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

[Tomasz Figa <tfiga@chromium.org>]

On Tue, Mar 10, 2015 at 12:41 PM, Yingjoe Chen
<yingjoe.chen@mediatek.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@mediatek.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