Message-ID: <20230615210800.lvmekpvxjiszkrh4@synopsys.com>
Date:   Thu, 15 Jun 2023 21:08:01 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Krishna Kurapati PSSNV <quic_kriskura@...cinc.com>
CC:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Johan Hovold <johan@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Felipe Balbi <balbi@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "quic_pkondeti@...cinc.com" <quic_pkondeti@...cinc.com>,
        "quic_ppratap@...cinc.com" <quic_ppratap@...cinc.com>,
        "quic_wcheng@...cinc.com" <quic_wcheng@...cinc.com>,
        "quic_jackp@...cinc.com" <quic_jackp@...cinc.com>,
        "quic_harshq@...cinc.com" <quic_harshq@...cinc.com>,
        "ahalaney@...hat.com" <ahalaney@...hat.com>
Subject: Re: [PATCH v8 6/9] usb: dwc3: qcom: Add multiport controller support
 for qcom wrapper

On Thu, Jun 15, 2023, Krishna Kurapati PSSNV wrote:
> 
> 
> On 6/9/2023 11:46 PM, Thinh Nguyen wrote:
> > On Fri, Jun 09, 2023, Johan Hovold wrote:
> > > On Thu, Jun 08, 2023 at 05:57:23PM +0000, Thinh Nguyen wrote:
> > > > On Thu, Jun 08, 2023, Krishna Kurapati PSSNV wrote:
> > > > > On 6/8/2023 3:12 PM, Johan Hovold wrote:
> > > > > > On Thu, Jun 08, 2023 at 01:21:02AM +0530, Krishna Kurapati PSSNV wrote:
> > > > > > > On 6/7/2023 5:07 PM, Johan Hovold wrote:
> > > > > > 
> > > > > > > > So there at least two issues with this series:
> > > > > > > > 
> > > > > > > > 	1. accessing xhci registers from the dwc3 core
> > > > > > > > 	2. accessing driver data of a child device
> > > > > > > > 
> > > > > > > > 1. The first part about accessing xhci registers goes against the clear
> > > > > > > > separation between glue, core and xhci that Felipe tried to maintain.
> > > > > > > > 
> > > > > > > > I'm not entirely against doing this from the core driver before
> > > > > > > > registering the xhci platform device as the registers are unmapped
> > > > > > > > afterwards. But if this is to be allowed, then the implementation should
> > > > > > > > be shared with xhci rather than copied verbatim.
> > > > 
> > > > The core will just be looking at the HW capability registers and
> > > > accessing the ports capability. Our programming guide also listed the
> > > > host capability registers in its documentation. We're not driving the
> > > > xhci controller here. We're initializing some of the core configs base
> > > > on its capability.
> > > > 
> > > > We're duplicating the logic here and not exactly doing it verbatim.
> > > > Let's try not to share the whole xhci header where we should not have
> > > > visibility over. Perhaps it makes sense in some other driver, but let's
> > > > not do it here.
> > > 
> > > The patch series even copied the kernel doc verbatim. This is just not
> > > the way things are supposed to be done upstream. We share defines and
> > > implementations all the time, but we should not be making copies of
> > > them.
> > 
> >   We had some fixes to the kernel doc as it's incorrect description.
> >   Perhaps we can fully rewrite the kernel-doc if that what makes it
> >   better. We can share define implementations if they are meant to be
> >   shared. However, with the current way xhci header is implemented, it's
> >   not meant to be shared with dwc3. You agreed that we are violating this
> >   in some driver, but you're also insistent that we should not duplicate
> >   the logic to avoid this violation. Perhaps I'm not a maintainer here
> >   long enough to know some violation is better kept. If sharing the xhci
> >   header is what it takes to get this through, then fine.
> > 
> > > 
> > > > > > > > 
> > > > > > > > The alternative that avoids this issue entirely could indeed be to
> > > > > > > > simply count the number of PHYs described in DT as Rob initially
> > > > > > > > suggested. Why would that not work?
> > > > 
> > > > See below.
> > > > 
> > > > > > > > 
> > > > > > > The reason why I didn't want to read the Phy's from DT is explained in
> > > > > > > [1]. I felt it makes the code unreadable and its very tricky to read the
> > > > > > > phy's properly, so we decided we would initialize phy's for all ports
> > > > > > > and if a phy is missing in DT, the corresponding member in
> > > > > > > dwc->usbX_generic_phy[] would be NULL and any phy op on it would be a NOP.
> > > > > > 
> > > > > > That doesn't sound too convincing. Can't you just iterate over the PHYs
> > > > > > described in DT and determine the maximum port number used for HS and
> > > > > > SS?
> > > > > > > Also as per Krzysztof suggestion on [2], we can add a compatible to read
> > > > > > > number of phy's / ports present. This avoids accessing xhci members
> > > > > > > atleast in driver core. But the layering violations would still be present.
> > > > > > 
> > > > > > Yes, but if the information is already available in DT it's better to use
> > > > > > it rather than re-encode it in the driver.
> > > 
> > > > >    Are you suggesting that we just do something like
> > > > > num_ports = max( highest usb2 portnum, highest usb3 port num)
> > > > 
> > > > Why do we want to do this? This makes num_ports ambiguous. Let's not
> > > > sacrifice clarity for some lines of code.
> > > 
> > > This is not about lines of code, but avoiding the bad practice of
> > > copying code around and, to some degree, maintaining the separation
> > > between the glue, core, and xhci which Felipe (perhaps mistakingly) has
> > > fought for.
> > 
> > We're talking about combining num_usb3_ports and num_usb2_ports here,
> > what does that have to do with layer separation?
> > 
> > > 
> > > If you just need to know how many PHYs you have in DT so that you can
> > > iterate over that internal array, you can just look at the max index in
> > > DT where the indexes are specified in the first place.
> > > 
> > > Don't get hung up on the current variable names, those can be renamed to
> > > match the implementation. Call it max_ports or whatever.
> > 
> > It doesn't matter what variable name is given, it doesn't change the
> > fact that this "num_ports" or "max_ports" obfuscated usb2 vs usb3 ports
> > just for this specific implementation. So, don't do that.
> > 
> > > 
> > > > > If so, incase the usb2 phy of quad port controller is missing in DT, we
> > > > > would still read num_usb2_ports as 4 but the usb2_generic_phy[1] would be
> > > > > NULL and any phy ops would still be NOP. But we would be getting rid of
> > > > > reading the xhci registers compeltely in core driver.
> > > > > 
> > > > > Thinh, Bjorn, can you also let us know your views on this.
> > > > > 
> > > > > 1. Read:
> > > > >    num_usb3_ports = highest usb3 port index in DT
> > > > >    num_usb2_ports = max( highest usb2 port index, num_usb3_ports)
> > > > > 
> > > > > 2. Read the same by parsing xhci registers as done in recent versions of
> > > > > this series.
> > > > 
> > > > DT is not reliable to get this info. As noted, the DT may skip some
> > > > ports and still be fine. However, the driver doesn't know which port
> > > > reflects which port config index without the exact port count.
> > > 
> > > That's not correct. DT provides the port indexes already, for example:
> > > 
> > > 	phy-names = "usb2-port0", "usb3-port0",
> > > 		    "usb2-port1", "usb3-port1",
> > > 		    "usb2-port2",
> > > 		    "usb2-port3";
> > > 
> > > So if you just need this to iterate over the PHYs all the information
> > > needed is here.
> > > 
> > > If you need to access ports which do not have a PHY described in DT,
> > > then this is not going to suffice, but I have not seen anyone claim that
> > > that is needed yet.
> > 
> > Perhaps I misunderstand the conversation. However, there isn't a method
> > that everyone's agree on yet regarding DT [*]. Perhaps this indicates it
> > may not be the best approach. You can resume the conversation if you
> > want to:
> > 
> > [*] https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/9671cade-1820-22e1-9db9-5c9836414908@quicinc.com/*t__;Iw!!A4F2R9G_pg!YNb76pwkiNunnVGWfpM33LmCTJQNL7zPRooIIygA5rsUzkPGglyrsj5SLCy2raqkqwtjizd5js2wJ_OAP1Pp0N6mN4myMg$
> > 
> > > > More importantly, the host controller that lives on the PCI bus will not
> > > > use DT. This can be useful for some re-configurations if the controller
> > > > is a PCI device and that goes through the dwc3 code path.
> > > 
> > > Ok, this is a bit hand wavy, but if this ever turns out to be needed it
> > > can also be implemented then.
> > 
> > What does hand wavy mean? We have case where it's useful outside of
> > this, and it would be useful for PCI device too:
> > 
> > https://urldefense.com/v3/__https://lore.kernel.org/linux-usb/20230517233218.rjfmvptrexgkpam3@synopsys.com/__;!!A4F2R9G_pg!YNb76pwkiNunnVGWfpM33LmCTJQNL7zPRooIIygA5rsUzkPGglyrsj5SLCy2raqkqwtjizd5js2wJ_OAP1Pp0N4CJPF7cQ$
> > 
> > > 
> > > Or just generalise the xhci implementation for parsing these registers
> > > and reuse that from the start. (As a bonus you'd shrink the kernel text
> > > size by getting rid of that iffy inline implementation.)
> > > 
> > 
> > I don't like the iffy inline function either. We changed that here. To
> > rework the xhci header and define its global header seems a bit
> > excessive just for dwc3 to get the port capability. Regardless, as I've
> > said, if we _must_, perhaps we can just import xhci-ext-caps.h instead
> > of the whole xhci.h.
> 
> Hi Thinh, Johan,
> 
>  How about we add compatible data indicating the number of usb2/usb3 ports.
> That way we needn't parse the DT or read xhci registers atleast as a
> temporary solution to unblock other patches. Once this series is merged, we
> can get back to fixing the port count calculation. Does it seem fine ?
> 

Temporary solution should not involve DT as it's not easily reverted or
changed. Just include xhci-ext-caps.h and use the inline function. I
think Johan is fine with that. If not, he can provide more feedback.

Thanks,
Thinh

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