Hello, Adrian McMenamin wrote: > I am working on a driver for the CD Rom drive on the Sega Dreamcast > (the so-called "GD Rom" drive). This device is electorically > compatible with IDE-3 devices and has a pretty good match in terms of > the control block registers but it implements its own packet command > interface. > > I now have a working driver but the performance is lousy. > > The driver reads data off the disk using DMA and the target for the > DMA has to be a contiguous. Therefore I have set: > > /* using DMA so memory will need to be contiguous */ > blk_queue_max_hw_segments(gd.gdrom_rq, 1); > /* set a large max size to get most from DMA */ > blk_queue_max_segment_size(gd.gdrom_rq, 0x40000); Ah... > ie only one segment per request but a big (for a small device) maximum > size for the segment. > > A priori I can see no performance advantage in allowing each request > to include multiple segments because then I'd only have to reshake > them so they went in one at a time. But from the looks of it if I do > set the maximum number of segments to 1 then each request is limited > to the smallest size - ie what I set in blk_queue_hardsect_size. There just isn't much room for maneuver w/ just one segment. Large contiguous memory region isn't too common these days. That said, there was a bug recently spotted by Mark Lord which made contiguous memory regions even rarer. Which kernel version are you using? > Is that right? What is the best way to go here? If you can spare some memory and cpu cycles, preparing a contiguous buffer and staging data there might help. It will eat up some cpu cycles but it won't be too much compared to PIO cycles. -- tejun - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
