linux-kernel - Re: [PATCH 0/6] usb: dwc3: Calculate REFCLKPER et. al. from reference clock

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Date:   Tue, 18 Jan 2022 21:10:24 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Sean Anderson <sean.anderson@...o.com>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Baruch Siach <baruch@...s.co.il>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Robert Hancock <robert.hancock@...ian.com>,
        Felipe Balbi <balbi@...nel.org>,
        Balaji Prakash J <bjagadee@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Michal Simek <michal.simek@...inx.com>,
        Rob Herring <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH 0/6] usb: dwc3: Calculate REFCLKPER et. al. from reference
 clock

Hi Sean,

Sean Anderson wrote:
> Hi Thinh,
> 
> On 1/18/22 3:00 PM, Thinh Nguyen wrote:
>> Sean Anderson wrote:
>>> Hi Thinh,
>>>
>>> On 1/18/22 2:46 PM, Thinh Nguyen wrote:
>>>> Hi Sean,
>>>>
>>>> Baruch Siach wrote:
>>>>> Hi Sean, Thinh,
>>>>>
>>>>> On Fri, Jan 14 2022, Sean Anderson wrote:
>>>>>> This is a rework of patches 3-5 of [1]. It attempts to correctly
>>>>>> program
>>>>>> REFCLKPER and REFCLK_FLADJ based on the reference clock frequency.
>>>>>> Since
>>>>>> we no longer need a special property duplicating this configuration,
>>>>>> snps,ref-clock-period-ns is deprecated.
>>>>>>
>>>>>> Please test this! Patches 3/4 in this series have the effect of
>>>>>> programming REFCLKPER and REFCLK_FLADJ on boards which already
>>>>>> configure
>>>>>> the "ref" clock. I have build tested, but not much else.
>>>>>>
>>>>>> [1]
>>>>>> https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20220114044230.2677283-1-robert.hancock@calian.com/__;!!A4F2R9G_pg!M3zKxDZC9a_etqzXo7GSEMTHRWfc1wR_84wwM4-fShiA35CsGcxcTEffHPbprbdC4d2R$
>>>>>>
>>>>>>
>>>>>
>>>>> Thinh, you suggested the dedicated DT property for the reference
>>>>> clock:
>>>>>
>>>>>
>>>>> https://urldefense.com/v3/__https://lore.kernel.org/all/d5acb192-80b9-36f7-43f5-81f21c4e6ba0@synopsys.com/__;!!A4F2R9G_pg!M3zKxDZC9a_etqzXo7GSEMTHRWfc1wR_84wwM4-fShiA35CsGcxcTEffHPbprbpOFmvX$
>>>>>
>>>>>
>>>>>
>>>>> Can you comment on this series?
>>>>>
>>>>
>>>> Unless there's a good way to pass this information for PCI devices, my
>>>> opinion hasn't changed. (Btw, I don't think creating a dummy clock
>>>> provider and its dummy ops is a good solution as seems to complicate
>>>> and
>>>> bloat the PCI glue drivers).
>>>
>>> Can you explain your situation a bit more? I'm not sure how you can
>>> access a device tree property but not add a fixed-rate clock.
>>>
>>> --Sean
>>
>> Currently for dwc3 pci devices, we have glue drivers that create a
>> platform_device with specific properties to pass to the dwc3 core
>> driver. Without a ref clock property, we would need another way to pass
>> this information to the core driver or another way for the dwc3 core
>> driver to check for specific pci device's properties and quirks.
> 
> The primary problem with the existing binding is that it does not
> contain enough information to calculate the fractional period. With the
> frequency, we can calculate the correct values for the registers without
> needing an additional binding. So we need to transition to some kind of
> frequency-based system. So perhaps we should add a "ref-clock-frequency"
> property and use that as a default for when the clock is missing.
> 

Not sure about others, but that's fine with me. The other solution is to
rework the dwc3 drivers as Robert noted, but that requires some work.

Thanks,
Thinh