Re: [PATCH v5 2/7] dmaengine: add a bus for DMA channel devices

[Frank Li <Frank.li@oss.nxp.com>]

On Thu, Nov 12, 2026 at 08:27:32AM +0800, Peng Fan wrote:
> On Wed, Nov 11, 2026 at 11:31:32AM -0600, Frank Li wrote:
> >On Wed, Nov 11, 2026 at 09:47:30PM +0800, Peng Fan (OSS) wrote:
> >> From: Peng Fan <peng.fan@nxp.com>
> >>
> >> DMA channel devices are currently created as class devices without a bus.
> >> This works for sysfs, but it prevents per-channel DMA/IOMMU configuration:
> >> of_dma_configure() only probes the IOMMU when dev->bus is set, and the
> >> IOMMU core only installs probe/release notifiers for buses listed in
> >> iommu_buses[].
> >>
> >> Drivers that need per-channel IOMMU support currently work around this by
> >> assigning dev->bus after device_register(). That leaves the bus subsystem
> >> reference count unbalanced because bus_add_device() already ran with a NULL
> >> bus, and bus_remove_device() will later drop a reference that was never
> >> taken.
> >>
> >> Add a dedicated bus for DMA channel devices and register it during
> >> dmaengine initialisation.  Use core_initcall so the bus exists before any
> >> IOMMU driver can call iommu_device_register(), which iterates iommu_buses[]
> >> and fails on an unregistered bus.  The match callback always reports no
> >> match, so no driver can bind to these devices.  A follow-up patch will
> >> register the channel devices on this bus.
> >>
> >> Clean up the unmap pools if dmaengine initialisation fails after they have
> >> been allocated.
> >>
> >> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> >> ---
> >>  drivers/dma/dmaengine.c   | 28 +++++++++++++++++++++++++---
> >>  include/linux/dmaengine.h |  4 ++++
> >>  2 files changed, 29 insertions(+), 3 deletions(-)
> >>
> >...
> >
> >> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> >> index 805668fbe437..14d8f182c7cb 100644
> >> --- a/include/linux/dmaengine.h
> >> +++ b/include/linux/dmaengine.h
> >> @@ -412,6 +412,10 @@ struct dma_chan_dev {
> >>  	bool chan_dma_dev;
> >>  };
> >>
> >> +#ifdef CONFIG_DMA_ENGINE
> >> +extern const struct bus_type dma_chan_bus_type;
> >
> >New API, need use dmaengine prefix, dmaengine_chan_bus_type
>
> Fix in v6.
>
> >
> >Not sure how other dmaengine support it before? Maybe already block for
> >long time.
>
> In drivers/dma/tegra186-gpc-dma.c:
> 1543       if (use_iommu_map) {
> 1544                chdev->bus = pdev->dev.bus;
>
> I used similar method, but Sashiko spotted issue for v1 [1], we have to
> add a new bus to handle it.
>
> "
> Does this illegally modify the bus type of an already-registered device?
>
> This is executed after dmaenginem_async_device_register() has already called
> device_add(). Since bus_add_device() is never executed, won't device_del()
> invoke bus_remove_device() during driver unbinding or removal, causing a fatal
> reference count underflow and use-after-free on the global platform_bus_type
> subsystem?
> "
>
> [1] https://lore.kernel.org/dmaengine/20260916160608.099201F000FF@smtp.kernel.org/

Can you add it to commit message to know why need add new bus type

Frank

>
> Thanks
> Peng
>
> >
> >Frank
> >
> >> +#endif
> >> +
> >>  /**
> >>   * enum dma_slave_buswidth - defines bus width of the DMA slave
> >>   * device, source or target buses
> >>
> >> --
> >> 2.34.1
> >>
> >