true. Yes, it can be fixed /
removable / unknown (whatever the bus decides) if the
device_type->supports_location is true.

One of my primary considerations was also that the existing UAPI for
the USB's "removable" attribute shouldn't be changed. Currently, it
exists for all USB devices, so I think the current code / check is OK.

>
> [...]
> > +enum device_removable {
> > +     DEVICE_REMOVABLE_UNKNOWN = 0,
> > +     DEVICE_REMOVABLE,
> > +     DEVICE_FIXED,
> > +};
>
> I know this was moved from the USB core, but I personally find it
> a little bit awkward to read, would something like that be acceptable?
>
> enum device_removable {
>         DEVICE_STATE_UNKNOWN = 0,
>         DEVICE_STATE_REMOVABLE,
>         DEVICE_STATE_FIXED,
> };
>
> The addition of state to the name follows the removable_show() function
> where the local variable is called "state", and I think it makes sense
> to call this as such.  What do you think?

I think I made a mistake by using the "state" as the local variable
there. I will change it to "location". I'm happy to change the enums
above to DEVICE_LOCATION_REMOVABLE* etc if there is a wider consensus
on this. IMHO, the current shorter one also looks OK.

>
> > +static inline bool dev_is_removable(struct device *dev)
> > +{
> > +     return dev && dev->type && dev->type->supports_removable
> > +         && dev->removable == DEVICE_REMOVABLE;
> > +}
>
> Similarly to my question about - would a simple check to see if
> "dev->removable" is set to "DEVICE_REMOVABLE" here be enough?

No, as I mentioned above, the dev->removable field should be
considered meaningful only if device_type->supports_location is true.
So the check for supports_removable is needed here.

Please feel free to send me more thoughts.

Thanks & Best Regards,

Rajat


>
> Krzysztof