Re: [PATCH v3] media: uvc: don't do DMA on stack

On Tue, Jun 22, 2021 at 02:21:27PM +0000, David Laight wrote:

From: Alan Stern

quoted

Sent: 22 June 2021 14:29

...

quoted

quoted

Thought...

Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into
a cache line that will not be accessed by any other code?
(This is slightly weaker than requiring a cache-line aligned
pointer - but very similar.)

As I understand it, on architectures that do not have cache-coherent
I/O, kmalloc is guaranteed to return a buffer that is
cacheline-aligned and whose length is a multiple of the cacheline
size.

Now, whether that buffer ends up being accessed by any other code
depends on what your driver does with the pointer it gets from
kmalloc.  :-)

Thanks for the clarification.

Most of the small allocates in the usb stack are for transmits
where it is only necessary to ensure a cache write-back.

I know there has been some confusion because one of the
allocators can add a small header to every allocation.
This can lead to unexpectedly inadequately aligned pointers.
If it is updated when the preceding block is freed (as some
user-space mallocs do) then it would need to be in a
completely separate cache line.

If you really want to find out what the true story is, you should ask 
on the linux-mm mailing list.  The rest of us are not experts on this 
stuff.

Alan Stern