linux-kernel - Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?

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Message-Id: <200907252341.48526.laurent.pinchart@skynet.be>
Date:	Sat, 25 Jul 2009 23:41:47 +0200
From:	Laurent Pinchart <laurent.pinchart@...net.be>
To:	linux-kernel@...r.kernel.org
Subject: Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?

Hi everybody,

I'm trying to implement the USERPTR V4L2 streaming I/O method in the uvcvideo 
driver. In a nutshell, that methods uses userspace-allocated buffers to 
transfer video data from kernelspace to userspace. The buffers are reused 
during the whole video stream in a circular fashion.

The kernel driver receives userspace pointers to those buffers, which can have 
been allocated using different methods (malloc, Xv alloc, mmap() on a reserved 
memory region, ...). I get the list of underlying pages using 
get_user_pages(), and I then need to map those pages to the kernel virtual 
address space.

Pages will be written to from the kernel in USB interrupt context. I can then 
either kmap_atomic() pages before copying data and kunmap_atomic() them right 
after, or kmap() them once at the beginning of the video stream and keep them 
mapped until the end.

As I don't have much experience with the memory management subsystem, I'd 
appreciate if someone could give me a few advices regarding the best way to 
proceed. kmap() is expensive, but it would only be called once, while 
kmap_atomic() would be called many times (4500 times per second for a 640x480 
30fps video stream). On the other hand, I'm not sure how much pressure keeping 
all those kmap() pages mapped into kernel virtual memory for a long time would 
put on the memory subsystem.

Please CC me on answers.

Regards,

Laurent Pinchart

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