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Sat, 10 Oct 2020 03:36:42 -0700 (PDT) X-Gm-Message-State: AOAM530ktt6w57XWQHQiZpyzkxozDoyLVcxVzRYCA9jNkXNP0N65LzMz 33eo3MU/q9xPypkrhr4q1maBUf9VtsETqfxfqQI= X-Google-Smtp-Source: ABdhPJyRUmME/uNQCpEvmy5TmuigC5HqxkAmy4br3cKJSuYPDqK1iFTDzWqlHrlHAracvofdjaHYJW7dOhpWJoeIxWo= X-Received: by 2002:a9d:6a85:: with SMTP id l5mr12561956otq.77.1602326201362; Sat, 10 Oct 2020 03:36:41 -0700 (PDT) MIME-Version: 1.0 References: <20201001172320.GQ21544@gaia> <20201002115541.GC7034@gaia> <12f33d487eabd626db4c07ded5a1447795eed355.camel@suse.de> <20201009071013.GA12208@lst.de> <513833810c15b5efeab7c3cbae1963a78c71a79f.camel@suse.de> <20201009152433.GA19953@e121166-lin.cambridge.arm.com> <20201009171051.GL23638@gaia> In-Reply-To: <20201009171051.GL23638@gaia> From: Ard Biesheuvel Date: Sat, 10 Oct 2020 12:36:30 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711 To: Catalin Marinas Cc: Linux ARM , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , Frank Rowand , Linux Kernel Mailing List , Linux Memory Management List , iommu@lists.linux-foundation.org, Rob Herring , linux-rpi-kernel@lists.infradead.org, Will Deacon , Christoph Hellwig , Robin Murphy X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Fri, 9 Oct 2020 at 19:10, Catalin Marinas wrote: > > On Fri, Oct 09, 2020 at 06:23:06PM +0200, Ard Biesheuvel wrote: > > On Fri, 9 Oct 2020 at 17:24, Lorenzo Pieralisi > > wrote: > > > We can move this check to IORT code and call it from arm64 if it > > > can be made to work. > > > > Finding the smallest value in the IORT, and assigning it to > > zone_dma_bits if it is < 32 should be easy. But as I understand it, > > having these separate DMA and DMA32 zones is what breaks kdump, no? So > > how is this going to fix the underlying issue? > > If zone_dma_bits is 32, ZONE_DMA32 disappears into ZONE_DMA (GFP_DMA32 > allocations fall back to ZONE_DMA). > > kdump wants DMA-able memory and, without a 30-bit ZONE_DMA, that would > be the bottom 32-bit. With the introduction of ZONE_DMA, this suddenly > became 1GB. We could change kdump to allocate ZONE_DMA32 but this one > may also be small as it lost 1GB to ZONE_DMA. However, the kdump kernel > would need to be rebuilt without ZONE_DMA since it won't have any. IIRC > (it's been a while since I looked), the kdump allocation couldn't span > multiple zones. > > In a separate thread, we try to fix kdump to use allocations above 4G as > a fallback but this only fixes platforms with enough RAM (and maybe it's > only those platforms that care about kdump). > One thing that strikes me as odd is that we are applying the same shifting logic to ZONE_DMA as we are applying to ZONE_DMA32, i.e., if DRAM starts outside of the zone, it is shifted upwards. On a typical ARM box, this gives me [ 0.000000] Zone ranges: [ 0.000000] DMA [mem 0x0000000080000000-0x00000000bfffffff] [ 0.000000] DMA32 [mem 0x00000000c0000000-0x00000000ffffffff] [ 0.000000] Normal [mem 0x0000000100000000-0x0000000fffffffff] i.e., the 30-bit addressable range has bit 31 set, which is weird. I wonder if it wouldn't be better (and less problematic in the general case) to drop this logic for ZONE_DMA, and simply let it remain empty unless there is really some memory there. _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu