linux-kernel - Re: [PATCH 1/9] iommu/dma-iommu: Add iommu_map

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Date:   Sun, 14 Apr 2019 23:30:58 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tom Murphy <tmurphy@...sta.com>
Cc:     iommu@...ts.linux-foundation.org, Heiko Stuebner <heiko@...ech.de>,
        Will Deacon <will.deacon@....com>,
        David Brown <david.brown@...aro.org>,
        linux-samsung-soc@...r.kernel.org, dima@...sta.com,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-rockchip@...ts.infradead.org, Kukjin Kim <kgene@...nel.org>,
        Andy Gross <andy.gross@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-msm@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        murphyt7@....ie, Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 1/9] iommu/dma-iommu: Add iommu_map_atomic

On Thu, Apr 11, 2019 at 07:47:30PM +0100, Tom Murphy via iommu wrote:
> The iommu ops .map function (or the iommu_map function which calls it)
> was always supposed to be sleepable (according to Joerg's comment in
> this thread: https://lore.kernel.org/patchwork/patch/977520/ ) and so
> should probably have had a "might_sleep()" since it was written. However
> currently the dma-iommu api can call iommu_map in an atomic context,
> which it shouldn't do. This doesn't cause any problems because any iommu
> driver which uses the dma-iommu api uses gfp_atomic in it's iommu_ops
> .map function. But doing this wastes the memory allocators atomic pools.
> 
> Add a new function iommu_map_atomic, use it in the dma-iommu api and add
> “might_sleep()” to the iommu_map function.
> 
> After this change all drivers which use the dma-iommu api need to
> implement the new iommu ops .map_atomic function. For the moment just
> reuse the driver's iommus ops .map function for .map_atomic.

Instead of duplicating the callchain shouldn't we just pass down a flag
to indicate if the context can sleep or not?  Especially as all the
existing arm drivers already implement the atomic behavior, which is
a majority of struct iommu_ops based iommu drivers.