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Message-ID: <1405979809.25580.133.camel@deneb.redhat.com> Date: Mon, 21 Jul 2014 17:56:49 -0400 From: Mark Salter <msalter@...hat.com> To: Catalin Marinas <catalin.marinas@....com> Cc: Will Deacon <Will.Deacon@....com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] arm64: make CONFIG_ZONE_DMA user settable On Fri, 2014-07-18 at 12:07 +0100, Catalin Marinas wrote: > On Tue, Jun 24, 2014 at 03:38:34PM +0100, Mark Salter wrote: > > On Tue, 2014-06-24 at 15:14 +0100, Catalin Marinas wrote: > > > On Mon, Jun 23, 2014 at 02:17:03PM +0100, Mark Salter wrote: > > > > On Mon, 2014-06-23 at 12:09 +0100, Catalin Marinas wrote: > > > > > My proposal (in the absence of any kind of description) is to still > > > > > create a ZONE_DMA if we have DMA memory below 32-bit, otherwise just add > > > > > everything (>32-bit) to ZONE_DMA. Basically an extension from your CMA > > > > > patch, make dma_phys_limit static in that file and set it to > > > > > memblock_end_of_DRAM() if no 32-bit DMA. Re-use it in the > > > > > zone_sizes_init() function for ZONE_DMA (maybe with a pr_info for no > > > > > 32-bit only DMA zone). > > > > > > > > There's a performance issue with all memory being in ZONE_DMA. It means > > > > all normal allocations will fail on ZONE_NORMAL and then have to fall > > > > back to ZONE_DMA. It would be better to put some percentage of memory > > > > in ZONE_DMA. > > > > > > Is the performance penalty real or just theoretical? I haven't run any > > > benchmarks myself. > > > > It is real insofar as you must eat cycles eliminating ZONE_NORMAL from > > consideration in the page allocation hot path. How much that really > > costs, I don't know. But it seems like it could be easily avoided by > > limiting ZONE_DMA size. Is there any reason it needs to be larger than > > 4GiB? > > Basically ZONE_DMA should allow a 32-bit dma mask. When memory starts > above 4G, in the absence of an IOMMU, it is likely that 32-bit devices > get some offset for the top bits to be able to address the bottom of the > memory. The problem is that dma_to_phys() that early in the kernel has > no idea about DMA offsets until later (they can be specified in DT per > device). > > The patch belows tries to guess a DMA offset and use the bottom 32-bit > of the DRAM as ZONE_DMA. > > -------8<----------------------- > > From 133656f8378dbb838ad5f12ea29aa9303d7ca922 Mon Sep 17 00:00:00 2001 > From: Catalin Marinas <catalin.marinas@....com> > Date: Fri, 18 Jul 2014 11:54:37 +0100 > Subject: [PATCH] arm64: Create non-empty ZONE_DMA when DRAM starts above 4GB > > ZONE_DMA is created to allow 32-bit only devices to access memory in the > absence of an IOMMU. On systems where the memory starts above 4GB, it is > expected that some devices have a DMA offset hardwired to be able to > access the bottom of the memory. Linux currently supports DT bindings > for the DMA offsets but they are not (easily) available early during > boot. > > This patch tries to guess a DMA offset and assumes that ZONE_DMA > corresponds to the 32-bit mask above the start of DRAM. > > Signed-off-by: Catalin Marinas <catalin.marinas@....com> > Cc: Mark Salter <msalter@...hat.com> > --- Tested-by: Mark Salter <msalter@...hat.com> Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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