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

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Message-ID: <vn7nrxpga7ewyqr7hpiczsn63qo2z4qzenif6powryqrtmnwjs@23jp6c7qdvxt>
Date: Fri, 7 Mar 2025 08:41:33 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Bjorn Andersson <bjorn.andersson@....qualcomm.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, Felipe Balbi <balbi@...nel.org>, 
	Wesley Cheng <quic_wcheng@...cinc.com>, Saravana Kannan <saravanak@...gle.com>, 
	Thinh Nguyen <Thinh.Nguyen@...opsys.com>, Philipp Zabel <p.zabel@...gutronix.de>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	Frank Li <Frank.li@....com>, linux-arm-msm@...r.kernel.org, linux-usb@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] usb: dwc3: qcom: Transition to flattened model

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@....qualcomm.com>
> ---
>  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?

>  #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