Hi,
On 05/13/2018 07:54 PM, Adam Baker wrote:
On 05/05/18 09:22, Hans de Goede wrote:
The recent "x86 ZONE_DMA love" discussion at LSF/MM pointed out that some
gspca sub-drivvers are using GFP_DMA to allocate buffers which are used
for USB bulk transfers, there is absolutely no need for this, drop it.
The documentation for kmalloc() says
GFP_DMA - Allocation suitable for DMA.
end at least in sq905.c the allocation is passed to the USB stack that
then uses it for DMA.
Looking a bit closer the "suitable for DMA" label that GFP_DMA promises
is not really a sensible thing for kmalloc() to determine as it is
dependent on the DMA controller in question. The USB stack now ensures
that everything works correctly as long as the memory is allocated with
kmalloc() so acked by me for sq905.c but, is anyone taking care of
fixing the kmalloc() documentation?
The whole GFP_DMA flag use in the kernel is a mess and fixing the
doucmentation is not easy and likely also not the solution, see:
https://lwn.net/Articles/753273/
Note this article is currently only available to LWN subscribers
(it will become freely available in a week).
I'll send you a private mail with a link which will allow you
to read it.
Regards,
Hans
