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

[David Laight <David.Laight@ACULAB.COM>]

From: Alan Stern
> Sent: 22 June 2021 14:29
...
> > 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.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)