linux-kernel - Re: [PATCH 2/5] iommu/mediatek: Add mt8173 IOMMU driver

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Date:	Tue, 10 Mar 2015 13:06:31 +0900
From:	Tomasz Figa <tfiga@...omium.org>
To:	Yingjoe Chen <yingjoe.chen@...iatek.com>
Cc:	Joerg Roedel <joro@...tes.org>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, srv_heupstream@...iatek.com,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	iommu@...ts.linux-foundation.org, Rob Herring <robh+dt@...nel.org>,
	Daniel Kurtz <djkurtz@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Sasha Hauer <kernel@...gutronix.de>,
	Matthias Brugger <matthias.bgg@...il.com>,
	linux-mediatek@...ts.infradead.org,
	Robin Murphy <robin.murphy@....com>,
	Yong Wu (吴勇) <yong.wu@...iatek.com>,
	Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [PATCH 2/5] iommu/mediatek: Add mt8173 IOMMU driver

On Tue, Mar 10, 2015 at 12:41 PM, Yingjoe Chen
<yingjoe.chen@...iatek.com> wrote:
> On Tue, 2015-03-10 at 02:00 +0900, Tomasz Figa wrote:
>> On Mon, Mar 9, 2015 at 11:46 PM, Yingjoe Chen <yingjoe.chen@...iatek.com> wrote:
>> > On Mon, 2015-03-09 at 20:11 +0900, Tomasz Figa wrote:
>> > <...>
>> >> > +/*
>> >> > + * pimudev is a global var for dma_alloc_coherent.
>> >> > + * It is not accepatable, we will delete it if "domain_alloc" is enabled
>> >> > + */
>> >> > +static struct device *pimudev;
>> >>
>> >> This is indeed not acceptable. Could you replace dma_alloc_coherent()
>> >> with something that doesn't require device pointer, e.g.
>> >> alloc_pages()? (Although that would require you to handle cache
>> >> maintenance in the driver, due to cached memory allocated.) I need to
>> >> think about a better solution for this.
>> >
>> > Hi,
>> >
>> > For 2nd level page table, we use cached memory now. Currently we are
>> > using __dma_flush_range to flush the cache, which is also unacceptable.
>> >
>> > For proper cache management, we'll need to use dma_map_single or
>> > dma_sync_*, which still need a deivce*.
>>
>> Looking at how already mainlined drivers do this, they either use
>> dmac_flush_range()
>> (https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/iommu/msm_iommu.c?id=refs/tags/v4.0-rc3#n80)
>> or directly __cpuc_flush_dcache_area() and outer_flush_range()
>> (https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/iommu/rockchip-iommu.c?id=refs/tags/v4.0-rc3#n93).
>
> Hi,
>
> These only exist in arch/arm, not arm64. I think we should avoid using
> API start with __ in drivers. This driver might be used in both
> arm/arm64, I think the only option for us is DMA APIs.
>
> Actually, I'm thinking that we should change to use coherent memory for
> 2nd level page table as well and totally skip the cache flush. It seems
> dma_pool_create is suitable to replace kmem_cache we are using right
> now. However it still need a device*, which we have to fix anyway.

That sounds like a reasonable option, because this is what we have DMA
mapping API for.

Do you expect to have more than one M4U block inside a SoC? Maybe this
static variable actually isn't that bad, with a comment added
explaining that there is always only one such block and that a rework
will be needed if future SoCs will have more.

Best regards,
Tomasz
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