> On our MIPS based embedded board, we do not have swap space. In
> our driver we need to pass huge data from user land to kernel land
> and vice versa. Without using copy in and copy out, can we use the
> vitual address of the user land directly in the driver. Our
> understanding is that copy in and copy out is required under 2
> conditions: we have swapping or we use the buffers in interrupts
> and we do not have both these conditions imposed on our driver.
>
> On looking at the way copy_in and copy_out happens, it looks like
> it ultimately does a mem copy; so from kernel land kernel is
> accessing user's virtual addresses. So shouldn't just accessing
> the virtual addresses work?
You can avoid copy in/out using an mmaped buffer. You allocate memory in kernel,
mmap it to user-space (you implement mmap for chardev or proc file) and have
the user-mode app write the data there. It works even if there is swap, because
you can make the buffer is unswappable.
(It should be possible to use any user-space data from kernel, just you have
to make sure, that it's locked in memory while you work with it - and that's
often hard because pointers may be kept somewhere deep in some driver).
--------------------------------------------------------------------------------
- Jan Hudec `Bulb' <bulb@ucw.cz>
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