On Saturday 01 July 2006 14:34, Peter Jeremy wrote: > On Sat, 2006-Jul-01 10:44:54 +0200, Hans Petter Selasky wrote: > >Yes, but scatter and gather will add extra complexity to the driver, and > > maybe an extra memory copy in most cases. The idea is to allocate less > > than or equal to a page of memory, and then avoid the problem? > > The idea is to allocate multiple disjoint pages of physical memory. > The USB hardware is capable of supporting this and page translation > hides the lack of contiguousness from userland. I don't see why > additional memory copies would be required.
Because most USB device drivers depend on a contigous buffer. In your case one would have to implement something similar to "uiomove()" for copying, that will do the scatter and gathering ? > >The most important thing is to keep memory allocations of constant size. > > For example under my USB system, all memory is allocated at attach. There > > is no longer allocation and freeing of memory during usage, with a few > > exceptions. > > Are you talking about USB device attach - which could happen at any > time whilst the system is running - or USB bus attach? I am talking about the USB device attach. > > >I was thinking I could pre-allocate 2-4MB for the USB system, then make a > >list of freed memory blocks, and then search this list first, before > >allocating new memory. > > That strikes me as wasteful. Currently, umass needs something like > 64K (or maybe 128K) and ulpt needs a few KB (they are the only USB > devices I normally use). I would be surprised if the peak USB RAM > requirement was more than 256K for most people. Yes, but don't forget high-speed USB transfers. They require larger buffers. For example 1024 bytes for ULPT is too little. The interrupt rate will be so high, that it is unrealistic to transfer 20MB/s using 1024 byte interrupts. My rewritten ULPT now uses "2*(1<<17)" buffers. > "vmstat -m" on my > systems shows that the current biggest consumer is devbuf - with > 3-4MB. Most other consumers are orders of magnitude smaller than this > (though my video capture card grabs about 4MB RAM when it's in use). --HPS _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
