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

Yong Wu
2015-04-14T06:31:28Z

Hi Tomasz,

Thanks very much for you suggestion and explain so detail.
please help check below.

On Fri, 2015-03-27 at 18:41 +0900, Tomasz Figa wrote:
> Hi Yong Wu,
>
> Sorry for long delay, I had to figure out some time to look at this again.
>
> On Wed, Mar 18, 2015 at 8:22 PM, Yong Wu <yong.wu@mediatek.com> wrote:
> >>
> >> > + imudev = piommu->dev;
> >> > +
> >> > + spin_lock_irqsave(&priv->portlock, flags);
> >>
> >> What is protected by this spinlock?
> > We will write a register of the local arbiter while config port. If
> > some modules are in the same local arbiter, it may be overwrite. so I
> > add it here.
> >>
>
> OK. Maybe it could be called larb_lock then? It would be good to have
> structures or code that should be running under this spinlock
> annotated with proper comments. And purpose of the lock documented in
> a comment as well (probably in a kerneldoc-style documentation of
> priv).
Thanks. I have move the spinlock into the smi driver, it will lock
for writing the local arbiter regsiter only.
>
> >> > +static void mtk_iommu_detach_device(struct iommu_domain *domain,
> >> > + struct device *dev)
> >> > +{
> >>
> >> No hardware (de)configuration or clean-up necessary?
> > I will add it. Actually we design like this:If a device have attached to
> > iommu domain, it won't detach from it.
>
> Isn't proper clean-up required for module removal? Some drivers might
> be required to be loadable modules, which should be unloadable.
>
> >>
> >> > +
> >> > + piommu->protect_va = devm_kmalloc(piommu->dev, MTK_PROTECT_PA_ALIGN*2,
> >>
> >> style: Operators like * should have space on both sides.
> >>
> >> > + GFP_KERNEL);
> >>
> >> Shouldn't dma_alloc_coherent() be used for this?
> > We don't care the data in it. I think they are the same. Could you
> > help tell me why dma_alloc_coherent may be better.
>
> Can you guarantee that at the time you allocate the memory using
> devm_kmalloc() the memory is not dirty (i.e. some write back data are
> stored in CPU cache) and is not going to be written back in some time,
> overwriting data put there by IOMMU hardware?
>
As I noted in the function "mtk_iommu_hw_init":

/* protect memory,HW will write here while translation fault */
protectpa = __virt_to_phys(piommu->protect_va);

We don’t care the content of this buffer, It is ok even though its
data is dirty.
It seem to be a the protect memory. While a translation fault
happened, The iommu HW will overwrite here instead of writing to the
fault physical address which may be 0 or some random address.

> >> > +
> >> > + iommu_set_fault_handler(domain, mtk_iommu_fault_handler, piommu);
> >>
> >> I don't see any other drivers doing this. Isn't this for upper layers,
> >> so that they can set their own generic fault handlers?
> > I think that this function is related with the iommu domain, we
> > have only one multimedia iommu domain. so I add it after the iommu
> > domain are created.
>
> No, this function is for drivers of IOMMU clients (i.e. master IP
> blocks) which want to subscribe to page fault to do things like paging
> on demand and so on. It shouldn't be called by IOMMU driver. Please
> see other IOMMU drivers, for example rockchip-iommmu.c.
Thanks. I have read it. I will delete it and print the error info
in the ISR. Also call the report_iommu_fault in the ISR.

> Best regards,
> Tomasz