Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?
- From: Laurent Pinchart <laurent.pinchart@xxxxxxxxx>
- Date: Sat, 25 Jul 2009 23:41:47 +0200
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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