Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 17 May 2023 10:28:01 +0200
From:   Johan Hovold <johan@kernel.org>
To:     Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= <u.kleine-koenig@pengutronix.de>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, kernel@pengutronix.de
Subject: Re: [PATCH] driver core: Call pm_runtime_put_sync() only after
 device_remove()
Message-ID: <ZGSQEapCl5HfQpY8@hovoldconsulting.com>
References: <20230511073428.10264-1-u.kleine-koenig@pengutronix.de>
 <CAJZ5v0gNPt=rq+pQtmoGL5nxzDQboOK4d6h7=NoY=LueVhZjAQ@mail.gmail.com>
 <20230511103923.hvibdyo5ges4bab2@pengutronix.de>
 <ZFzWCey825wSlr2v@hovoldconsulting.com>
 <CAJZ5v0jvJT4JkHtO3RCUEzkfawxLCwR=QO2Y2CsL=cYN9s4hXw@mail.gmail.com>
 <ZF3tUQFTeILXV_VT@hovoldconsulting.com>
 <20230512184925.d7w3j4r7oajtpsxi@pengutronix.de>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha256;
        protocol="application/pgp-signature"; boundary="u3d0lRLJWdny4Pqi"
Content-Disposition: inline
In-Reply-To: <20230512184925.d7w3j4r7oajtpsxi@pengutronix.de>
Precedence: bulk


--u3d0lRLJWdny4Pqi
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Fri, May 12, 2023 at 08:49:25PM +0200, Uwe Kleine-K=C3=B6nig wrote:
> On Fri, May 12, 2023 at 09:40:01AM +0200, Johan Hovold wrote:
> > On Thu, May 11, 2023 at 04:44:25PM +0200, Rafael J. Wysocki wrote:
> > > On Thu, May 11, 2023 at 1:48=E2=80=AFPM Johan Hovold <johan@kernel.or=
g> wrote:
> >=20
> > > > No, this seems like very bad idea and even violates the documentati=
on
> > > > which clearly states that the usage counter is balanced before call=
ing
> > > > remove() so that drivers can use pm_runtime_suspend() to put devices
> > > > into suspended state.
> > >=20
> > > I missed that, sorry.
> > >=20
> > > > There's is really no good reason to even try to change as this is i=
n no
> > > > way a fast path.
> > >=20
> > > Still, I think that while the "put" part needs to be done before
> > > device_remove(), the actual state change can be carried out later.
> > >=20
> > > So something like
> > >=20
> > >     pm_runtime_put_noidle(dev);
> > >=20
> > >     device_remove(dev);
> > >=20
> > >     pm_runtime_suspend(dev);
> > >=20
> > > would generally work, wouldn't it?
> >=20
> > No, as drivers typically disable runtime pm in their remove callbacks,
> > that pm_runtime_suspend() would amount to a no-op (and calling the
> > driver pm ops post unbind and the driver having freed its data would
> > not work either).
>=20
> However if a driver author also cares for the CONFIG_PM=3Dn case, calling
> pm_runtime_suspend() doesn't have the intended effect and so it's
> unfortunately complicated to rely on runtime-pm to power down your
> device and you have to do it by hand anyhow (unless you let your driver
> depend on CONFIG_PM). So I'm not convinced that "A driver can call
> pm_runtime_suspend() to power down" is a useful thing to have.

Right, but we do have drivers that have CONFIG_PM as an explicit
dependency.

> In the end something like 72362dcdf654 ("can: mcp251xfd:
> mcp251xfd_unregister(): simplify runtime PM handling") might be an
> approach. But IMHO it's more complicated than it should be and honestly
> I'm not sure if it's safe and correct this way.

Yeah, unfortunately runtime PM is fairly underspecified so we end up
with this multitude of implementations, many of which are broken in
various ways. A smaller API with documented best-practices may have
helped, but that's not where we are right now.

Looks like 72362dcdf654 ("can: mcp251xfd: mcp251xfd_unregister():
simplify runtime PM handling") introduces yet another way to do things,
and which will break if anyone enables (or tries to use this pattern in
another driver with) autosuspend...

Johan

--u3d0lRLJWdny4Pqi
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iHUEABYIAB0WIQQHbPq+cpGvN/peuzMLxc3C7H1lCAUCZGSQDQAKCRALxc3C7H1l
CCUxAQCBMVdykLUbY8F9CWXd5BBDXfshKIcDX1jXyL2+ftKfpAD/RWsfIRH06GYz
7PlUJ/q2Y7mzBVV4JEzZlWr8lX7/+g8=
=o2gs
-----END PGP SIGNATURE-----

--u3d0lRLJWdny4Pqi--