On Wed, 2008-11-26 at 17:13 +0000, Russell King wrote: > I've not had any response to this, so in liu of any response by this > coming weekend, I'm going to assume that everyone's happy with this > change (at which point it's going to become buried under a lot of > merges with other trees.) > > ----- Forwarded message from Russell King <rmk+lkml@xxxxxxxxxxxxxxxx> ----- > > Date: Thu, 20 Nov 2008 17:50:17 +0000 > From: Russell King <rmk+lkml@xxxxxxxxxxxxxxxx> > To: linux-arch@xxxxxxxxxxxxxxx, > Linux Kernel List <linux-kernel@xxxxxxxxxxxxxxx>, > linux-mm@xxxxxxxxx > Subject: [PATCH] [ARM] clearpage: provide our own clear_user_highpage() > > This patch is part of a larger ARM specific patch set cleaning up > aliasing VIPT cache support. > > With aliasing VIPT cache support, our implementation of clear_user_page() > and copy_user_page() sets up a temporary kernel space mapping such that > we have the same cache colour as the userspace page. This avoids having > to consider any userspace aliases from this operation. > > However, when highmem is enabled, kmap_atomic() have to setup mappings. > The copy_user_highpage() and clear_user_highpage() call these functions > before delegating the copies to copy_user_page() and clear_user_page(). > > The effect of this is that each of the *_user_highpage() functions setup > their own kmap mapping, followed by the *_user_page() functions setting > up another mapping. This is rather wasteful. > > Thankfully, copy_user_highpage() can be overriden by architectures by > defining __HAVE_ARCH_COPY_USER_HIGHPAGE. However, replacement of > clear_user_highpage() is more difficult because its inline definition > is not conditional. It seems that you're expected to define > __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE and provide a replacement > __alloc_zeroed_user_highpage() implementation instead. > > The allocation itself is fine, so we don't want to override that. What > we really want to do is to override clear_user_highpage() with our own > version which doesn't kmap_atomic() unnecessarily. > > However, there are two drivers (drivers/media/video/videobuf-dma-sg.c > and drivers/staging/go7007/go7007-v4l2.c) which want to provide non- > highmem clear_user_page()'d pages to userspace. > > Requiring an architecture to provide __alloc_zeroed_user_highpage(), > a sub-optimal clear_user_page(), and keep the sub-optimal > clear_user_highpage() around seems rather silly and potentially > error prone. > > So, what this patch below does is allow clear_user_highpage() itself > to be overriden by architectures, so that they can provide just one > implementation. > > What needs to follow on from this is converting those two drivers to > use clear_user_highpage() instead of clear_user_page() - that should > be a trivial patch. > > Are there any objections to this approach? Can I get any acked-by's > from any MM folk for the include/linux/highmem.h change? We'd like to pull this trick on parisc as well (another VIPT architecture), so you can add my ack. James -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
