Ben Nizette wrote: > On Mon, 2008-10-20 at 03:06 -0800, Nicholas Mc Guire wrote: >>> On Mon, 2008-10-20 at 10:55 +0100, Douglas, Jim (Jim) wrote: >>>> We are contemplating porting a large number of device drivers to Linux. >>>> The pragmatic solution is to keep them in user mode (using the UIO >>>> framework) where possible ... they are written in C++ for a start. >>>> >>>> The obvious disadvantages of user mode device drivers are security / >>>> isolation. The main benefit is ease of development. >>>> >>>> Do you know what the *technical* disadvantages of this approach might >>>> be? I am most concerned about possible impact on interrupt handling. >>>> >>>> For example, I assume the context switching overhead is higher, and that >>>> interrupt latency is more difficult to predict? >>> Userspace drivers certainly aren't first class citizens; uio and kernel >>> mode drivers generally aren't really interchangeable. >>> >>> The technical disadvantages of userspace drivers are that you don't have >>> access to kernel subsystems, you can't run any userspace content in irq >>> context so everything needs to be scheduled before it can be dealt with. >>> A UIO driver still needs a kernel component to do acknowledge the >>> interrupt. As such when you say "interrupt latency" you need to define >>> the end point. A UIO driver will have it's in-kernel handler called >>> just as quickly as any other driver but the userspace app will need to >>> be scheduled before it receives notification that the IRQ has fired. >>> >>> The technical advantage of a UIO driver is that devices which only need >>> to shift data don't have to double-handle it. e.g. an ADC card doesn't >>> need to move ADC results from hardware to kernel, kernel to userspace, >>> it's just one fluid movement. >>> >>> What kind of device drivers are you talking about? They have to be of a >>> fairly specific flavour to fit in to a UIO model. Linux isn't a >>> microkernel, userspace drivers are quite restricted in their power. >>> >> are these claims based on benchmarks of a specific driver ? I only know >> of a singe UIO driver for a Hilscher CIF card and one for a SMX >> Cryptengine (I guess thats yours any way) but none for a AD/DIO card - if >> you know of such a driver I would be interested in seeing its performance. > > When UIO was being discussed for inclusion, the example case being > thrown around was for such an ADC card. They claimed to have seen > significant improvements in speed by avoiding the double-handling of > data. Come to think of it, I can't see that this specific driver has > shown up... > > But what kind of benchmarks do you want? When I say "restricted in > their power" I mean more in a feature-set kind of way than a raw speed > way. Userspace drivers can't plug in to kernel subsystems so can't, for > example, be SPI hosts or terminal devices or network hardware or > anything else which sits in the middle of a standard stack. All they > can do is be notified of an interrupt and have direct access to a lump > of memory. > > As I asked before, what's your use-case? It tends to be fairly obvious > whether the hardware is suitable for a UIO-based driver or whether it's > going to have to live in kernel. > >> Also if you know of any simple UIO sample drivers that would also help. > > As in examples of the userspace half? Unfortunately uio-smx isn't ready > to fly thanks to some significant production delays but the userspace > half of the Hilscher CIF driver can be found at > http://www.osadl.org/projects/downloads/UIO/user/ As I see it, mainly the license conditions attract people to use UIO. Performance is not that important. Wolfgang. -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
