linux-kernel - Re: [PATCH v7] usb: misc: qcom_eud: Access EUD_MODE

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Message-ID: <CAPHGfUMnaJ8HPX=CC_q6m2nbq-ODP=MY_NkcvHjXR8mD9b=tBQ@mail.gmail.com>
Date: Fri, 25 Jul 2025 18:19:21 +0530
From: Komal Bajaj <komal.bajaj@....qualcomm.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Souradeep Chowdhury <quic_schowdhu@...cinc.com>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Melody Olvera <quic_molvera@...cinc.com>,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Subject: Re: [PATCH v7] usb: misc: qcom_eud: Access EUD_MODE_MANAGER2 through
 secure calls

On Thu, Jul 24, 2025 at 3:06 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Tue, Jul 22, 2025 at 05:01:53PM +0530, Komal Bajaj wrote:
> > EUD_MODE_MANAGER2 register is mapped to a memory region that is marked
> > as read-only for operating system running at EL1, enforcing access
> > restrictions that prohibit direct memory-mapped writes via writel().
> >
> > Attempts to write to this region from HLOS can result in silent failures
> > or memory access violations, particularly when toggling EUD (Embedded
> > USB Debugger) state. To ensure secure register access, modify the driver
> > to use qcom_scm_io_writel(), which routes the write operation to Qualcomm
> > Secure Channel Monitor (SCM). SCM has the necessary permissions to access
> > protected memory regions, enabling reliable control over EUD state.
> >
> > SC7280, the only user of EUD is also affected, indicating that this could
> > never have worked on a properly fused device.
> >
> > Fixes: 9a1bf58ccd44 ("usb: misc: eud: Add driver support for Embedded USB Debugger(EUD)")
> > Signed-off-by: Melody Olvera <quic_molvera@...cinc.com>
> > Signed-off-by: Komal Bajaj <komal.bajaj@....qualcomm.com>
> > Reviewed-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> > ---
> > Changes in v7:
> > - Updated the commit message as per Greg's comment
> > - Link to v6: https://lore.kernel.org/r/20250721-eud_mode_manager_secure_access-v6-1-fe603325ac04@oss.qualcomm.com
> >
> > Changes in v6:
> > - Propagating the error code from disable_eud(), per Dmitry's suggestion
> > - Link to v5: https://lore.kernel.org/r/20250715-eud_mode_manager_secure_access-v5-1-e769be308d4a@oss.qualcomm.com
> >
> > usb: misc: qcom_eud: Access EUD_MODE_MANAGER2 through secure calls
> >
> > Changes in v5:
> > * Changed select QCOM_SCM to depends on QCOM_SCM in Kconfig per Greg's review
> > * Link to v4: https://lore.kernel.org/all/20250709065533.25724-1-komal.bajaj@oss.qualcomm.com/
> >
> > Changes in v4:
> > * Added error logging in disable_eud() for SCM write failures, per Konrad’s suggestion
> > * Link to v3: https://lore.kernel.org/all/20250708085208.19089-1-komal.bajaj@oss.qualcomm.com/
> >
> > Changes in v3:
> > * Moved secure write before normal writes
> > * Added error checking in disable_eud()
> > * Use ENOMEM error code if platform_get_resource() fails
> > * Select QCOM_SCM driver if USB_QCOM_EUD is enabled
> > * Link to v2: https://lore.kernel.org/all/20250627125131.27606-1-komal.bajaj@oss.qualcomm.com/
> >
> > Changes in v2:
> > * Drop separate compatible to be added for secure eud
> > * Use secure call to access EUD mode manager register
> > * Link to v1: https://lore.kernel.org/all/20240807183205.803847-1-quic_molvera@quicinc.com/
> > ---
> >  drivers/usb/misc/Kconfig    |  1 +
> >  drivers/usb/misc/qcom_eud.c | 33 ++++++++++++++++++++++++---------
> >  2 files changed, 25 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/usb/misc/Kconfig b/drivers/usb/misc/Kconfig
> > index 6497c4e81e951a14201ad965dadc29f9888f8254..73ebd3257625e4567f33636cdfd756344b9ed4e7 100644
> > --- a/drivers/usb/misc/Kconfig
> > +++ b/drivers/usb/misc/Kconfig
> > @@ -147,6 +147,7 @@ config USB_APPLEDISPLAY
> >  config USB_QCOM_EUD
> >       tristate "QCOM Embedded USB Debugger(EUD) Driver"
> >       depends on ARCH_QCOM || COMPILE_TEST
> > +     depends on QCOM_SCM
>
> You now are preventing this code from ever being able to be built in any
> testing systems, including mine, so I don't even know if this patch
> builds or not.
>
> You did not even document this in the changelog :(

QCOM_SCM is essential for QCOM_EUD for its functionality.
One option I'm considering is using select QCOM_SCM, which ensures
dependency is enabled when QCOM_EUD is selected. QCOM_SCM facilitates
communication with secure world, this approach should not cause issues even
when COMPILE_TEST is enabled on non-ARCH_QCOM platforms.

Alternatively, I could use a conditional depends expression like:
depends on (ARCH_QCOM && QCOM_SCM) || COMPILE_TEST

This would allow the driver to be built under COMPILE_TEST while ensuring
QCOM_SCM is present on actual QCOM platforms. However, this would
require proper stubbing in the qcom_scm driver to avoid build failures during
compile testing.

Thanks
Komal

>
> {sigh}
>
> greg k-h