Re: [Bug 1412] Copy from USB1 CF/SM reader stalls, no actual content is read (only directory structure)

From: Jens Axboe (axboe_at_suse.de)
Date: 11/07/03

  • Next message: Jean Delvare: "Re: I2C parallel port adapters drivers" 
    Date:	Fri, 7 Nov 2003 22:22:19 +0100
To: Alan Stern <stern@rowland.harvard.edu>

    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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  • Next message: Jean Delvare: "Re: I2C parallel port adapters drivers"

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