linux-kernel - Re: [PATCH v4 1/3] dt-bindings: pwm: Add Xilinx AXI Timer

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Message-ID: <45b3c713-952e-610f-0a98-ed8e48825e97@seco.com>
Date:   Fri, 2 Jul 2021 13:31:54 -0400
From:   Sean Anderson <sean.anderson@...o.com>
To:     Michal Simek <michal.simek@...inx.com>,
        Michal Simek <monstr@...str.eu>, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Sascha Hauer <s.hauer@...gutronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Alvaro Gamez <alvaro.gamez@...ent.com>,
        linux-arm-kernel@...ts.infradead.org, Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v4 1/3] dt-bindings: pwm: Add Xilinx AXI Timer



On 7/2/21 8:40 AM, Michal Simek wrote:
>
>
> On 7/1/21 5:32 PM, Sean Anderson wrote:
>>
>>
>> On 6/30/21 9:47 AM, Michal Simek wrote:
>>>
>>>
>>> On 5/28/21 11:45 PM, Sean Anderson wrote:
>>>> This adds a binding for the Xilinx LogiCORE IP AXI Timer. This device is
>>>> a "soft" block, so it has many parameters which would not be
>>>> configurable in most hardware. This binding is usually automatically
>>>> generated by Xilinx's tools, so the names and values of some properties
>>>> must be kept as they are. Replacement properties have been provided for
>>>> new device trees.
>>>>
>>>> Because we need to init timer devices so early in boot, the easiest way
>>>> to configure things is to use a device tree property. For the moment
>>>> this is 'xlnx,pwm', but this could be extended/renamed/etc. in the
>>>> future if these is a need for a generic property.
>>>>
>>>> Signed-off-by: Sean Anderson <sean.anderson@...o.com>
>>>> ---
>>>>
>>>> Changes in v4:
>>>> - Remove references to generate polarity so this can get merged
>>>> - Predicate PWM driver on the presence of #pwm-cells
>>>> - Make some properties optional for clocksource drivers
>>>>
>>>> Changes in v3:
>>>> - Mark all boolean-as-int properties as deprecated
>>>> - Add xlnx,pwm and xlnx,gen?-active-low properties.
>>>> - Make newer replacement properties mutually-exclusive with what they
>>>>   replace
>>>> - Add an example with non-deprecated properties only.
>>>>
>>>> Changes in v2:
>>>> - Use 32-bit addresses for example binding
>>>>
>>>>  .../bindings/pwm/xlnx,axi-timer.yaml          | 85 +++++++++++++++++++
>>>>  1 file changed, 85 insertions(+)
>>>>  create mode 100644
>> Documentation/devicetree/bindings/pwm/xlnx,axi-timer.yaml
>>>>
>>>> diff --git
>> a/Documentation/devicetree/bindings/pwm/xlnx,axi-timer.yaml
>> b/Documentation/devicetree/bindings/pwm/xlnx,axi-timer.yaml
>>>> new file mode 100644
>>>> index 000000000000..48a280f96e63
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/pwm/xlnx,axi-timer.yaml
>>>
>>> I don't think this is the right location for this.
>>>
>>> I have done some grepping and I think this should be done in a different
>>> way. I pretty much like solution around "ti,omap3430-timer" which is
>>> calling dmtimer_systimer_select_best() and later dmtimer_is_preferred()
>>> which in this case would allow us to get rid of cases which are not
>>> suitable for clocksource and clockevent.
>>>
>>> And there is drivers/pwm/pwm-omap-dmtimer.c which has link to timer
>>> which is providing functions for it's functionality.
>>>
>>> I have also looked at
>>> Documentation/devicetree/bindings/timer/nxp,tpm-timer.yaml which is also
>>> the same device.
>>
>> Ok, I will move this under bindings/timer.
>>
>>>
>>> And sort of curious if you look at
>>>
>> https://www.xilinx.com/support/documentation/ip_documentation/axi_timer/v2_0/pg079-axi-timer.pdf
>>
>>> ( Figure 1-1)
>>> that PWM is taking input from generate out 0 and generate out 1 which is
>>> maybe can be modeled is any output and pwm driver can register inputs
>>> for pwm driver.
>>
>> I don't think that is a good model, since several bits (GENERATE, PWM,
>> etc) need to be set in the TCSR, and we need to coordinate changes
>> between timers closely to keep our contract for apply_state(). Although
>> that is how the hardware is organized, the requirements of the
>> clocksource and pwm subsystems are very different.
>
> There is another upstream solution done by samsung. Where they use
> samsung,pwm-outputs property to identify PWMs.

As I understand it, the samsung PWM/timer has 5 timers, four of which
may be independently configured as PWMs. To contrast, this device has at
most two timers, both of which must be used for a single PWM output.
Because of this, it is sufficient to have a single property whose
presence indicates that the device is to be configured as a PWM.

> I think that make sense to consider to identify which timer should be
> clocksource/clockevent because with MB SMP this has to be done to pair
> timer with cpu for clockevents.

This is not done by the current driver. The first timer in the system
always binds itself to CPU 0.

--Sean