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:   Fri, 23 Sep 2022 09:09:32 -0400
From:   William Breathitt Gray <william.gray@linaro.org>
To:     Julien Panis <jpanis@baylibre.com>
Cc:     robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, mranostay@ti.com
Subject: Re: [PATCH v8 3/4] counter: ti-ecap-capture: capture driver support
 for ECAP
Message-ID: <Yy2wDPITCZ/J01Xr@fedora>
References: <20220922170402.403683-1-jpanis@baylibre.com>
 <20220922170402.403683-4-jpanis@baylibre.com>
 <Yy0G9a5S3OzwyEwW@fedora>
 <2f3e5036-caab-f892-a4ad-b852f72db331@baylibre.com>
 <Yy2aAMv5PRjsJ4s2@fedora>
 <923f5876-d692-7e0e-f351-f0e05869ccd8@baylibre.com>
 <8a60b6a0-9289-0d89-c543-02386a721bca@baylibre.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
        protocol="application/pgp-signature"; boundary="XPKB3zJM2TKbw2gr"
Content-Disposition: inline
In-Reply-To: <8a60b6a0-9289-0d89-c543-02386a721bca@baylibre.com>
Precedence: bulk


--XPKB3zJM2TKbw2gr
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Fri, Sep 23, 2022 at 02:32:26PM +0200, Julien Panis wrote:
>=20
>=20
> On 23/09/2022 14:17, Julien Panis wrote:
> >=20
> >=20
> > On 23/09/2022 13:35, William Breathitt Gray wrote:
> > > On Fri, Sep 23, 2022 at 09:23:26AM +0200, Julien Panis wrote:
> > > >=20
> > > > On 23/09/2022 03:08, William Breathitt Gray wrote:
> > > > > On Thu, Sep 22, 2022 at 07:04:01PM +0200, Julien Panis wrote:
> > > > > > ECAP hardware on TI AM62x SoC supports capture feature.
> > > > > > It can be used
> > > > > > to timestamp events (falling/rising edges) detected on input si=
gnal.
> > > > > >=20
> > > > > > This commit adds capture driver support for ECAP
> > > > > > hardware on AM62x SoC.
> > > > > >=20
> > > > > > In the ECAP hardware, capture pin can also be configured to be =
in
> > > > > > PWM mode. Current implementation only supports capture
> > > > > > operating mode.
> > > > > > Hardware also supports timebase sync between multiple instances=
, but
> > > > > > this driver supports simple independent capture functionality.
> > > > > >=20
> > > > > > Signed-off-by: Julien Panis <jpanis@baylibre.com>
> > > > > Hello Julien,
> > > > >=20
> > > > > Comments follow inline below.
> > > > >=20
> > > > > > +/**
> > > > > > + * struct ecap_cnt_dev - device private data structure
> > > > > > + * @enabled: device state
> > > > > > + * @clk:=A0=A0=A0=A0 device clock
> > > > > > + * @regmap:=A0 device register map
> > > > > > + * @nb_ovf:=A0 number of overflows since capture start
> > > > > > + * @pm_ctx:=A0 device context for PM operations
> > > > > > + */
> > > > > > +struct ecap_cnt_dev {
> > > > > > +=A0=A0=A0 bool enabled;
> > > > > > +=A0=A0=A0 struct clk *clk;
> > > > > > +=A0=A0=A0 struct regmap *regmap;
> > > > > > +=A0=A0=A0 atomic_t nb_ovf;
> > > > > > +=A0=A0=A0 struct {
> > > > > > +=A0=A0=A0=A0=A0=A0=A0 u8 ev_mode;
> > > > > > +=A0=A0=A0=A0=A0=A0=A0 u32 time_cntr;
> > > > > > +=A0=A0=A0 } pm_ctx;
> > > > > > +};
> > > > > Provide documentation for the ev_mode and time_cntr members. You
> > > > > probably need a lock as well to protect access to this structure =
or
> > > > > you'll end up with race problems.
> > > > Hi William,
> > > >=20
> > > > How can I end up with race problems ? pm_ctx members are only
> > > > accessed at
> > > > suspend (after capture/IRQ are disabled) and resume (before
> > > > capture/IRQ are
> > > > re-enabled).
> > > > Is there any risk I did not identify ?
> > > >=20
> > > > Julien
> > > I was thinking of the ecap_cnt_dev enabled member. The Counter callba=
cks
> > > may execute in concurrent threads, so races can appear when you access
> > > members of the ecap_cnt_dev structure in these callbacks.
> > >=20
> > > Take for example this section of ecap_cnt_enable_write():
> > >=20
> > > =A0=A0=A0=A0=A0=A0=A0=A0 if (enable =3D=3D ecap_dev->enabled)
> > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 return 0;
> > > =A0=A0=A0=A0=A0=A0=A0=A0 if (enable)
> > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 ecap_cnt_capture_ena=
ble(counter);
> > > =A0=A0=A0=A0=A0=A0=A0=A0 else
> > > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 ecap_cnt_capture_dis=
able(counter);
> > > =A0=A0=A0=A0=A0=A0=A0=A0 ecap_dev->enabled =3D enable
> > >=20
> > > Suppose two threads try to enable the count capture. A race condition=
 is
> > > present where the two threads could see ecap_dev->enabled as false and
> > > both proceed to call ecap_cnt_capture_enable(). This results in
> > > pm_runtime_get_sync() bumping the usage count twice and we're left wi=
th
> > > a mismatch the next time ecap_cnt_capture_disable() is called.
> > >=20
> > > William Breathitt Gray
> >=20
> > OK, If I understand well there's the same problem with IO access with
> > regmap ?
> > Julien
>=20
> [ERRATUM] It seems that some spinlock is already used by regmap API.
> So, only the 'enabled' member needs a lock.

Keep in mind that although individual regmap API calls can be considered
atomic, the order of the calls made could be a point of race if there
are concurrent threads. However, in your particular driver it looks
you're fine as long as access to 'enabled' is protected.

William Breathitt Gray

--XPKB3zJM2TKbw2gr
Content-Type: application/pgp-signature; name="signature.asc"

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

iHUEARYKAB0WIQSNN83d4NIlKPjon7a1SFbKvhIjKwUCYy2wDAAKCRC1SFbKvhIj
KyNpAQCvH/rPZZz1R4Vh3ZDkM0LwoAjB6xLQsH7OrXS24uapFgD+L+lRKRh0hTw2
DUe0T8mhULo6o9b3NMlQEXOY3diuQQU=
=RAH6
-----END PGP SIGNATURE-----

--XPKB3zJM2TKbw2gr--