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

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Message-ID: <8e8094f148920388b81a2a58bd7cd85cd4316d92.camel@calian.com>
Date:   Tue, 18 Jan 2022 23:21:55 +0000
From:   Robert Hancock <robert.hancock@...ian.com>
To:     "sean.anderson@...o.com" <sean.anderson@...o.com>,
        "Thinh.Nguyen@...opsys.com" <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>,
        "robh@...nel.org" <robh@...nel.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

On Tue, 2022-01-18 at 21:10 +0000, Thinh Nguyen wrote:
> 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.
> 

Seems reasonable enough for a short-term fix anyway.

> Thanks,
> Thinh
> 
-- 
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com