linux-kernel - Re: [PATCH v8 2/2] iommu/exynos: Add iommu driver for Exynos Platforms

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Message-ID: <CAHQjnOMyoOH2cFjjGSzMvefZTQGROmQgaA8S0MmxnKGHWyQhyA@mail.gmail.com>
Date:	Wed, 25 Jan 2012 15:51:58 +0900
From:	KyongHo Cho <pullip.cho@...sung.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Kukjin Kim <kgene.kim@...sung.com>,
	Subash Patel <subash.ramaswamy@...aro.org>,
	linux-kernel@...r.kernel.org,
	Younglak Kim <younglak1004.kim@...sung.com>,
	iommu@...ts.linux-foundation.org,
	linux-samsung-soc@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Sanghyun Lee <sanghyun75.lee@...sung.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v8 2/2] iommu/exynos: Add iommu driver for Exynos Platforms

Hi,
On Mon, Jan 23, 2012 at 11:27 PM, Joerg Roedel <joro@...tes.org> wrote:
> Hi,
>
> please also get and inclue Acks from the Exynos maintainer for the next
> post.
> Since I have a compiling config for exynos now I will merge the patches
> when you have the Acks and addressed or explained the issues I pointed
> out below.
>
Thanks for review!
I will include the Acks in the next patchset.
I will post the next patchset with corrections by the day after tomorrow.

And sorry for late reply.
I had holidays for the new year's day based on Lunar system.

> On Thu, Dec 29, 2011 at 09:26:08PM +0900, KyongHo Cho wrote:
>> +static void exynos_iommu_domain_destroy(struct iommu_domain *domain)
>> +{
>> +     struct exynos_iommu_domain *priv = domain->priv;
>> +     struct list_head *pos, *n;
>> +     unsigned long flags;
>> +     int i;
>> +
>> +     WARN_ON(!list_empty(&priv->clients));
>
> This isn't really a problem. We allow destroying a domain with devices
> attached. So this WARN_ON is not necessary.
>
OK.
BTW, Isn't it a problem when a device driver does not know that its
iommu domain is destroyed?
Can we regards that it is the faulty use of iommu API?

>> +static int exynos_iommu_map(struct iommu_domain *domain, unsigned long iova,
>> +                      phys_addr_t paddr, size_t size, int prot)
>> +{
>> +     struct exynos_iommu_domain *priv = domain->priv;
>> +     unsigned long *entry;
>> +     unsigned long flags;
>> +     int ret = -ENOMEM;
>> +
>> +     BUG_ON(priv->pgtable == NULL);
>> +
>> +     spin_lock_irqsave(&priv->pgtablelock, flags);
>> +
>> +     entry = section_entry(priv->pgtable, iova);
>> +
>> +     if (size >= SECT_SIZE) {
>> +             ret = lv1set_section(entry, paddr, size >> SECT_ORDER,
>> +                                     &priv->lv2entcnt[lv1ent_offset(iova)]);
>
> This looks like you are partially re-implementing behavior of generic
> code because you are mapping multiple sections at once. The generic map
> code already splits up the address range correctly, so no need to do
> this in the driver (unless there is some benefit in the hardware, like
> an IOTLB entry that can cover multiple sections or something similar).
>
Yes, I wanted to avoid repeated function call by iommu_map().
s5p_iommu_map() maps once for the same page size since it is efficient
and simple.
That's why this driver initializes domain->pgsize_bitmap with 0xFFFFF000
even though our IOMMU driver just supports 3 different page sizes
including 4KB, 64KB and 1MB.

Do you think it is better for s5p_iommu_map() to map just one page at once?

>> +static size_t exynos_iommu_unmap(struct iommu_domain *domain,
>> +                                            unsigned long iova, size_t size)
>> +{
>> +     struct exynos_iommu_domain *priv = domain->priv;
>> +     struct iommu_client *client;
>> +     unsigned long flags;
>> +
>> +     BUG_ON(priv->pgtable == NULL);
>> +
>> +     spin_lock_irqsave(&priv->pgtablelock, flags);
>> +
>> +     while (size != 0) {
>> +             int i, nent, order;
>> +             unsigned long *pent, *sent;
>
> Same with this while-loop. This looks like it re-implements behavior
> from the generic code.
>
If a region to unmap consists of tens of pages
there is no way to avoid flushing IOTLB repeatedly.

Out iommu driver doesn't need to flush IOTLB more than once for a
region to unmap.

Do you think the driver is better to unmaps just one page at once
though flushing IOTLB repeatedly?


Thank you.

  KyongHo
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