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

From: Alan Stern
Date: Fri Nov 07 2003 - 19:59:43 EST

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()?

Alan Stern

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