On Thu, Jul 17, 2008 at 11:02:15AM +0200, Oliver Rutsch wrote: > Hi, > > We're using the latest 2.4.25-Kernel from DENX on a custom mpc5200b board. > We have a RS485 device which sends continuously its data at a rate of > 1.25 MBit/s. The problem here is that it takes nearly 90% of system time > just to read these data. I had a look into the driver > (arch/ppc/5xxx_io/psc.c) and the problem seems to be the RxRDY interrupt > of the PSCs. This interrupt is inserted every time when there's any data > in the FIFO. I found out, that most of the time the isr is reading only > 1 or 2 bytes out of the FIFO, so the system is very busy to serve > thousands of interrupts. > Then I set the FFULL flag as the interrupt source in the mr1 register of > the PSC. I set an rx alarm level of 112 bytes (means that an interrupt > will be generated when FIFO>=400 bytes!) and rx granularity of 7. > With this the system load drops down under 1 percent! > But this approach has one big drawback: There will be no interrupt if I > receive less than 400 bytes. So this is not usable for "normal" SIO > operation. > Although not tested it looks like the same problem is in the driver for > the 2.6.25-Kernel (correct me when I'm wrong here). > So I have two ideas for better performance of this driver: > > 1. The interrupt will be generated by the FFULL flag. An endless kernel > timer will have every jiffy a look at the PSC FIFO status and pull out > any data if necessary. A look on the PSC FIFO status is a very short > operation so this timer shouldn't affect the system performance too > much. But maybe this adds some more latency when receiving only a few > bytes.
This is the approach I would take. BTW, if you want to fix this in mainline, then you'll need to migrate to arch/powerpc. arch/ppc is now removed from mainline. g. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
