Re: [PATCH 3/3] misc: fastrpc: Use context device bus for compute banks

[Dmitry Baryshkov]

On Wed, Apr 15, 2026 at 09:35:47PM +0530, Vishnu Reddy wrote:
> 
> 
> On 4/14/2026 10:01 PM, Ekansh Gupta wrote:
> > Replace the platform driver approach for compute bank (CB) devices
> > with the generic context_device_bus_type. Compute bank devices are
> > synthetic IOMMU context banks, not real platform devices, so using
> > the context device bus provides a more accurate representation in
> > the device model.
> > 
> > Currently, fastrpc used of_platform_populate() to create platform
> > devices for each "qcom,fastrpc-compute-cb" DT node, with a platform
> > driver (fastrpc_cb_driver) to handle probe/remove. This approach
> > had a race condition: device nodes were created before channel
> > resources (like spin_lock) were initialized, and probe was async,
> > so applications could open the device before sessions were available.
> > 
> > This patch addresses the race by manually creating and configuring
> > CB devices synchronously during fastrpc_rpmsg_probe(), after all
> > channel resources are initialized. The approach follows the pattern
> > used in host1x_memory_context_list_init().
> > 
> > Signed-off-by: Ekansh Gupta <ekansh.gupta@xxxxxxxxxxxxxxxx>
> > ---
> >   drivers/misc/Kconfig   |   1 +
> >   drivers/misc/fastrpc.c | 180 ++++++++++++++++++++++++++++++++++---------------
> >   2 files changed, 125 insertions(+), 56 deletions(-)
> > 
> >   	}
> >   	dma_bits = cctx->soc_data->dma_addr_bits_default;
> > +	if (cctx->domain_id == CDSP_DOMAIN_ID)
> > +		dma_bits = cctx->soc_data->dma_addr_bits_cdsp;
> > +
> >   	sess = &cctx->session[cctx->sesscount++];
> >   	sess->used = false;
> >   	sess->valid = true;
> > -	sess->dev = dev;
> > -	dev_set_drvdata(dev, sess);
> > +	sess->sid = sid;
> > +	spin_unlock_irqrestore(&cctx->lock, flags);
> > -	if (cctx->domain_id == CDSP_DOMAIN_ID)
> > -		dma_bits = cctx->soc_data->dma_addr_bits_cdsp;
> > +	cb_dev = kzalloc_obj(*cb_dev);
> > +	if (!cb_dev)
> > +		return -ENOMEM;
> > -	if (of_property_read_u32(dev->of_node, "reg", &sess->sid))
> > -		dev_info(dev, "FastRPC Session ID not specified in DT\n");
> > +	cb_dev->sess = sess;
> > -	if (sessions > 0) {
> > -		struct fastrpc_session_ctx *dup_sess;
> > +	device_initialize(&cb_dev->dev);
> > +	cb_dev->dev.parent = parent;
> > +	cb_dev->dev.bus = &context_device_bus_type;
> > +	cb_dev->dev.release = fastrpc_cb_dev_release;
> > +	cb_dev->dev.of_node = of_node_get(cb_node);
> > +	cb_dev->dev.dma_mask = &cb_dev->dev.coherent_dma_mask;
> > +	cb_dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
> > +	dev_set_name(&cb_dev->dev, "%s:compute-cb@%u", dev_name(parent), sid);
> > +	rc = device_add(&cb_dev->dev);
> 
> device_initialize() and device_add() can be replaced with single
> device_register() call. You can refer the below patch,

Keep in mind that for several arches device_initialize() sets the
coherent_dma_mask. So one should set coherent_dma_mask (as it's done
here) after calling device_initialize().

> 
> https://lore.kernel.org/all/20260313-kaanapali-iris-v3-4-9c0d1a67af4b@xxxxxxxxxxxxxxxx/
> 

-- 
With best wishes
Dmitry