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

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Message-ID: <076fae75-d6e7-41bf-d9e6-c77d08aa51c4@synopsys.com>
Date:   Tue, 18 Jan 2022 20:54:08 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Robert Hancock <robert.hancock@...ian.com>,
        "sean.anderson@...o.com" <sean.anderson@...o.com>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "baruch@...s.co.il" <baruch@...s.co.il>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "bjagadee@...eaurora.org" <bjagadee@...eaurora.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "agross@...nel.org" <agross@...nel.org>
Subject: Re: [PATCH 0/6] usb: dwc3: Calculate REFCLKPER et. al. from reference
 clock

Robert Hancock wrote:
> On Tue, 2022-01-18 at 20:00 +0000, 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.
> 
> We've used the device tree to instantiate/configure devices inside of a PCI
> device, though obviously that only works on DT-based platforms, and for
> hardware that's part of the board itself, not an add-in card.
> 
> We've also used the MFD infrastructure to instantiate devices and device
> properties inside a PCI device on x86, which can be used if the driver you are
> instantiating uses the generic device property accessors and not the DT-
> specific ones. That gets a bit dirty however - I don't think there's an easy
> way to create properties that are references to other nodes, or more than a
> single level deep heirarchy of nodes.
> 
> For a use case like you're describing, it sounds like it would be better to
> abstract away some of the core DWC3 code from reading the settings from DT
> directly, so that the PCI devices can instantiate it and set the configuration
> however they want, without having to worry about creating fake properties for
> the core to read. I think that pattern has been used with some other drivers
> such as AHCI?
> 

Yes. It would be great if we can rework and abstract this. I'd need to
review how AHCI handles it. It doesn't seem like a small task as it
touches on multiple drivers. But if anyone can start this off, I can
help further contribute/review.

Thanks,
Thinh