Re: [PATCH v4 6/7] usb: dwc3: qcom: Transition to flattened model

[Bjorn Andersson]

On Tue, Mar 11, 2025 at 09:02:03PM +0200, Dmitry Baryshkov wrote:
> On Mon, Mar 10, 2025 at 09:46:24PM -0500, Bjorn Andersson wrote:
> > On Fri, Mar 07, 2025 at 08:41:33AM +0200, Dmitry Baryshkov wrote:
> > > On Wed, Feb 26, 2025 at 04:17:53PM -0800, Bjorn Andersson wrote:
> > > > The USB IP-block found in most Qualcomm platforms is modelled in the
> > > > Linux kernel as 3 different independent device drivers, but as shown by
> > > > the already existing layering violations in the Qualcomm glue driver
> > > > they can not be operated independently.
> > > > 
> > > > With the current implementation, the glue driver registers the core and
> > > > has no way to know when this is done. As a result, e.g. the suspend
> > > > callbacks needs to guard against NULL pointer dereferences when trying
> > > > to peek into the struct dwc3 found in the drvdata of the child.
> > > > Even with these checks, there are no way to fully protect ourselves from
> > > > the race conditions that occur if the DWC3 is unbound.
> > > > 
> > > > Missing from the upstream Qualcomm USB support is handling of role
> > > > switching, in which the glue needs to be notified upon DRD mode changes.
> > > > Several attempts has been made through the years to register callbacks
> > > > etc, but they always fall short when it comes to handling of the core's
> > > > probe deferral on resources etc.
> > > > 
> > > > Moving to a model where the DWC3 core is instantiated in a synchronous
> > > > fashion avoids above described race conditions.
> > > > 
> > > > It is however not feasible to do so without also flattening the
> > > > DeviceTree binding, as assumptions are made in the DWC3 core and
> > > > frameworks used that the device's associated of_node will the that of
> > > > the core. Furthermore, the DeviceTree binding is a direct
> > > > representation of the Linux driver model, and doesn't necessarily
> > > > describe "the USB IP-block".
> > > > 
> > > > The Qualcomm DWC3 glue driver is therefor transitioned to initialize and
> > > > operate the DWC3 within the one device context, in synchronous fashion.
> > > > 
> > > > To provide a limited time backwards compatibility, a snapshot of the
> > > > driver is retained in a previous commit. As such no care is taken in the
> > > > dwc3-qcom driver for the qcom,dwc3 backwards compatibility.
> > > > 
> > > > Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxxxxxxxx>
> > > > ---
> > > >  drivers/usb/dwc3/dwc3-qcom.c | 138 +++++++++++++++++++++----------------------
> > > >  1 file changed, 69 insertions(+), 69 deletions(-)
> > > > 
> > > > diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
> > > > index 9d04c2457433..63e60f15ceaa 100644
> > > > --- a/drivers/usb/dwc3/dwc3-qcom.c
> > > > +++ b/drivers/usb/dwc3/dwc3-qcom.c
> > > > @@ -4,7 +4,6 @@
> > > >   * Inspired by dwc3-of-simple.c
> > > >   */
> > > >  
> > > > -#include <linux/cleanup.h>
> > > >  #include <linux/io.h>
> > > >  #include <linux/of.h>
> > > >  #include <linux/clk.h>
> > > > @@ -14,7 +13,6 @@
> > > >  #include <linux/kernel.h>
> > > >  #include <linux/extcon.h>
> > > 
> > > As a heads up, would it make sense to also drop extcon support while we
> > > are transitioning to the new driver / DT bindings?
> > > 
> > 
> > Yes, I believe this code should be cleaned out in favor of relying on
> > the core's implementation thereof and callbacks into the qcom-code for
> > configuration the glue hardware - which looks to be the same callback we
> > want to add for the usb_role_switch.
> 
> Hmm. So do you want to clean it later? Or do you plan to drop extcon
> from the next iteration?
> 

It turns out that this was cleaner to do as a separate commit, but as we
need a role-switch callback from core to glue (Krishna has a patch ready
to go for this) and I don't have any extcon hardware at my desk right
now I decided to leave a TODO in the code for now.

This needs to be resolved before we transition the extcon-based dts
files.

Regards,
Bjorn

> > 
> > Regards,
> > Bjorn
> > 
> > > >  #include <linux/interconnect.h>
> > > > -#include <linux/of_platform.h>
> > > >  #include <linux/platform_device.h>
> > > >  #include <linux/phy/phy.h>
> > > >  #include <linux/usb/of.h>
> > > 
> > > -- 
> > > With best wishes
> > > Dmitry
> 
> -- 
> With best wishes
> Dmitry