Date:   Fri, 11 May 2018 13:32:54 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Dmitry Osipenko <digetx@...il.com>, Joerg Roedel <joro@...tes.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>
Cc:     linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 7/9] iommu/tegra: gart: Provide single domain and group
 for all devices

On 08/05/18 19:16, Dmitry Osipenko wrote:
> GART aperture is shared by all devices, hence there is a single IOMMU
> domain and group shared by these devices. Allocation of a group per
> device only wastes resources and allowance of having more than one domain
> is simply wrong because IOMMU mappings made by the users of "different"
> domains will stomp on each other.

Strictly, that reasoning is a bit backwards - allocating multiple groups 
is the conceptually-wrong thing if the GART cannot differentiate between 
different devices, whereas having multiple domains *exist* is no real 
problem, it's merely that only one can be active at any point in time 
(which will inherently become the case once all devices are grouped 
together).

> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> ---
>   drivers/iommu/tegra-gart.c | 107 +++++++++----------------------------
>   1 file changed, 24 insertions(+), 83 deletions(-)
> 
> diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
> index 5b2d27620350..ebc105c201bd 100644
> --- a/drivers/iommu/tegra-gart.c
> +++ b/drivers/iommu/tegra-gart.c
> @@ -19,7 +19,6 @@
>   
>   #include <linux/io.h>
>   #include <linux/iommu.h>
> -#include <linux/list.h>
>   #include <linux/module.h>
>   #include <linux/of_device.h>
>   #include <linux/slab.h>
> @@ -44,22 +43,17 @@
>   #define GART_PAGE_MASK						\
>   	(~(GART_PAGE_SIZE - 1) & ~GART_ENTRY_PHYS_ADDR_VALID)
>   
> -struct gart_client {
> -	struct device		*dev;
> -	struct list_head	list;
> -};
> -
>   struct gart_device {
>   	void __iomem		*regs;
>   	u32			*savedata;
>   	u32			page_count;	/* total remappable size */
>   	dma_addr_t		iovmm_base;	/* offset to vmm_area */
>   	spinlock_t		pte_lock;	/* for pagetable */
> -	struct list_head	client;
> -	spinlock_t		client_lock;	/* for client list */
>   	struct device		*dev;
>   
>   	struct iommu_device	iommu;		/* IOMMU Core handle */
> +	struct iommu_group	*group;		/* Common IOMMU group */
> +	struct gart_domain	*domain;	/* Unique IOMMU domain */
>   
>   	struct tegra_mc_gart_handle mc_gart_handle;
>   };
> @@ -169,81 +163,31 @@ static inline bool gart_iova_range_valid(struct gart_device *gart,
>   static int gart_iommu_attach_dev(struct iommu_domain *domain,
>   				 struct device *dev)
>   {
> -	struct gart_domain *gart_domain = to_gart_domain(domain);
> -	struct gart_device *gart = gart_domain->gart;
> -	struct gart_client *client, *c;
> -	int err = 0;
> -
> -	client = devm_kzalloc(gart->dev, sizeof(*c), GFP_KERNEL);
> -	if (!client)
> -		return -ENOMEM;
> -	client->dev = dev;
> -
> -	spin_lock(&gart->client_lock);
> -	list_for_each_entry(c, &gart->client, list) {
> -		if (c->dev == dev) {
> -			dev_err(gart->dev,
> -				"%s is already attached\n", dev_name(dev));
> -			err = -EINVAL;
> -			goto fail;
> -		}
> -	}
> -	list_add(&client->list, &gart->client);
> -	spin_unlock(&gart->client_lock);
> -	dev_dbg(gart->dev, "Attached %s\n", dev_name(dev));
>   	return 0;
> -
> -fail:
> -	devm_kfree(gart->dev, client);
> -	spin_unlock(&gart->client_lock);
> -	return err;
>   }
>   
>   static void gart_iommu_detach_dev(struct iommu_domain *domain,
>   				  struct device *dev)
>   {
> -	struct gart_domain *gart_domain = to_gart_domain(domain);
> -	struct gart_device *gart = gart_domain->gart;
> -	struct gart_client *c;
> -
> -	spin_lock(&gart->client_lock);
> -
> -	list_for_each_entry(c, &gart->client, list) {
> -		if (c->dev == dev) {
> -			list_del(&c->list);
> -			devm_kfree(gart->dev, c);
> -			dev_dbg(gart->dev, "Detached %s\n", dev_name(dev));
> -			goto out;
> -		}
> -	}
> -	dev_err(gart->dev, "Couldn't find\n");
> -out:
> -	spin_unlock(&gart->client_lock);
>   }

The .detach_dev callback is optional in the core API now, so you can 
just remove the whole thing.

>   static struct iommu_domain *gart_iommu_domain_alloc(unsigned type)
>   {
> -	struct gart_domain *gart_domain;
> -	struct gart_device *gart;
> -
> -	if (type != IOMMU_DOMAIN_UNMANAGED)
> -		return NULL;
> +	struct gart_device *gart = gart_handle;
>   
> -	gart = gart_handle;
> -	if (!gart)
> +	if (type != IOMMU_DOMAIN_UNMANAGED || gart->domain)

Singleton domains are a little unpleasant given the way the IOMMU API 
expects things to work, but it looks fairly simple to avoid needing that 
at all. AFAICS you could move gart->savedata to something like 
gart_domain->ptes and keep it up-to-date in .map/.unmap, then in 
.attach_dev you just need to do something like:

	if (gart_domain != gart->domain) {
		do_gart_setup(gart, gart_domain->ptes);
		gart->domain = gart_domain;
	}

to context-switch the hardware state when moving the group from one 
domain to another (and as a bonus you would no longer need to do 
anything for suspend, since resume can just look at the current domain 
too). If in practice there's only ever one domain allocated anyway, then 
there's no difference in memory overhead, but you still have the benefit 
of the driver being more consistent with others and allowing that 
flexibility if anyone ever did want to play with it.

>   		return NULL;
>   
> -	gart_domain = kzalloc(sizeof(*gart_domain), GFP_KERNEL);
> -	if (!gart_domain)
> -		return NULL;
> -
> -	gart_domain->gart = gart;
> -	gart_domain->domain.geometry.aperture_start = gart->iovmm_base;
> -	gart_domain->domain.geometry.aperture_end = gart->iovmm_base +
> +	gart->domain = kzalloc(sizeof(*gart->domain), GFP_KERNEL);
> +	if (gart->domain) {
> +		gart->domain->domain.geometry.aperture_start = gart->iovmm_base;
> +		gart->domain->domain.geometry.aperture_end = gart->iovmm_base +
>   					gart->page_count * GART_PAGE_SIZE - 1;
> -	gart_domain->domain.geometry.force_aperture = true;
> +		gart->domain->domain.geometry.force_aperture = true;
> +		gart->domain->gart = gart;
> +	}
>   
> -	return &gart_domain->domain;
> +	return &gart->domain->domain;
>   }
>   
>   static void gart_iommu_domain_free(struct iommu_domain *domain)
> @@ -251,18 +195,7 @@ static void gart_iommu_domain_free(struct iommu_domain *domain)
>   	struct gart_domain *gart_domain = to_gart_domain(domain);
>   	struct gart_device *gart = gart_domain->gart;
>   
> -	if (gart) {
> -		spin_lock(&gart->client_lock);
> -		if (!list_empty(&gart->client)) {
> -			struct gart_client *c;
> -
> -			list_for_each_entry(c, &gart->client, list)
> -				gart_iommu_detach_dev(domain, c->dev);
> -		}
> -		spin_unlock(&gart->client_lock);
> -	}
> -
> -	kfree(gart_domain);
> +	kfree(gart->domain);
>   }
>   
>   static int gart_iommu_map(struct iommu_domain *domain, unsigned long iova,
> @@ -377,7 +310,7 @@ struct iommu_group *gart_iommu_device_group(struct device *dev)
>   	if (err)
>   		return ERR_PTR(err);
>   
> -	return generic_device_group(dev);
> +	return gart_handle->group;

You should take a reference per device, i.e.:

	return iommu_group_ref_get(gart_handle->group);

otherwise removing devices could unbalance things and result in the 
group getting freed prematurely.

>   }
>   
>   static int gart_iommu_of_xlate(struct device *dev,
> @@ -502,8 +435,6 @@ static int tegra_gart_probe(struct platform_device *pdev)
>   
>   	gart->dev = &pdev->dev;
>   	spin_lock_init(&gart->pte_lock);
> -	spin_lock_init(&gart->client_lock);
> -	INIT_LIST_HEAD(&gart->client);
>   	gart->regs = gart_regs;
>   	gart->iovmm_base = (dma_addr_t)res_remap->start;
>   	gart->page_count = (resource_size(res_remap) >> GART_PAGE_SHIFT);
> @@ -517,6 +448,14 @@ static int tegra_gart_probe(struct platform_device *pdev)
>   		goto iommu_unregister;
>   	}
>   
> +	gart->group = iommu_group_alloc();
> +	if (IS_ERR(gart->group)) {
> +		ret = PTR_ERR(gart->group);
> +		goto free_savedata;
> +	}
> +
> +	iommu_group_ref_get(gart->group);

You already hold the initial reference from iommu_group_alloc(), so 
there's no need to take a second one at this point.

Robin.

> +
>   	platform_set_drvdata(pdev, gart);
>   	do_gart_setup(gart, NULL);
>   
> @@ -525,6 +464,8 @@ static int tegra_gart_probe(struct platform_device *pdev)
>   
>   	return 0;
>   
> +free_savedata:
> +	vfree(gart->savedata);
>   iommu_unregister:
>   	iommu_device_unregister(&gart->iommu);
>   remove_sysfs:
> 

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