On Fri, May 26, 2017 at 10:18:30AM +0200, Daniel Vetter wrote: > On Mon, May 22, 2017 at 10:13:44AM +0100, Chris Wilson wrote: > > On Mon, May 22, 2017 at 10:55:01AM +0200, Daniel Vetter wrote: > > > On Wed, May 17, 2017 at 02:02:50PM +0100, Chris Wilson wrote: > > > > Older gen use a physical address for the hardware status page, for which > > > > we use cache-coherent writes. As the writes are into the cpu cache, we use > > > > a normal WB mapped page to read the HWS, used for our seqno tracking. > > > > > > > > Anecdotally, I observed lost breadcrumbs writes into the HWS on i965gm, > > > > which so far have not reoccurred with this patch. How reliable that > > > > evidence is remains to be seen. > > > > > > > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > > > > > dma is assumed to be coherent, so this should give you the same thing in > > > the end. > > > > Hmm, DMA historically hasn't been coherent, to get coherency required > > UC - being able to snoop was an ability of the GMCH, I thought. Looking > > at drivers/base/dma-coherent.c it is allocating UC (WC if you are > > lucky) from a device region, though I'm lost working out how the dynamic > > allocaton is treated. > > Yes, gpu dma wasn't coherent, but x86 dma api always assumed that > everything is coherent (since for non-gpus that was the case). At least > that's how I thought things worked. But it can only be coherent by disabling the cpu cache for that range. I think Broadwell is the earliest that allowed pci-e devices to write into the CPU cache (specifically to support 10G nic iirc), but I can't remember if that came to pass. > > > Except that dma_map_page can use bounce buffers, whereas > > > dma_alloc_coherent will make sure you're not doing that. > > > > Hmm, on these machines we don't have DMAR, we really should be feeding > > in physical addresses. That was probably being too polite. Fancy > > without? > > Not sure what you mean ... page_to_pfn, plus comment? See v3, just feed in the physical address of the page (and not the kernel's page_address()) which is just its pfn << PAGE_SHIFT. Completely avoid passing it a dma mapped address, just in case it does something. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
