Messages in this thread | | | Date | Fri, 7 Nov 2003 22:22:19 +0100 | From | Jens Axboe <> | Subject | Re: [Bug 1412] Copy from USB1 CF/SM reader stalls, no actual content is read (only directory structure) |
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On Fri, Nov 07 2003, Alan Stern wrote: > On Fri, 7 Nov 2003, Jens Axboe wrote: > > > I've lost the original mail in the thread, but I'm quite sure it listed > > sg[0].page as being valid. Maybe the driver cleared it somewhere too > > early? page_address() will not have returned NULL for a valid page. > > Unless it's a highmem page, in that case you have to map it and unmap > > after use. For drivers like this that aren't performance critical and > > are by no means highmem ready, it's easier just to set your dma mask low > > enough that you wont be handed such pages. > > I think you may have put your finger on the problem. Our difficulty is > that we have no control over where these sg pages are allocated; that > depends on the capabilities of the USB host controller that our device > happens to be plugged into. Probably all the spots in the usb-storage > driver that directly access those sg pages will have to be rewritten to > take into account that they may not be located in mapped memory. > > So I will need to learn the proper way of doing that. Is it as simple as > calling page_address(), and if the result is 0 then calling kmap() > followed by kunmap()?
Forget page_address(), you should never use it. _Always_ kmap the page (or kmap_atomic() in irq context) and it will do the right thing regardless.
-- Jens Axboe
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