Re: [PATCH v2 2/2] i2c: mux: pca954x: Support multiple devices on a single reset line

[Rob Herring]

On Wed, Aug 4, 2021 at 1:50 AM Peter Rosin <peda@xxxxxxxxxx> wrote:
>
> On 2021-08-02 23:51, Eddie James wrote:
> > On Mon, 2021-08-02 at 14:46 -0600, Rob Herring wrote:
> >> On Tue, Jul 27, 2021 at 11:03:15AM -0500, Eddie James wrote:
> >>> Some systems connect several PCA954x devices to a single reset
> >>> GPIO. For
> >>> these devices to get out of reset and probe successfully, each
> >>> device must
> >>> defer the probe until the GPIO has been hogged. Accomplish this by
> >>> attempting to grab a new "reset-shared-hogged" devicetree property,
> >>> but
> >>> expect it to fail with EPROBE_DEFER or EBUSY.
> >>>
> >>> Signed-off-by: Eddie James <eajames@xxxxxxxxxxxxx>
> >>> ---
> >>>  drivers/i2c/muxes/i2c-mux-pca954x.c | 46 +++++++++++++++++++++++
> >>> ------
> >>>  1 file changed, 37 insertions(+), 9 deletions(-)
> >>>
> >>> diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c
> >>> b/drivers/i2c/muxes/i2c-mux-pca954x.c
> >>> index 4ad665757dd8..376b54ffb590 100644
> >>> --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> >>> +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> >>> @@ -434,15 +434,43 @@ static int pca954x_probe(struct i2c_client
> >>> *client,
> >>>     i2c_set_clientdata(client, muxc);
> >>>     data->client = client;
> >>>
> >>> -   /* Reset the mux if a reset GPIO is specified. */
> >>> -   gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
> >>> -   if (IS_ERR(gpio))
> >>> -           return PTR_ERR(gpio);
> >>> -   if (gpio) {
> >>> -           udelay(1);
> >>> -           gpiod_set_value_cansleep(gpio, 0);
> >>> -           /* Give the chip some time to recover. */
> >>> -           udelay(1);
> >>> +   /*
> >>> +    * Grab the shared, hogged gpio that controls the mux reset. We
> >>> expect
> >>> +    * this to fail with either EPROBE_DEFER or EBUSY. The only
> >>> purpose of
> >>> +    * trying to get it is to make sure the gpio controller has
> >>> probed up
> >>> +    * and hogged the line to take the mux out of reset, meaning
> >>> that the
> >>> +    * mux is ready to be probed up. Don't try and set the line any
> >>> way; in
> >>> +    * the event we actually successfully get the line (if it
> >>> wasn't
> >>> +    * hogged) then we immediately release it, since there is no
> >>> way to
> >>> +    * sync up the line between muxes.
> >>> +    */
> >>> +   gpio = gpiod_get_optional(dev, "reset-shared-hogged", 0);
> >>> +   if (IS_ERR(gpio)) {
> >>> +           ret = PTR_ERR(gpio);
> >>> +           if (ret != -EBUSY)
> >>> +                   return ret;
> >>
> >> Why can't you just do this with the existing 'reset-gpios' property?
> >> What's the usecase where you'd want to fail probe because EBUSY
> >> other
> >> than an error in your DT.
> >
> > Hi, thanks for the reply.
> >
> > Are you suggesting I use "reset-gpios" and change the driver to ignore
> > EBUSY? I don't know any other usecase, I just didn't think it would be
> > acceptable to ignore EBUSY on that, but perhaps it is a better
> > solution.
>
> Hi!
>
> From a device-tree point of view that might seem simple. But it becomes
> a mess when several driver instances need to coordinate. If one instance
> is grabbing the reset line but is then stalled while other instances
> race ahead, they might be clobbered by a late reset from the stalled
> first instance.
>
> And while it might be possible to arrange the code such that those dragons
> are dodged and that the reset is properly coordinated, what if the gpio is
> supposed to be shared with some other totally unrelated driver? It might
> seem to work when everything is normal, but as soon as anything out of the
> ordinary happens, all bets are off. I expect subtle problems in the
> furture.

All of this is true, but a different reset GPIO property name does
nothing to solve it.

> I see no simple solution to this, and I also expect that if gpios need
> to be shared, there will eventually need to be some kind of layer that
> helps with coordination such that it becomes explicit rather than
> implicit and fragile.

Yes, like making the reset subsystem handle 'reset-gpios' properties
as I suggested.

Rob