Re: [PATCH v9 1/5] dt-bindings: reserved-memory: Document iommu-addresses

[Robin Murphy <robin.murphy@arm.com>]

On 2022-10-07 14:54, Thierry Reding wrote:
> On Fri, Oct 07, 2022 at 02:45:31PM +0100, Robin Murphy wrote:
>> On 2022-09-23 13:35, Thierry Reding wrote:
>>> From: Thierry Reding <treding@nvidia.com>
>>>
>>> This adds the "iommu-addresses" property to reserved-memory nodes, which
>>> allow describing the interaction of memory regions with IOMMUs. Two use-
>>> cases are supported:
>>>
>>>     1. Static mappings can be described by pairing the "iommu-addresses"
>>>        property with a "reg" property. This is mostly useful for adopting
>>>        firmware-allocated buffers via identity mappings. One common use-
>>>        case where this is required is if early firmware or bootloaders
>>>        have set up a bootsplash framebuffer that a display controller is
>>>        actively scanning out from during the operating system boot
>>>        process.
>>>
>>>     2. If an "iommu-addresses" property exists without a "reg" property,
>>>        the reserved-memory node describes an IOVA reservation. Such memory
>>>        regions are excluded from the IOVA space available to operating
>>>        system drivers and can be used for regions that must not be used to
>>>        map arbitrary buffers.
>>
>> Bah, I've only just realised: don't we also need to change the "oneOf:
>> required: ..." schema to permit "iommu-addresses" without "reg" or "size"?
> 
> Hm... good point. I think at least we'll want another:
> 
>       - required:
>           - iommu-addresses
> 
> in there. I wonder if we also need to avoid the combination of "size"
> and "iommu-addresses". When "size" is specified, is it guaranteed that
> those regions will be allocated before the direct mapping needs to be
> created?

Well, it couldn't really be a direct mapping anyway. In general I don't 
think that combination makes any sense, since the presence of 
"iommu-addresses" means one of two things; either it says the IOVA range 
is carved out for some special purpose or just unusable, in which case 
allocating any memory to back it would surely be pointless, or it's 
saying don't touch these addresses because the device is already 
accessing them, thus the underlying physical memory must be allocated 
somewhere already.

Thanks,
Robin.