linux-kernel - Re: [PATCH] ARM: dts: dra7: Add bus_dma

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Date:   Tue, 10 Mar 2020 16:16:14 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Tony Lindgren <tony@...mide.com>, Tero Kristo <t-kristo@...com>
Cc:     Roger Quadros <rogerq@...com>, hch@....de, robh+dt@...nel.org,
        nm@...com, nsekhar@...com, linux-omap@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: dts: dra7: Add bus_dma_limit for L3 bus

On 10/03/2020 3:48 pm, Tony Lindgren wrote:
> * Tero Kristo <t-kristo@...com> [200310 14:46]:
>> On 10/03/2020 13:53, Roger Quadros wrote:
>>> The L3 interconnect can access only 32-bits of address.
>>> Add the dma-ranges property to reflect this limit.
>>>
>>> This will ensure that no device under L3 is
>>> given > 32-bit address for DMA.
>>>
>>> Issue was observed only with SATA on DRA7-EVM with 4GB RAM
>>> and CONFIG_ARM_LPAE enabled. This is because the controller
>>> can perform 64-bit DMA and was setting the dma_mask to 64-bit.
>>>
>>> Setting the correct bus_dma_limit fixes the issue.
>>
>> This seems kind of messy to modify almost every DT node because of this....
>> Are you sure this is the only way to get it done? No way to modify the sata
>> node only which is impacted somehow?
>>
>> Also, what if you just pass 0xffffffff to the dma-ranges property? That
>> would avoid modifying every node I guess.
> 
> Also, I think these interconnects are not limited to 32-bit access.
> So yeah I too would prefer a top level dma-ranges property assuming
> that works.
> 
> I guess there dma-ranges should not be 0xffffffff though if
> limited to 2GB :)

It should work fine to just describe the Q3 and Q4 DDR regions as the 
DMA range, i.e.:

	ocp {
		...
		dma-ranges = <0x80000000 0 0x80000000 0x80000000>;
		...
	};

That would certainly be far less invasive :)

Robin.