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

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Date:   Tue, 19 Nov 2019 13:57:43 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     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>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Burton <paulburton@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        phil@...pberrypi.org, linux-acpi@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        James Hogan <jhogan@...nel.org>, Len Brown <lenb@...nel.org>,
        devicetree@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] dma-mapping: treat dev->bus_dma_mask as a DMA limit

On Wed, 2019-11-13 at 17:13 +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 is
> meant to contain the higher accesible DMA address.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> 

Hi Rob & Christoph,
do you mind if I append v2 of this into my upcoming v3 RPi4 PCIe support
series, I didn't do it initially as I thought this was going to be a
contentious patch.  But as it turned out better than expected, I think it
should go into the PCIe series. In the end it's the first explicit user of the
bus DMA limit.

Here's v2 in case you don't know what I'm talking about:
https://www.spinics.net/lists/arm-kernel/msg768459.html

Regards,
Nicolas


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