On Fri, Nov 22, 2019 at 12:56 PM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: > > > > On 2019-11-22 1:50 p.m., Dan Williams wrote: > > On Fri, Nov 22, 2019 at 8:53 AM Dave Jiang <dave.jiang@xxxxxxxxx> wrote: > >> > >> > >> > >> On 11/21/19 10:20 PM, Vinod Koul wrote: > >>> On 14-11-19, 10:03, Logan Gunthorpe wrote: > >>>> > >>>> > >>>> On 2019-11-13 9:55 p.m., Vinod Koul wrote: > >>>>>> But that's the problem. We can't expect our users to be "nice" and not > >>>>>> unbind when the driver is in use. Killing the kernel if the user > >>>>>> unexpectedly unbinds is not acceptable. > >>>>> > >>>>> And that is why we review the code and ensure this does not happen and > >>>>> behaviour is as expected > >>>> > >>>> Yes, but the current code can kill the kernel when the driver is unbound. > >>>> > >>>>>>>> I suspect this is less of an issue for most devices as they wouldn't > >>>>>>>> normally be unbound while in use (for example there's really no reason > >>>>>>>> to ever unbind IOAT seeing it's built into the system). Though, the fact > >>>>>>>> is, the user could unbind these devices at anytime and we don't want to > >>>>>>>> panic if they do. > >>>>>>> > >>>>>>> There are many drivers which do modules so yes I am expecting unbind and > >>>>>>> even a bind following that to work > >>>>>> > >>>>>> Except they will panic if they unbind while in use, so that's a > >>>>>> questionable definition of "work". > >>>>> > >>>>> dmaengine core has module reference so while they are being used they > >>>>> won't be removed (unless I complete misread the driver core behaviour) > >>>> > >>>> Yes, as I mentioned in my other email, holding a module reference does > >>>> not prevent the driver from being unbound. Any driver can be unbound by > >>>> the user at any time without the module being removed. > >>> > >>> That sounds okay then. > >> > >> I'm actually glad Logan is putting some work in addressing this. I also > >> ran into the same issue as well dealing with unbinds on my new driver. > > > > This was an original mistake of the dmaengine implementation that > > Vinod inherited. Module pinning is distinct from preventing device > > unbind which ultimately can't be prevented. Longer term I think we > > need to audit dmaengine consumers to make sure they are prepared for > > the driver to be removed similar to how other request based drivers > > can gracefully return an error status when the device goes away rather > > than crashing. > > Yes, but that will be a big project because there are a lot of drivers. Oh yes, in fact I think it's something that can only reasonably be considered for new consumers. > But I think the dmaengine common code needs to support removal properly, > which essentially means changing how all the drivers allocate and free > their structures, among other things. > > The one saving grace is that most of the drivers are for SOCs which > can't be physically removed and there's really no use-case for the user > to call unbind. Yes, the SOC case is not so much my concern as the generic offload use cases, especially if those offloads are in a similar hotplug domain as a cpu.
