linux-kernel - Re: [RFC PATCH 1/2] dt-bindings: pci: Add drop mask property for MSI and IOMMU

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Date:   Fri, 7 Jul 2017 16:42:48 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Scott Branden <scott.branden@...adcom.com>,
        Mark Rutland <mark.rutland@....com>,
        Srinath Mannam <srinath.mannam@...adcom.com>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, Rob Herring <robh+dt@...nel.org>,
        bcm-kernel-feedback-list@...adcom.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pci: Add drop mask property for MSI
 and IOMMU

On 07/07/17 16:22, Scott Branden wrote:
> Hi Robin,
> 
> On 17-07-07 07:55 AM, Robin Murphy wrote:
>> On 07/07/17 14:30, Mark Rutland wrote:
[...]
>>> Your mapping can be expressed today using a number of msi-map entries,
>>> which you can easily generate programmatically with a trivial perl
>>> script, without requiring a new binding or any new kernel code.
>>>
>>> Please do that instead.
>> Indeed. The systems I'm aware of which need to express non-trivial RID
>> to SID mappings tend to have the bootloader probe PCI and dynamically
>> generate map entries per discovered RID, but even if you wanted to
>> statically generate the whole lot for the worst-case bus range that's
>> still only 512 entries, which is not unmanageable. Notably, it's also
>> what would have to be done (in equivalent) for IORT, although I assume
>> this is an embedded platform for which nobody cares about ACPI.
> Actually we will care about ACPI and need to add it (doesn't need to be
> in this patchet unless easy to do so...)

Ah, OK, that's an even stronger argument for not adding anything new
then - DT "iommu-map" is already marginally more expressive than IORT ID
mappings can be, so there doesn't seem to be much justification for
diverging them further.

Robin.