Re: [PATCH 1/3] HID: logitech-hidpp: use devres to manage FF private data

[Andrey Smirnov]

On Fri, Oct 11, 2019 at 1:33 PM Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
>
> On Fri, Oct 11, 2019 at 09:25:52PM +0200, Benjamin Tissoires wrote:
> > On Fri, Oct 11, 2019 at 8:26 PM Dmitry Torokhov
> > <dmitry.torokhov@xxxxxxxxx> wrote:
> > >
> > > On Fri, Oct 11, 2019 at 04:52:04PM +0200, Benjamin Tissoires wrote:
> > > > Hi Andrey,
> > > >
> > > > On Mon, Oct 7, 2019 at 7:13 AM Andrey Smirnov <andrew.smirnov@xxxxxxxxx> wrote:
> > > > >
> > > > > To simplify resource management in commit that follows as well as to
> > > > > save a couple of extra kfree()s and simplify hidpp_ff_deinit() switch
> > > > > driver code to use devres to manage the life-cycle of FF private data.
> > > > >
> > > > > Signed-off-by: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
> > > > > Cc: Jiri Kosina <jikos@xxxxxxxxxx>
> > > > > Cc: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> > > > > Cc: Henrik Rydberg <rydberg@xxxxxxxxxxx>
> > > > > Cc: Sam Bazely <sambazley@xxxxxxxxxxxx>
> > > > > Cc: Pierre-Loup A. Griffais <pgriffais@xxxxxxxxxxxxxxxxx>
> > > > > Cc: Austin Palmer <austinp@xxxxxxxxxxxxxxxxx>
> > > > > Cc: linux-input@xxxxxxxxxxxxxxx
> > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > >
> > > > This patch doesn't seem to fix any error, is there a reason to send it
> > > > to stable? (besides as a dependency of the rest of the series).
> > > >
> > > > > ---
> > > > >  drivers/hid/hid-logitech-hidpp.c | 53 +++++++++++++++++---------------
> > > > >  1 file changed, 29 insertions(+), 24 deletions(-)
> > > > >
> > > > > diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
> > > > > index 0179f7ed77e5..58eb928224e5 100644
> > > > > --- a/drivers/hid/hid-logitech-hidpp.c
> > > > > +++ b/drivers/hid/hid-logitech-hidpp.c
> > > > > @@ -2079,6 +2079,11 @@ static void hidpp_ff_destroy(struct ff_device *ff)
> > > > >         struct hidpp_ff_private_data *data = ff->private;
> > > > >
> > > > >         kfree(data->effect_ids);
> > > >
> > > > Is there any reasons we can not also devm alloc data->effect_ids?
> > > >
> > > > > +       /*
> > > > > +        * Set private to NULL to prevent input_ff_destroy() from
> > > > > +        * freeing our devres allocated memory
> > > >
> > > > Ouch. There is something wrong here: input_ff_destroy() calls
> > > > kfree(ff->private), when the data has not been allocated by
> > > > input_ff_create(). This seems to lack a little bit of symmetry.
> > >
> > > Yeah, ff and ff-memless essentially take over the private data assigned
> > > to them. They were done before devm and the lifetime of the "private"
> > > data pieces was tied to the lifetime of the input device to simplify
> > > error handling and teardown.
> >
> > Yeah, that stealing of the pointer is not good :)
> > But OTOH, it helps
> >
> > >
> > > Maybe we should clean it up a bit... I'm open to suggestions.
> >
> > The problem I had when doing the review was that there is no easy way
> > to have a "devm_input_ff_create_()", because the way it's built is
> > already "devres-compatible": the destroy gets called by input core.
>
> I do not think we want devm_input_ff_create() explicitly, I think the
> fact that you can "build up" an input device by allocating it, then
> adding slots, poller, ff support, etc, and input core cleans it up is
> all good. It is just the ownership if the driver-private data block is
> not very obvious and is not compatible with allocating via devm.
>

What do you think of creating input_ff_create_with_private() which
would take a pointer to private memory and set a new
"borrowed_private" flag in struct ff_device which input_ff_destroy()
can check to determine if it needs to free ->private or not?

Thanks,
Andrey Smirnov