On Tue, 5 Jun 2018, Alexey Brodkin wrote: > Hi Mikulas, > > On Sun, 2018-06-03 at 16:41 +0200, Mikulas Patocka wrote: > > Modern processors can detect linear memory accesses and prefetch data > > automatically, so there's no need to use prefetch. > > Not each and every CPU that's capable of running Linux has prefetch > functionality :) > > Still read-on... > > > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > > --- > > drivers/gpu/drm/udl/udl_transfer.c | 7 ------- > > 1 file changed, 7 deletions(-) > > > > Index: linux-4.16.12/drivers/gpu/drm/udl/udl_transfer.c > > =================================================================== > > --- linux-4.16.12.orig/drivers/gpu/drm/udl/udl_transfer.c 2018-05-31 14:48:12.000000000 +0200 > > +++ linux-4.16.12/drivers/gpu/drm/udl/udl_transfer.c 2018-05-31 14:48:12.000000000 +0200 > > @@ -13,7 +13,6 @@ > > #include <linux/module.h> > > #include <linux/slab.h> > > #include <linux/fb.h> > > -#include <linux/prefetch.h> > > #include <asm/unaligned.h> > > > > #include <drm/drmP.h> > > @@ -51,9 +50,6 @@ static int udl_trim_hline(const u8 *bbac > > int start = width; > > int end = width; > > > > - prefetch((void *) front); > > - prefetch((void *) back); > > AFAIK prefetcher fetches new data according to a known history... i.e. based on previously > used pattern we'll trying to get the next batch of data. > > But the code above is in the very beginning of the data processing routine where > prefetcher doesn't yet have any history to know what and where to prefetch. > > So I'd say this particular usage is good. > At least those prefetches shouldn't hurt because typically it > would be just 1 instruction if those exist or nothing if CPU/compiler doesn't > support it. See this post https://lwn.net/Articles/444336/ where they measured that prefetch hurts performance. Prefetch shouldn't be used unless you have a proof that it improves performance. The problem is that the prefetch instruction causes stalls in the pipeline when it encounters TLB miss and the automatic prefetcher doesn't. Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
