Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

From: Olav Haugan <ohaugan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
To: Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>
Cc: Mark Rutland <Mark.Rutland-5wv7dgnIgG8@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"linux-samsung-soc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-samsung-soc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>,
	Pawel Moll <Pawel.Moll-5wv7dgnIgG8@public.gmane.org>,
	Ian Campbell
	<ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>,
	Grant Grundler <grundler-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
	Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>,
	"linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Marc Zyngier <Marc.Zyngier-5wv7dgnIgG8@public.gmane.org>,
	Linux IOMMU
	<iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>,
	Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Thierry Reding
	<thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Kumar Gala <galak-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>,
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	<linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Cho KyongHo <pullip.cho-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>,
	Dave P Martin <Dave.Martin-5wv7dgnIgG8@public.gmane.org>,
	"linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org"
	<linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org>
Subject: Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings
Date: Fri, 27 Jun 2014 15:23:27 -0700	[thread overview]
Message-ID: <53ADEEDF.7060902@codeaurora.org> (raw)
In-Reply-To: <20140625091858.GG6153-5wv7dgnIgG8@public.gmane.org>

On 6/25/2014 2:18 AM, Will Deacon wrote:
> On Tue, Jun 24, 2014 at 10:35:54PM +0100, Olav Haugan wrote:
>> On 6/24/2014 11:11 AM, Will Deacon wrote:
>>> On Tue, Jun 24, 2014 at 06:57:44PM +0100, Olav Haugan wrote:
>>>> On 6/24/2014 2:18 AM, Will Deacon wrote:
>>>>> On Sat, Jun 21, 2014 at 12:16:25AM +0100, Olav Haugan wrote:
>>>>>> We have multiple-master SMMUs and each master emits a variable number of
>>>>>> StreamIDs. However, we have to apply a mask (the ARM SMMU spec allows
>>>>>> for this) to the StreamIDs due to limited number of StreamID 2 Context
>>>>>> Bank entries in the SMMU. If my understanding is correct we would
>>>>>> represent this in the DT like this:
>>>>>>
>>>>>> 	iommu {
>>>>>> 		#address-cells = <2>;
>>>>>> 		#size-cells = <0>;
>>>>>> 	};
>>>>>>
>>>>>> 	master@a {
>>>>>> 		...
>>>>>> 		iommus = <&iommu StreamID0 MASK0>,
>>>>>> 			 <&iommu StreamID1 MASK1>,
>>>>>> 			 <&iommu StreamID2 MASK2>;
>>>>>> 	};
>>>>>
>>>>> Stupid question, but why not simply describe the masked IDs? What use does
>>>>> the `raw' ID have to Linux?
>>>>
>>>> We do describe the masked StreamID (SID) but we need to specify the mask
>>>> that the SMMU should apply to the incoming SIDs, right?
>>>>
>>>> We have a bus master that emits 43 unique SIDs. However, we have only 40
>>>> SMMU_SMRn registers in the SMMU. So we need to mask out some of the
>>>> incoming SID bits so that the 43 SIDs can match one of 40 entries in the
>>>> SMR.
>>>
>>> Hmm, so you're talking about stream matching, right? That doesn't belong in
>>> the device-tree. I appreciate that the current driver does a terrible job at
>>> allocating the SMRs (it's bloody difficult!), but we should try to improve
>>> the dynamic behaviour instead of moving configuration of the SMMU out into
>>> device-tree, where it's inflexible at best.
>>
>> I am talking about SMMU_SMRn[MASK] register bits. This is not something
>> that can be dynamically detected at run-time. It is configuration at the
>> same level as the actual StreamIDs.
> 
> Why can't it be dynamically detected? Whilst the StreamIDs are fixed in
> hardware (from the SMMU architecture perspective), the SMRs are completely
> programmable. Why doesn't something like Andreas's proposal work for you?
> The idea there was to find the constant bits among the StreamIDs for a
> master and create the mask accordingly.
> 
Lets say I have an IOMMU with 2 masters and 2 SMRn slots with the
following stream IDs coming from the masters:

Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
Master 2: 0x30

To make this work I would program SMR[0] with StreamID 0x20 and mask 0xF
to ignore lower 4 bits. SMR[1] would just be StreamID 0x30 with mask 0x0.

However, I could also have an IOMMU with 2 masters and 9 SMRn slots with
the following stream IDs:

Master 1: 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28
Master 2: 0x29

Here I would program all SMRn and leave the mask to be 0 for all SMRn's.
So how do I detect when to apply a mask or not?

I am not familiar with Andreas's proposal. Do you have a link?

Thanks,

Olav Haugan

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