Re: [PATCHv2 03/16] Documentation: dt: add OMAP iommu bindings

[Suman Anna <s-anna-l0cyMroinI0@public.gmane.org>]

Hi Laurent,

On 02/25/2014 03:26 PM, Laurent Pinchart wrote:
> Hi Suman,
>
> Thank you for the patch.
>
> On Thursday 13 February 2014 12:15:34 Suman Anna wrote:
>> From: Florian Vaussard <florian.vaussard-p8DiymsW2f8@public.gmane.org>
>>
>> This patch adds the iommu bindings for all OMAP2+ SoCs. Apart from
>> the standard bindings used by OMAP peripherals, this patch uses a
>> 'dma-window' (already used by Tegra SMMU) and adds two OMAP custom
>> bindings - 'ti,#tlb-entries' and 'ti,iommu-bus-err-back'.
>>
>> Signed-off-by: Florian Vaussard <florian.vaussard-p8DiymsW2f8@public.gmane.org>
>> [s-anna-l0cyMroinI0@public.gmane.org: split bindings document, add dra7 and bus error back]
>> Signed-off-by: Suman Anna <s-anna-l0cyMroinI0@public.gmane.org>
>> ---
>>   .../devicetree/bindings/iommu/ti,omap-iommu.txt    | 28 +++++++++++++++++++
>>   1 file changed, 28 insertions(+)
>>   create mode 100644
>>   Documentation/devicetree/bindings/iommu/ti,omap-iommu.txt
>>
>> diff --git a/Documentation/devicetree/bindings/iommu/ti,omap-iommu.txt
>> b/Documentation/devicetree/bindings/iommu/ti,omap-iommu.txt new file mode
>> 100644
>> index 0000000..116492d
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/iommu/ti,omap-iommu.txt
>> @@ -0,0 +1,28 @@
>> +OMAP2+ IOMMU
>> +
>> +Required properties:
>> +- compatible : Should be one of,
>> +		"ti,omap2-iommu" for OMAP2/OMAP3 IOMMU instances
>> +		"ti,omap4-iommu" for OMAP4/OMAP5 IOMMU instances
>> +		"ti,dra7-iommu" for DRA7xx IOMMU instances
>> +- ti,hwmods  : Name of the hwmod associated with the IOMMU instance
>> +- reg        : Address space for the configuration registers
>> +- interrupts : Interrupt specifier for the IOMMU instance
>> +- dma-window : IOVA start address and length
>
> Isn't the dma window more of a system configuration property than a hardware
> property ? How do you expect it to be set?

We are setting it based on the addressable range for the MMU. We are 
reusing the existing defined property and it allows us to get rid of the 
IOVA start and end addresses defined in the pre-DT OMAP iommu platform data.

>
>> +Optional properties:
>> +- ti,#tlb-entries : Number of entries in the translation look-aside buffer.
>> +                    Should be either 8 or 32 (default: 32)
>> +- ti,iommu-bus-err-back : Indicates the IOMMU instance supports throwing
>> +		          back a bus error response on MMU faults.
>
> Do these features vary per IOMMU instance or per IOMMU model ? In the latter
> case they could be inferred from the compatible string by the driver without
> requiring them to be explicit in DT (whether you want to do so is left to you
> though).

Well, these are fixed features given an IOMMU instance, like the OMAP3 
ISP is the only one that has 8 TLB entries, all the remaining ones have 
32, and the IPU iommu instances are the only ones that support the bus 
error response back. I have no preference to any particular way, and 
sure the driver can infer these easily based on unique compatible 
strings per subsystem per SoC. I just happened to go with defining 
compatible strings per SoC, with the optional properties differentiating 
the fixed behavior between different IOMMU instances on that SoC. This 
is where I was looking for some inputs/guidance from the DT bindings 
maintainers on what is the preferred method.

regards
Suman

>
>> +Example:
>> +	/* OMAP3 ISP MMU */
>> +	mmu_isp: mmu@480bd400 {
>> +		compatible = "ti,omap2-iommu";
>> +		reg = <0x480bd400 0x80>;
>> +		interrupts = <24>;
>> +		ti,hwmods = "mmu_isp";
>> +		ti,#tlb-entries = <8>;
>> +		dma-window = <0 0xfffff000>;
>> +	};
>