Re: [PATCH v2] drm/tegra: Stop using iommu_present()

From: Robin Murphy <robin.murphy@arm.com>
To: Dmitry Osipenko <dmitry.osipenko@collabora.com>,
	thierry.reding@gmail.com
Cc: jonathanh@nvidia.com, dri-devel@lists.freedesktop.org,
	linux-tegra@vger.kernel.org, iommu@lists.linux-foundation.org
Subject: Re: [PATCH v2] drm/tegra: Stop using iommu_present()
Date: Wed, 4 May 2022 12:52:03 +0100	[thread overview]
Message-ID: <63c021c4-57de-38fe-e48f-a308088d235b@arm.com> (raw)
In-Reply-To: <add31812-50d5-6cb0-3908-143c523abd37@collabora.com>

On 2022-05-04 01:52, Dmitry Osipenko wrote:
> On 4/11/22 16:46, Robin Murphy wrote:
>> @@ -1092,6 +1092,19 @@ static bool host1x_drm_wants_iommu(struct host1x_device *dev)
>>   	struct host1x *host1x = dev_get_drvdata(dev->dev.parent);
>>   	struct iommu_domain *domain;
>>   
>> +	/* For starters, this is moot if no IOMMU is available */
>> +	if (!device_iommu_mapped(&dev->dev))
>> +		return false;
> 
> Unfortunately this returns false on T30 with enabled IOMMU because we
> don't use IOMMU for Host1x on T30 [1] to optimize performance. We can't
> change it until we will update drivers to support Host1x-dedicated buffers.

Huh, so is dev->dev here not the DRM device? If it is, and 
device_iommu_mapped() returns false, then the later iommu_attach_group() 
call is going to fail anyway, so there's not much point allocating a 
domain. If it's not, then what the heck is host1x_drm_wants_iommu() 
actually testing for?

In the not-too-distant future we'll need to pass an appropriate IOMMU 
client device to iommu_domain_alloc() as well, so the sooner we can get 
this code straight the better.

Thanks,
Robin.

> 
> [1]
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/host1x/dev.c#L258
> 