linux-kernel - Re: [PATCH v2] dma-mapping: treat dev->bus_dma

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Date:   Thu, 21 Nov 2019 15:55:39 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paulburton@...nel.org>,
        James Hogan <jhogan@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-pci@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ide@...r.kernel.org,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2] dma-mapping: treat dev->bus_dma_mask as a DMA limit

On 21/11/2019 3:26 pm, Christoph Hellwig wrote:
> On Thu, Nov 21, 2019 at 04:24:57PM +0100, Christoph Hellwig wrote:
>> On Thu, Nov 21, 2019 at 10:26:44AM +0100, Nicolas Saenz Julienne wrote:
>>> Using a mask to represent bus DMA constraints has a set of limitations.
>>> The biggest one being it can only hold a power of two (minus one). The
>>> DMA mapping code is already aware of this and treats dev->bus_dma_mask
>>> as a limit. This quirk is already used by some architectures although
>>> still rare.
>>>
>>> With the introduction of the Raspberry Pi 4 we've found a new contender
>>> for the use of bus DMA limits, as its PCIe bus can only address the
>>> lower 3GB of memory (of a total of 4GB). This is impossible to represent
>>> with a mask. To make things worse the device-tree code rounds non power
>>> of two bus DMA limits to the next power of two, which is unacceptable in
>>> this case.
>>>
>>> In the light of this, rename dev->bus_dma_mask to dev->bus_dma_limit all
>>> over the tree and treat it as such. Note that dev->bus_dma_limit should
>>> contain the higher accesible DMA address.
>>>
>>> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
>>
>> I've tentatively added this patch to the dma-mapping tree based on
>> Robins principal approval of the last version.  That way tomorrows
>> linux-next run should still pick it up.
> 
> Actually.  This doesn't apply because the dma-mapping tree doesn't
> have you zone_dma_bits change.  I guess we'll need to wait for the
> next merge window, or maybe post rc1 if this happens to fix the
> powerpc problem that Christian reported.

Hmm, there's no functional dependency though, is there? AFAICS it's 
essentially just a context conflict. Is it worth simply dropping (or 
postponing) the local renaming in __dma_direct_optimal_gfp_mask(), or 
perhaps even cross-merging arm64/for-next/zone-dma into dma/for-next?

Robin.