On Thu, Jan 04, 2024 at 08:08:50PM +0100, Krzysztof Kozlowski wrote:
> On 04/01/2024 17:30, Sean Anderson wrote:

> > device post-assert delay post-deassert delay
> > ====== ================= ===================
> > A 500us 1ms
> > B 1ms 300us

...

> Now, if we are back to realistic cases - use just the longest reset time.

Isn't the main concern here that when one device probes we don't yet
know the times for the other devices?

> > If we leave things up to the drivers, then whoever probes first will get
> > to decide the reset sequence.

> In current design yes, but it's not a problem to change it. Where is the
> limitation? Just read other values and update the reset time.

We might have already done a reset by that time and earlier devices
might prevent later devices from resetting again. It shouldn't be such
an issue for the post delay, but might be one for the pre delay.

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2024-01-06 15:32:52

by Krzysztof Kozlowski

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Subject: Re: [PATCH 2/4] reset: add GPIO-based reset controller

On 05/01/2024 15:33, Mark Brown wrote:
> On Thu, Jan 04, 2024 at 08:08:50PM +0100, Krzysztof Kozlowski wrote:
>> On 04/01/2024 17:30, Sean Anderson wrote:
>
>>> device post-assert delay post-deassert delay
>>> ====== ================= ===================
>>> A 500us 1ms
>>> B 1ms 300us
>
> ...
>
>> Now, if we are back to realistic cases - use just the longest reset time.
>
> Isn't the main concern here that when one device probes we don't yet
> know the times for the other devices?

You can never know when second device will appear. It might come from
modules, DTB overlay etc. If we want to satisfy all users, then we need
to wait till all users appear, which I think is not even possible,
considering runtime loaded overlays.

>
>>> If we leave things up to the drivers, then whoever probes first will get
>>> to decide the reset sequence.
>
>> In current design yes, but it's not a problem to change it. Where is the
>> limitation? Just read other values and update the reset time.
>
> We might have already done a reset by that time and earlier devices
> might prevent later devices from resetting again. It shouldn't be such
> an issue for the post delay, but might be one for the pre delay.

If the reset deassert (or assert, depending what's the default state) is
triggered in the probe, then it will happen with the probe of the first
device. If the delays of that reset are not suitable for the second -
not yet probed - then what do you propose? I have the answer: do not use
the simple, generic solution. The simple and generic solutions work for
simple and generic cases.

Best regards,
Krzysztof

2024-01-09 09:41:37

by Krzysztof Kozlowski

[permalink] [raw]

Subject: Re: [PATCH 2/4] reset: add GPIO-based reset controller

On 05/01/2024 15:31, Philipp Zabel wrote:
>>>> Sorry, then I don't get what you refer to. The driver calls deassert
>>>> when it is safe for it to do it, so the driver *knows*. Now, you claim
>>>> that driver does not know that... core also does not know, so no one knows.
>>>
>>> Yes! That is the problem with this design. Someone has to coordinate the
>>> reset, and it can't be the driver. But the core also doesn't have enough
>>> information. So no one can do it.
>>
>> The point is that the driver coordinates.
>
> Currently the reset controller API supports two types of shared resets.
> I hope distinguishing the two types and illustrating them helps the
> discussion:
>
> 1) For devices that just require the reset to be deasserted while they
> are active, and don't care otherwise, there is the clk-like behavior
> described in [1].
>
> requested reset signal via reset_control_deassert/assert():
> device A: ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
> device B: ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
>
> actual reset signal to both devices:
> ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
>
> In this scenario, there should be no delays in the reset controller
> driver. reset_control_deassert() may return as soon as the physical
> reset signal is deasserted [2]. Any post-deassert delays required by
> the devices are handled in the device drivers, and they can be
> different for each device. The devices have to be able to cope with a
> (much) longer post-deassert delay than expected (e.g. device B in this
> case). It is assumed that the reset signal is initially asserted.
>
> The reset-gpio patchset supports this.

Yep! :)

>
> 2) The second type is for devices that require a single reset pulse for
> initialization, at any time before they become active. This is
> described in [3].
>
> requested reset signal via reset_control_reset/rearm():
> device A: ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
> device B: ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
>
> actual reset signal to both devices:
> ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
>
> Here the reset controller needs to know the delay between assertion and
> deassertion - either baked into the hardware or as a delay call in the
> .reset callback.
>
> This is not supported by the reset-gpio patchset. It could be

Yep, as well.

> implemented via a delay property in the device tree that would have to
> be the same for all devices sharing the reset line, and by adding the

Or through dedicated node to which reset-gpio binds, just like in Sean's
code some years ago. Nothing stops achieving that, except of course
convincing Rob. The point is that although my design does not solve it,
it also does not prevent it in the future.

> .reset callback to the reset controller driver. The only issue is that
> the initial state of the reset line should be deasserted, and at
> reset_control_get() time, when the reset-gpio controller is
> instantiated, it is not yet known which type the driver will use.
>
> Sharing a reset line between devices of different type is not
> supported. Unfortunately, this will only fail at
> reset_control_deassert() / reset_control_reset() time when the second
> device tries to use the reset control in a different way than the
> first.
>
> [1] https://docs.kernel.org/driver-api/reset.html#assertion-and-deassertion
> [2] https://docs.kernel.org/driver-api/reset.html#c.reset_control_deassert
> [3] https://docs.kernel.org/driver-api/reset.html#triggering
>
>>> For example, say we want to share a reset GPIO between two devices. Each
>>> device has the following constraints:
>>>
>>> device post-assert delay post-deassert delay
>>> ====== ================= ===================
>>> A 500us 1ms
>>> B 1ms 300us
>>
>> And now imagine that these values are incompatible between them, so
>> using 1ms on device A is wrong - too long.
>>
>> This is just not doable. You invented some imaginary case to prove that
>> hardware is broken.
>>
>> Now, if we are back to realistic cases - use just the longest reset time.
>
> Right. This all only works if no device has an upper bound to the
> allowed delays on the shared reset line.

If device had an upper bound, it would be quite a conflicting design,
tricky to implement. I don't think we should target such case with
generic solution.

>
> I interpret the post-assert delay to be the desired length of the reset
> pulse between the rising edge and the falling edge in case 2) above,
> since in case 1) a post-assert delay is not useful.
>
> The post-deassert delays are not supposed to be handled by the reset
> controller drivers at all, except where they are needed to reach the
> deasserted state on the reset line. Reset drivers that do have post-
> deassert delays in the .deassert callback might be bending the rules a
> bit for convenience.

Best regards,
Krzysztof

2024-01-09 15:54:00

by Sean Anderson

[permalink] [raw]

Subject: Re: [PATCH 2/4] reset: add GPIO-based reset controller

On 1/9/24 04:41, Krzysztof Kozlowski wrote:
> On 05/01/2024 15:31, Philipp Zabel wrote:
>>>>> Sorry, then I don't get what you refer to. The driver calls deassert
>>>>> when it is safe for it to do it, so the driver *knows*. Now, you claim
>>>>> that driver does not know that... core also does not know, so no one knows.
>>>>
>>>> Yes! That is the problem with this design. Someone has to coordinate the
>>>> reset, and it can't be the driver. But the core also doesn't have enough
>>>> information. So no one can do it.
>>>
>>> The point is that the driver coordinates.
>>
>> Currently the reset controller API supports two types of shared resets.
>> I hope distinguishing the two types and illustrating them helps the
>> discussion:
>>
>> 1) For devices that just require the reset to be deasserted while they
>> are active, and don't care otherwise, there is the clk-like behavior
>> described in [1].
>>
>> requested reset signal via reset_control_deassert/assert():
>> device A: ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
>> device B: ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
>>
>> actual reset signal to both devices:
>> ⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺⎺
>>
>> In this scenario, there should be no delays in the reset controller
>> driver. reset_control_deassert() may return as soon as the physical
>> reset signal is deasserted [2]. Any post-deassert delays required by
>> the devices are handled in the device drivers, and they can be
>> different for each device. The devices have to be able to cope with a
>> (much) longer post-deassert delay than expected (e.g. device B in this
>> case). It is assumed that the reset signal is initially asserted.
>>
>> The reset-gpio patchset supports this.
>
> Yep! :)
>
>>
>> 2) The second type is for devices that require a single reset pulse for
>> initialization, at any time before they become active. This is
>> described in [3].
>>
>> requested reset signal via reset_control_reset/rearm():
>> device A: ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
>> device B: ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
>>
>> actual reset signal to both devices:
>> ⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽/⎺⎺\⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽⎽
>>
>> Here the reset controller needs to know the delay between assertion and
>> deassertion - either baked into the hardware or as a delay call in the
>> .reset callback.
>>
>> This is not supported by the reset-gpio patchset. It could be
>
> Yep, as well.
>
>> implemented via a delay property in the device tree that would have to
>> be the same for all devices sharing the reset line, and by adding the
>
> Or through dedicated node to which reset-gpio binds, just like in Sean's
> code some years ago. Nothing stops achieving that, except of course
> convincing Rob. The point is that although my design does not solve it,
> it also does not prevent it in the future.

Given this and

> If the reset deassert (or assert, depending what's the default state) is
> triggered in the probe, then it will happen with the probe of the first
> device. If the delays of that reset are not suitable for the second -
> not yet probed - then what do you propose? I have the answer: do not use
> the simple, generic solution. The simple and generic solutions work for
> simple and generic cases.

I think a separate pseudo-device is necessary a generic solution. So I
guess I will revive my patchset.

>> .reset callback to the reset controller driver. The only issue is that
>> the initial state of the reset line should be deasserted, and at
>> reset_control_get() time, when the reset-gpio controller is
>> instantiated, it is not yet known which type the driver will use.
>>
>> Sharing a reset line between devices of different type is not
>> supported. Unfortunately, this will only fail at
>> reset_control_deassert() / reset_control_reset() time when the second
>> device tries to use the reset control in a different way than the
>> first.
>>
>> [1] https://cas5-0-urlprotect.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fdocs.kernel.org%2fdriver%2dapi%2freset.html%23assertion%2dand%2ddeassertion&umid=0ba4c26a-9b7a-4e4b-8dba-ac7f2f194fcd&auth=d807158c60b7d2502abde8a2fc01f40662980862-4bd780a2a258eadb798324d4af563d691f01efb6
>> [2] https://cas5-0-urlprotect.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fdocs.kernel.org%2fdriver%2dapi%2freset.html%23c.reset%5fcontrol%5fdeassert&umid=0ba4c26a-9b7a-4e4b-8dba-ac7f2f194fcd&auth=d807158c60b7d2502abde8a2fc01f40662980862-d6349120c92e1887c5765842e10b274192584bde
>> [3] https://cas5-0-urlprotect.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2fdocs.kernel.org%2fdriver%2dapi%2freset.html%23triggering&umid=0ba4c26a-9b7a-4e4b-8dba-ac7f2f194fcd&auth=d807158c60b7d2502abde8a2fc01f40662980862-973380c68f114aad02c47e69b5ceb92a23759963
>>
>>>> For example, say we want to share a reset GPIO between two devices. Each
>>>> device has the following constraints:
>>>>
>>>> device post-assert delay post-deassert delay
>>>> ====== ================= ===================
>>>> A 500us 1ms
>>>> B 1ms 300us
>>>
>>> And now imagine that these values are incompatible between them, so
>>> using 1ms on device A is wrong - too long.
>>>
>>> This is just not doable. You invented some imaginary case to prove that
>>> hardware is broken.
>>>
>>> Now, if we are back to realistic cases - use just the longest reset time.
>>
>> Right. This all only works if no device has an upper bound to the
>> allowed delays on the shared reset line.
>
> If device had an upper bound, it would be quite a conflicting design,
> tricky to implement. I don't think we should target such case with
> generic solution.

This is why I had explicit properties for the various durations. That
way the system integrator can go through the reset requirements
and specify something which satisfies all devices.

--Sean

2024-02-29 17:27:35

by Tim Harvey

[permalink] [raw]

Subject: Re: [PATCH 2/4] reset: add GPIO-based reset controller

> On 1/9/24 04:41, Krzysztof Kozlowski wrote:
>
> I think a separate pseudo-device is necessary a generic solution. So I
> guess I will revive my patchset.
>

Sean and Krzysztof,

I see a lot of value in a generic reset-gpio driver that you have both
tried to get accepted in the past. I support boards that have a number
of SPI and I2C devices that often have GPIO resets wired to them that
are pulled in hardware to the in-reset state and find no support in
the various drivers for reset handling. I've often wondered why a
generic gpio reset wasn't supported in the SPI/I2C cores like it is
for some other subsystems.

The approach of a gpios-reset solution makes sense to me.

Will you be submitting a follow-on series to your previous attempts?

Best Regards,

Tim

2024-02-29 17:45:55

by Krzysztof Kozlowski

[permalink] [raw]

Subject: Re: [PATCH 2/4] reset: add GPIO-based reset controller

On 29/02/2024 18:26, Tim Harvey wrote:
>> On 1/9/24 04:41, Krzysztof Kozlowski wrote:
>>
>> I think a separate pseudo-device is necessary a generic solution. So I
>> guess I will revive my patchset.
>>
>
> Sean and Krzysztof,
>
> I see a lot of value in a generic reset-gpio driver that you have both
> tried to get accepted in the past. I support boards that have a number
> of SPI and I2C devices that often have GPIO resets wired to them that
> are pulled in hardware to the in-reset state and find no support in
> the various drivers for reset handling. I've often wondered why a
> generic gpio reset wasn't supported in the SPI/I2C cores like it is
> for some other subsystems.
>
> The approach of a gpios-reset solution makes sense to me.
>
> Will you be submitting a follow-on series to your previous attempts?

Sorry, don't get. Why do you revive this old thread after code was
merged? If my patchset which was finally merged a week ago or so does
not work for you, please comment there or send follow-up work extending it.

Best regards,
Krzysztof