On 10/13/2015 08:48 PM, Dan Williams wrote: > On Tue, Oct 13, 2015 at 11:37 AM, Thomas Hellstrom > <thellstrom@xxxxxxxxxx> wrote: >> On 10/13/2015 06:35 PM, Dan Williams wrote: >>> On Mon, Oct 12, 2015 at 10:18 PM, Thomas Hellstrom >>> <thellstrom@xxxxxxxxxx> wrote: >>>> Hi! >>>> >>>> On 10/13/2015 12:35 AM, Dan Williams wrote: >>>>> Per commit 2e586a7e017a "drm/vmwgfx: Map the fifo as cached" the driver >>>>> expects the fifo registers to be cacheable. In preparation for >>>>> deprecating ioremap_cache() convert its usage in vmwgfx to memremap(). >>>>> >>>>> Cc: David Airlie <airlied@xxxxxxxx> >>>>> Cc: Thomas Hellstrom <thellstrom@xxxxxxxxxx> >>>>> Cc: Sinclair Yeh <syeh@xxxxxxxxxx> >>>>> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx >>>>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> >>>> While I have nothing against the conversion, what's stopping the >>>> compiler from reordering writes on a generic architecture and caching >>>> and reordering reads on x86 in particular? At the very least it looks to >>>> me like the memory accesses of the memremap'd memory needs to be >>>> encapsulated within READ_ONCE and WRITE_ONCE. >>> Hmm, currently the code is using ioread32/iowrite32 which only do >>> volatile accesses, whereas READ_ONCE / WRITE_ONCE have a memory >>> clobber on entry and exit. So, I'm assuming all you need is the >>> guarantee of "no compiler re-ordering" and not the stronger >>> READ_ONCE/WRITE_ONCE guarantees, but that still seems broken compared >>> to explicit fencing where it matters. >> I'm not quite sure I follow you here, it looks to me like READ_ONCE() >> and WRITE_ONCE() are implemented as >> volatile accesses, > Ah, sorry, I was looking at the default case... > >> https://urldefense.proofpoint.com/v2/url?u=http-3A__lxr.free-2Delectrons.com_source_include_linux_compiler.h-23L215&d=BQIBaQ&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=JRxebmjcR4J-yhD0wROjKrAKyto5OeetIvqt7MqV_WA&s=zn7YmnS74zjM3Sd5Dp9mZnSL27jqel6cwRHwYV6gU3U&e= >> >> just like ioread32 and iowrite32 >> >> https://urldefense.proofpoint.com/v2/url?u=http-3A__lxr.free-2Delectrons.com_source_include_asm-2Dgeneric_io.h-23L54&d=BQIBaQ&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=JRxebmjcR4J-yhD0wROjKrAKyto5OeetIvqt7MqV_WA&s=y4dD2GUpcZVHljnThYugF-YLTgeP6En4JwoOnkaLg7A&e= >> >> which would minimize any potential impact of this change. >> IMO optimizing the memory accesses can be done as a later step. >> > Ok, I'll make local read_fifo() and write_fifo() macros to make this > explicit. Are these names ok with you? Sure. Thanks, Thomas _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
