linux-kernel - Re: [PATCH v7 04/16] drivers: iommu: make of_iommu_set/get

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Message-ID: <41e3eff1-9ce6-bcfb-5716-c65ef38add63@arm.com>
Date:   Mon, 14 Nov 2016 12:00:47 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org,
        Will Deacon <will.deacon@....com>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Tomasz Nowicki <tn@...ihalf.com>, Jon Masters <jcm@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        Nate Watterson <nwatters@...eaurora.org>,
        Prem Mallappa <prem.mallappa@...adcom.com>,
        Dennis Chen <dennis.chen@....com>, linux-acpi@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v7 04/16] drivers: iommu: make of_iommu_set/get_ops() DT
 agnostic

On 14/11/16 10:26, Lorenzo Pieralisi wrote:
> Hi Robin, Joerg,
> 
> On Fri, Nov 11, 2016 at 05:43:39PM +0000, Robin Murphy wrote:
>> On 11/11/16 16:27, Joerg Roedel wrote:
>>> On Fri, Nov 11, 2016 at 04:17:37PM +0000, Robin Murphy wrote:
>>>> In the original of_iommu_configure design, the thought was that an ops
>>>> structure could be IOMMU-instance-specific (hence the later-removed
>>>> "priv" member), so I suppose right now it is mostly a hangover from
>>>> that. However, it's also what we initialise a device's fwspec with, so
>>>> becomes important again if we're ever going to get past the limitations
>>>> of buses-which-are-not-actually-buses[1].
>>>
>>> Yeah, I discussed this with a few others at LPC. My current idea is to
>>> tell the iommu-core which hardware-iommus exist in the system and a
>>> seperate iommu_ops ptr for each of them. Then every struct device can
>>> link to the iommu-instance it is translated by.
>>
>> Er, that sounds very much like a description of what we already have in
>> 4.9-rc. Every struct device now has an iommu_fwspec which encapsulates
>> both an iommu_ops pointer (which can perfectly well be per-instance if
>> the IOMMU driver wants) and a place for the IOMMU-private data to
>> replace the mess of archdata.iommu and driver-internal globals.
>>
>>> We are not there yet, but this will give you the same per-device
>>> iommu-ops as implemented here.
>>
>> With those two patches I linked to, which make the bulk of the IOMMU
>> core code per-device-ops-aware off the bat, I'd say we *are* already
>> pretty much there. It's only iommu_domain_alloc() which needs a
>> device-based alternative, and the non-of_xlate-based IOMMU drivers to
>> either call iommu_fwspec_init() for themselves, or perhaps for x86
>> plumbing in DMAR/IVRS equivalents of the IORT parsing to the
>> infrastructure provided by this series.
> 
> I think it all boils down to how we end up implementing the per-device
> iommu_ops look-up/binding, question is what do you want me to do with
> this patch, it should be fine to drop it and use dev->bus->iommu_ops
> for the look-up but I should know sooner rather than later to make
> sure the series get another good round of testing.

If we've already made the decision to move away from bus ops, I don't
see that it makes sense to deliberately introduce new dependencies on
them. Besides, as it stands, this patch literally implements "tell the
iommu-core which hardware-iommus exist in the system and a seperate
iommu_ops ptr for each of them" straight off.

Robin.

> 
> Please let me know, thank you very much.
> 
> Lorenzo
>