linux-kernel - Re: [PATCH v2] misc: fastrpc: Fix channel resource access in device

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Message-ID: <golcrcr6voafr3fqsnihyjyut36sii55vzws4josfhkjjg3nie@ur43qq2kvlsv>
Date: Wed, 25 Jun 2025 02:45:27 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Ekansh Gupta <ekansh.gupta@....qualcomm.com>,
        srinivas.kandagatla@...aro.org, linux-arm-msm@...r.kernel.org,
        quic_bkumar@...cinc.com, linux-kernel@...r.kernel.org,
        quic_chennak@...cinc.com, dri-devel@...ts.freedesktop.org,
        arnd@...db.de, stable@...nel.org
Subject: Re: [PATCH v2] misc: fastrpc: Fix channel resource access in
 device_open

On Tue, Jun 24, 2025 at 04:38:25PM +0100, Greg KH wrote:
> On Tue, Jun 24, 2025 at 04:36:35PM +0100, Greg KH wrote:
> > On Tue, Jun 24, 2025 at 04:27:21PM +0300, Dmitry Baryshkov wrote:
> > > On Thu, Jun 19, 2025 at 10:40:26AM +0530, Ekansh Gupta wrote:
> > > > During rpmsg_probe, fastrpc device nodes are created first, then
> > > > channel specific resources are initialized, followed by
> > > > of_platform_populate, which triggers context bank probing. This
> > > > sequence can cause issues as applications might open the device
> > > > node before channel resources are initialized or the session is
> > > > available, leading to problems. For example, spin_lock is initialized
> > > > after the device node creation, but it is used in device_open,
> > > > potentially before initialization. Move device registration after
> > > > channel resource initialization in fastrpc_rpmsg_probe.
> > > 
> > > You've moved device init, however there is still a possibility for the
> > > context devices to be created, but not bound to the driver (because all
> > > the probings are async). I think instead we should drop the extra
> > > platform driver layer and create and set up corresponding devices
> > > manually. For example, see how it is handled in
> > > host1x_memory_context_list_init(). That function uses iommu-maps, but we
> > > can use OF nodes and iommus instead.
> > 
> > Is this a real platform device?  If so, why do you need a second
> > platform driver, what makes this so unique?  If this isn't a platform
> > device, then why not just use the faux bus instead?
> > 
> > It seems that "number of sessions" is a DT property, is that something
> > that is really defined by the hardware?  Or is it just a virtual thing
> > that people are abusing in the DT?

Purely software value.

> > 
> > And if you really have all these sessions, why not make them real
> > devices, wouldn't that make things simpler?
> 
> Oh wait, these are "fake" platform devices under the parent (i.e. real)
> platform device.  That's not good, please don't do that, use the faux
> bus code now instead to properly handle this.  Attempting to create a
> device when open() is called is really really odd...

The driver doesn't created devices during open(). It creates them
earlier, then another driver probes an populates the data. I suggest to
follow Tegra approach, remove the sub-driver completely and instead of
calling of_platform_populate() create necessary devices manually and set
corresponding IOMMU configuration from the main driver's probe path.

-- 
With best wishes
Dmitry