Hi Peter,

On 18/11/21 2:54 am, Peter Rosin wrote:

Hi!

On 2021-11-15 07:31, Aswath Govindraju wrote:

quoted

Hi Peter,

On 13/11/21 12:45 am, Peter Rosin wrote:

quoted

Hi!

On 2021-11-12 14:48, Aswath Govindraju wrote:

quoted

Hi Marc,

On 12/11/21 2:10 pm, Marc Kleine-Budde wrote:

quoted

On 11.11.2021 22:13:12, Aswath Govindraju wrote:

quoted

On some boards, for routing CAN signals from controller to transceiver,
muxes might need to be set. Therefore, add support for setting the mux by
reading the mux-controls property from the device tree node.

Signed-off-by: Aswath Govindraju <redacted>
---
 drivers/phy/phy-can-transceiver.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/phy/phy-can-transceiver.c b/drivers/phy/phy-can-transceiver.c
index 6f3fe37dee0e..3d8da5226e27 100644
--- a/drivers/phy/phy-can-transceiver.c
+++ b/drivers/phy/phy-can-transceiver.c

@@ -10,6 +10,7 @@
 #include<linux/module.h>
 #include<linux/gpio.h>
 #include<linux/gpio/consumer.h>
+#include <linux/mux/consumer.h>
 
 struct can_transceiver_data {
 	u32 flags;

@@ -21,13 +22,22 @@ struct can_transceiver_phy {
 	struct phy *generic_phy;
 	struct gpio_desc *standby_gpio;
 	struct gpio_desc *enable_gpio;
+	struct mux_control *mux_ctrl;
 };
 
 /* Power on function */
 static int can_transceiver_phy_power_on(struct phy *phy)
 {
+	int ret;
 	struct can_transceiver_phy *can_transceiver_phy = phy_get_drvdata(phy);
 
+	if (can_transceiver_phy->mux_ctrl) {
+		ret = mux_control_select(can_transceiver_phy->mux_ctrl, 1);

Hard coding the "1" looks wrong here. I have seen some boards where you
can select between a CAN-2.0 and a single wire CAN transceiver with a
mux. So I think we cannot hard code the "1" here.

Yes, as you mentioned it is not ideal to hard code "1". I feel that, it
would be much better to read the state of the mux to be set from the
mux-controls property. The issue that I see with this approach is that
the current implementation in the mux framework only allows for one
argument, which is for indicating the line to be toggled in the mux. If
more arguments are added then an error is returned from the
"mux_control_get". I am not sure why this limitation was added.

The only current use of the first argument is for mux chips that contain
more than one mux control. The limit in the mux core is there since no
mux driver need more than this one argument. The number of mux-control
property arguments is fixed by the #mux-control-cells property in the
mux-control node. I don't see any way to and a new optional mux-control
property argument that specifies a specific state. How would that not
break all existing users?

My idea was to use the second argument for reading the state of mux to
be set after increasing the #mux-control-cells value to 2. I don't think
this will break the existing mux controller users as the second argument
was not used till now, would be equivalent to adding an additional feature.

Ok, I see what you mean now, sorry for being dense. If we allow this then
there is a need to add a special value that means all/many states (such as
-1 or something such) so that a mux-control can be used simultaneously by
drivers "pointing at" a specific state like you want to do, and by the
existing "application" style drivers that wraps the whole mux control.

I.e. something like this

	mux: mux {
		compatible = "mux-gpio";
		...

		#mux-control-cells = <1>; /* one more than previously */
	};

	phy {
		...

		mux-control = <&mux 3>; /* point to specific state */
	};

	i2c-mux {
		compatible = "i2c-mux-gpmux";
		parent = <&i2c0>
		mux-control = <&mux (-1)>; /* many states needed */

		...

		i2c@1 {
			eeprom@50 {
				...
			};
		};

		i2c@2 {
			...
		};
	};

Yes, I realize that accesses to the eeprom cannot happen if the mux is
constantly selected and locked in state 3 by the phy, and that a mux with
one channel being a phy and other channels being I2C might not be
realistic, but the same gpio lines might control several muxes that are
used for separate signals solving at least the latter "problem" with this
completely made up example. Anyway, the above is in principle, and HW
designs are sometimes too weird for words.

This is almost exactly what I was intending to implement except for one
more change. The state of the mux will always be represented using the
second argument(i.e. #mux-control-cells = <2>).

For example,
mux-controls = <&mux 0 1>, <&mux 1 0>;


With this I think we wouldn't need a special value for all or many states.

quoted

One more question that I had is, if the number of arguments match the
#mux-control-cells and if the number of arguments are greater than 1 why
is an error being returned?

Changing that would require a bindings update anyway, so I simply
disallowed it as an error. Not much thought went into the decision,
as it couldn't be wrong to do what is being done with the bindings
that exist. That said, I have no problem lifting this restriction,
if there's a matching bindings update that makes it all fit.

Sure, I think making a change in

Documentation/devicetree/bindings/mux/gpio-mux.yaml, should be good
enough I assume.


Thank you for the comments. I'll post a respin of this series, with the
above changes.

Thanks,
Aswath

quoted

quoted

The current mux interface is designed around the idea that you wrap a
mux control in a mux (lacking better name) application. There are
several such mux applications in the tree, those for I2C, IIO and SPI
pops into my head, and that you then tie the end user consumer to this
muxing application. The mux state comes as a part of how you have tied
the end user consumer to the mux application and is not really something
that the mux-control is involved in.

In other words, a mux-control is not really designed to be used directly
by a driver that needs only one of the states.

However, I'm not saying that doing so isn't also a useful model. It
cetainly sound like it could be. However, the reason it's not done that
way is that I did not want to add muxing code to *all* drivers. I.e. it
would not be flexible to have to add boilerplate mux code to each and
every IIO driver that happen to be connected in a way that a mux has to
be in a certain state for the signal to reach the ADC (or whatever).
Instead, new IIO channels are created for the appropriate mux states
and the IIO mux is connected to the parent IIO channel. When one of the
muxed channels is accessed the mux is selected as needed, and the ADC
driver needs to know nothing about it. If two muxes need to be in a
certain position, you again have no need to "pollute" drivers with
double builerplate mux code. Instead, you simply add two levels of
muxing to the muxed IIO channel.

I think the same is probably true in this case too, and that it would
perhaps be better to create a mux application for phys? But I don't know
what the phy structure looks like, so I'm not in a position to say for
sure if this model fits. But I imagine that phys have providers and
consumers and that a mux can be jammed in there in some way and
intercept some api such that the needed mux state can be selected when
needed.

Yes, I understand that reading the state of the mux in drivers would not
be efficient as it would adding the boiler plate code in each of the
drivers. However, for phys as each of them can be used for a different
interface, I am not sure if a common mux phy wrapper can be introduced.
This is reason why I felt that drivers should be allowed to read the
state of the mux directly, when no mux wrapper application is suitable
for it.

It need not be one grand unifying phy mux, it could be one for each
kind of phy interface. But again, I don't know much about how phys
work nor their interfaces, not event roughly how many drivers there
are etc etc. I have simply never needed to look.

Hmm, wild idea, maybe there could be a mux "application" for pinctrl?
I mean such that you could tie pinctrl states to mux states. It doesn't
sound like too bad of a match to me?

Cheers,
Peter

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