Re: [PATCH v3 5/6] iommu/mediatek: Add mt8173 IOMMU driver

From: Robin Murphy <robin.murphy@arm.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Yong Wu <yong.wu@mediatek.com>, Joerg Roedel <joro@8bytes.org>,
	Thierry Reding <treding@nvidia.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"pebolle@tiscali.nl" <pebolle@tiscali.nl>,
	"arnd@arndb.de" <arnd@arndb.de>,
	"srv_heupstream@mediatek.com" <srv_heupstream@mediatek.com>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Will Deacon <Will.Deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Tomasz Figa <tfiga@google.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Rob Herring <robh+dt@kernel.org>,
	Daniel Kurtz <djkurtz@google.com>,
	Sasha Hauer <kernel@pengutronix.de>,
	"cloud.chou@mediatek.com" <cloud.chou@mediatek.com>,
	"linux-mediatek@lists.infradead.org" 
	<linux-mediatek@lists.infradead.org>,
	"frederic.chen@mediatek.com" <frederic.chen@mediatek.com>,
	"mitchelh@codeaurora.org" <mitchelh@codeaurora.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Lucas Stach <l.stach@pengutronix.de>
Subject: Re: [PATCH v3 5/6] iommu/mediatek: Add mt8173 IOMMU driver
Date: Mon, 27 Jul 2015 16:49:30 +0100	[thread overview]
Message-ID: <55B6530A.9050903@arm.com> (raw)
In-Reply-To: <20150727153148.GJ7557@n2100.arm.linux.org.uk>

On 27/07/15 16:31, Russell King - ARM Linux wrote:
> On Mon, Jul 27, 2015 at 02:23:26PM +0100, Robin Murphy wrote:
>> On 16/07/15 10:04, Yong Wu wrote:
>>> This patch adds support for mediatek m4u (MultiMedia Memory Management
>>> Unit).
>>>
>>> Signed-off-by: Yong Wu <yong.wu@mediatek.com>
>> [...]
>>> +static void mtk_iommu_flush_pgtable(void *ptr, size_t size, void *cookie)
>>> +{
>>> +       struct mtk_iommu_domain *domain = cookie;
>>> +       unsigned long offset = (unsigned long)ptr & ~PAGE_MASK;
>>> +
>>> +       dma_map_page(domain->data->dev, virt_to_page(ptr), offset,
>>> +                    size, DMA_TO_DEVICE);
>>
>> Nit: this looks like it may as well be dma_map_single.
>>
>> It would probably be worth following it with a matching unmap too, just to
>> avoid any possible leakage bugs (especially if this M4U ever appears in a
>> SoC supporting RAM above the 32-bit boundary).
>
> Why not do the job properly?  Take a look at how I implemented the
> streaming DMA API on Tegra SMMU (patch set recently sent out earlier
> today).
>
> There's no need for hacks like dma_map_page() (and discarding it's
> return value) or dma_map_page() followed by dma_unmap_page().

Indeed, as it happens I do have a branch where I prototyped that for the 
long-descriptor io-pgtable-arm code a while ago; this discussion has 
prompted me to dig it up again. Stay tuned, folks...

Robin.