Re: [PATCH v8 6/9] usb: dwc3: qcom: Add multiport controller support for qcom wrapper

[Thinh Nguyen]

On Wed, Jun 21, 2023, Johan Hovold wrote:
> Hi Thinh,
> 
> and sorry about the late reply. Was on holiday last week.
> 
> On Fri, Jun 09, 2023 at 06:16:13PM +0000, 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.
> 
> No, this is an example of why you should not copy code (and docs)
> around, for example, so you don't have to fix the same bug (or typo) in
> multiple places.
> 
> > 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.
> 
> Just because no-one has used these outside of xhci today, doesn't mean
> that you should copy the defines once they are needed outside that
> driver.
> 
> And in fact, they are used outside of the xhci driver proper already
> today, only that people took the lazy approach and shared the inline
> helper for that.
> 
> > > > > > > > 
> > > > > > > > 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?
> 
> I mentioned this is as an alternative if you're are hell-bent on not
> reusing the xhci register defines and parsing code.
> 
> > > 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.
> 
> Ok, then make sure you reuse the xhci implementation for that then.
> 
> > > > > 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@xxxxxxxxxxx/*t__;Iw!!A4F2R9G_pg!btc_fkrnAm1eFG5c5V0bWMnPnq4yvFyza1nDRJE8mPvSSrYIyxkUxjAARU51cRsoo2n0eLwTGt_SYvnQOaQ$ 
> 
> This was the approach suggested by Rob, the DT maintainer, in that very
> thread IIRC.
> 
> > > > 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@xxxxxxxxxxxx/__;!!A4F2R9G_pg!btc_fkrnAm1eFG5c5V0bWMnPnq4yvFyza1nDRJE8mPvSSrYIyxkUxjAARU51cRsoo2n0eLwTGt_SqPJ_Tn0$ 
> 
> Hand-wavy as in lacking detail which makes it hard to evaluate the
> argument.
> 
> > > 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.
> 
> No one said anything about importing the whole of xhci.h.
> 

This was how it was originally done in the earlier version of the
series. Perhaps that's why I was against it. It looks wrong for dwc3 to
know about xhci_hcd structure.

Thanks,
Thinh