On Thu, Jan 22, 2026 at 07:44:04PM -0400, Jason Gunthorpe wrote:
On Thu, Jan 22, 2026 at 12:32:03PM +0100, Christian König wrote:
What roughly happens is that each DMA-buf mapping through a couple of hoops keeps a reference on the device, so even after a hotplug event the device can only fully go away after all housekeeping structures are destroyed and buffers freed.
A simple reference on the device means nothing for these kinds of questions. It does not stop unloading and reloading a driver.
Well as far as I know it stops the PCIe address space from being re-used.
So when you do an "echo 1 > remove" and then an re-scan on the upstream bridge that works, but you get different addresses for your MMIO BARs!
That's pretty a niche scenario.. Most people don't rescan their PCI bus. If you just do rmmod/insmod then it will be re-used, there is no rescan to move the MMIO around on that case.
Ah I just remembered there is another important detail here.
It is illegal to call the DMA API after your driver is unprobed. The kernel can oops. So if a driver is allowing remove() to complete before all the dma_buf_unmaps have been called it is buggy and risks an oops.
https://lore.kernel.org/lkml/8067f204-1380-4d37-8ffd-007fc6f26738@kernel.org...
As calling a dma_buf_unmap() -> dma_unma_sg() after remove() returns is not allowed..
Jason