Date: Wed, 10 Mar 2010 10:07:26 -0800 From: Julian Elischer <julian@elischer.org> To: son goku <ryu.planka@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: physio and vmapbuf Message-ID: <4B97DFDE.10302@elischer.org> In-Reply-To: <b5a284501003100340r2962a15fwc8c5886da8b74714@mail.gmail.com> References: <b5a284501003100340r2962a15fwc8c5886da8b74714@mail.gmail.com>
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son goku wrote: > Hi hackers, > I have some experience with other UNIX kernels but none with FreeBSD. > FreeBSD interests me for a potential project I might be involved in , and > therefore I started researching it. > Browsing through the I/O layer code, I stumbled upon something that looked > strange, and perhaps you guys can help me with it. > Mainly, when reading the physio path, when the I/O buffer is from user-mode, > physio calls vmapbuf. The comments says that vmapbuf actually maps the > user-buffer into kernel address space. From my experience, such mapping is > needed only for very old devices that use PIO. yes, though to some of us that is relatively recent, and in fact we still need PIO mode for quite a few devices in the embedded space. (I have a soekris machine with a flash card that I need to use PIO mode to reach). We have been saying for some time (me personally for about 18 years) that we need to handle unmapped IO better. I don't believe that it has been done yet (*), but you certainly sound like the right person to take on that project :-) (*) I last looked at physio a few years ago so if someone snuck in unmapped IO in the mean-while, please excuse me. > Newer device just use DMA for > data copy, and therefore don't need the mapping, instead they just need to > pin the memory. Checking out vmapbuf only confused me further, I couldn't > find where such kernel mapping was taken place, nevertheless I did saw that > the buf->b_data was updated according to buf->b_saveaddr in some weird > offset calculation. > Adding to this confusion, there is a buf field called b_kvabase which > supposedly stores the kernel virtual address which wasn't touched at all!! > > Any help with understanding the flow and the usage of those fields/variables > will be greatly appreciated!! > _______________________________________________ > freebsd-hackers@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org"
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