On Sun, 05 Jun 2011, Jan Kiszka wrote: > On 2011-06-05 11:28, Nicholas Mc Guire wrote: > > On Sun, 05 Jun 2011, Armin Steinhoff wrote: > > > >> Nicholas Mc Guire wrote: > >>> On Sat, 04 Jun 2011, Monica Puig-Pey wrote: > >>> > >>>> Hello, > >>>> I'm studying how to develop drivers in a real time OS and how do they > >>>> work. I'm using Ubuntu 10.04 with the 2.6.31-11-rt patch installed. > >>>> I would like to know the priority when executing open(), read(), write() > >>>> and close() operations. > >>>> In my example the thread which is using the driver runs with 10 RTPRIO, > >>>> but I don't know what happens in kernel context with the priority when > >>>> running the I/O operations. > >>>> Thank you for your help, I don't know where to learn about this. > >>>> > >>> [] > >>> Also when using bottom half mechanisms you need to take into account the > >>> priority of the kernel thread that manages the defered work items, so > >>> rt-drivers may have a different structure than normal drivers. > >> > >> That's the reason why I prefer UIO based user space drivers ! > >> > > ...and how to resolve DMA ? if DMA were resolved cleanly I would agree. > > Regarding that limitation, there is some hope: "next-generation" UIO is > called VFIO. Useful for exclusively assigning virtual PCI functions of > network adapters etc. to user space stacks or hypervisors like QEMU/KVM > (for device pass-through). But it's not mainline yet. And it obviously > requires an IOMMU. > > But the key point remains: "exclusively". Anything else cannot be > modeled efficiently via UIO or VFIO. > for many RT apps that is an acceptable limit - in fact sometimes a requirement - having dynamic access and handling this at runtime is not necessarily an advantage for rt. Giving guarantees on timing for non-exclusive access in the general case would probably need to be based on a priority access model any way (i.e. running non-RT network traffic on a RT-link as "rt-idle" bandwidth usage - but full shared access at runtime would be very hard, if not impossible, to model at the device interface level without breaking RT properties. So I think its legitimate to keep it out of the device all together and let the user apps have a non-rt fight about access... Any pointers to VFIO ? hofrat -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
