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

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Message-ID: <7609007d-52f5-bb10-e8d5-96fadbfab46d@arm.com>
Date:   Tue, 19 Nov 2019 17:17:03 +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>,
        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 19/11/2019 5:00 pm, Christoph Hellwig wrote:
> On Tue, Nov 19, 2019 at 01:57:43PM +0100, Nicolas Saenz Julienne wrote:
>> 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
> 
> In principle I wouldn't mind, but I think this is going to conflict
> quite badly with other changes in the dma-mapping tree (including
> yours).  So I think we'll need a shared tree or I'll need to pull
> in the whole series through the dma-mapping tree if there are not
> other conflicts and the other maintainers are fine with it.

TBH I can't see it being a massive problem even if the DMA patch, driver 
and DTS patch went entirely separately via the respective DMA, PCI, and 
arm-soc trees in the same cycle. Bisecting over a merge window is a big 
enough pain in the bum as it is, and if the worst case is that someone 
trying to do that on a Pi4 has a wonky PCI controller appear for a 
couple of commits, they may as well just disable that driver for their 
bisection, because it wasn't there at the start so can't possibly be the 
thing they're looking for regressions in ;)

Robin.