Re: [PATCH] Revert "ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()"

From: Thierry Reding <treding@nvidia.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	linux-kernel@vger.kernel.org,
	Tobias Jakobi <tjakobi@math.uni-bielefeld.de>,
	iommu@lists.linux-foundation.org, Ben Skeggs <bskeggs@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	linux-arm-kernel@lists.infradead.org,
	Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [PATCH] Revert "ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()"
Date: Tue, 15 Jan 2019 16:29:11 +0100	[thread overview]
Message-ID: <20190115152910.GA12652@ulmo> (raw)
In-Reply-To: <4f7fd8db-33e4-9c8d-2fee-1ca9c98a9276@arm.com>

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On Mon, Jan 14, 2019 at 04:38:20PM +0000, Robin Murphy wrote:
> On 14/01/2019 16:09, Thierry Reding wrote:
> > On Mon, Jan 14, 2019 at 02:22:40PM +0100, Marek Szyprowski wrote:
> > > This reverts commit 1874619a7df4b14b23b14877e705ae15325773e3.
> > > 
> > > That patch broke IOMMU support for devices, which fails to probe for the
> > > first time and use deferred probe approach. When non-NULL dma_ops is set
> > > in arm_iommu_detach_device(), the given device later ignored by
> > > arch_setup_dma_ops() and stays with non-IOMMU dma_ops.
> > > 
> > > Reported-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
> > > Fixes: 1874619a7df4 "ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()"
> > > Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > > ---
> > >   arch/arm/mm/dma-mapping.c | 12 ++++++------
> > >   1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > Can you point out exactly what drivers break because of this change? We
> > need to find a solution that works for everyone. Reverting is only
> > marginally useful because somebody will just end up wanting to revert
> > the revert because a different driver is now broken.
> 
> At first glance, it sounds like what we really want is for
> arch_teardown_iommu_ops() to completely clear any ops that
> arch_setup_dma_ops() installed - does the below suffice?
> 
> Robin.
> 
> ----->8-----
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index f1e2922e447c..1e3e08a1c456 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -2390,4 +2390,6 @@ void arch_teardown_dma_ops(struct device *dev)
>  		return;
> 
>  	arm_teardown_iommu_dma_ops(dev);
> +	/* Let arch_setup_dma_ops() start again from scratch upon re-probe */
> +	set_dma_ops(dev, NULL);
>  }

Seems to work fine on Tegra:

Tested-by: Thierry Reding <treding@nvidia.com>

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