Re: [PoC] arm: dma-mapping: direct: Apply dma_pfn_offset only when it is valid

[Peter Ujfalusi]

Hi Christoph,

On 30/01/2020 18.40, Christoph Hellwig wrote:
> On Thu, Jan 30, 2020 at 03:04:37PM +0200, Peter Ujfalusi via iommu wrote:
>> On 30/01/2020 9.53, Christoph Hellwig wrote:
>>> [skipping the DT bits, as I'm everything but an expert on that..]
>>>
>>> On Mon, Jan 27, 2020 at 04:00:30PM +0200, Peter Ujfalusi wrote:
>>>> I agree on the phys_to_dma(). It should fail for addresses which does
>>>> not fall into any of the ranges.
>>>> It is just a that we in Linux don't have the concept atm for ranges, we
>>>> have only _one_ range which applies to every memory address.
>>>
>>> what does atm here mean?
>>
>> struct device have only single dma_pfn_offset, one can not have multiple
>> ranges defined. If we have then only the first is taken and the physical
>> address and dma address is discarded, only the dma_pfn_offset is stored
>> and used.
>>
>>> We have needed multi-range support for quite a while, as common broadcom
>>> SOCs do need it.  So patches for that are welcome at least from the
>>> DMA layer perspective (kinda similar to your pseudo code earlier)
>>
>> But do they have dma_pfn_offset != 0?
> 
> Well, with that I mean multiple ranges with different offsets.  Take
> a look at arch/mips/bmips/dma.c:__phys_to_dma() and friends.  This
> is an existing implementation for mips, but there are arm and arm64
> SOCs using the same logic on the market as well, and we'll want to
> support them eventually.

I see. My PoC patch was not too off then ;)
So the plan is to have a generic implementation for all of the
architecture, right?

>> The dma_pfn_offset is _still_ applied to the mask we are trying to set
>> (and validate) via dma-direct.
> 
> And for the general case that is exactly the right thing to do, we
> just need to deal with really odd ZONE_DMA placements like yours.

I'm still not convinced, the point of the DMA mask, at least how I see
it, to check that the dma address can be handled by the device (DMA,
peripheral with built in DMA, etc), it is not against physical address.
Doing phys_to_dma() on the mask from the dma_set_mask() is just wrong.

>>> We'll need to find the minimum change to make it work
>>> for now without switching ops, even if it isn't the correct one, and
>>> then work from there.
>>
>> Sure, but can we fix the regression by reverting to arm_ops for now only
>> if dma_pfn_offset is not 0? It used to work fine in the past at least it
>> appeared to work on K2 platforms.
> 
> But that will cause yet another regression in what we have just fixed
> with using the generic direct ops, at which points it turns into who
> screams louder.

Hehe, I see.
I genuinely curious why k2 platform worked just fine with LPAE (it needs
it), but guys had issues with LPAE on dra7/am5.
The fix for dra7/am5 broke k2.
As far as I can see the main (only) difference is that k2 have
dma_pfn_offset = 0x780000, while dra7/am5 have it 0 (really direct mapping).

> For now I'm tempted to throw something like this in, which is a bit
> of a hack, but actually 100% matches what various architectures have
> historically done:
> 
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 6af7ae83c4ad..6ba9ee6e20bd 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -482,6 +482,9 @@ int dma_direct_supported(struct device *dev, u64 mask)
>  {
>  	u64 min_mask;
>  
> +	if (mask >= DMA_BIT_MASK(32))
> +		return 1;
> +

Right, so skipping phys_to_dma() for the mask and believing that it will
work..

It does: audio and dmatest memcpy tests are just fine with this, MMC
also probed with ADMA enabled.

As far as I can tell it works as well as falling back to the old arm ops
in case of LPAE && dma_pfn_offset != 0

Fwiw:
Tested-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>

Would you be comfortable to send apply this patch to mainline with
Fixes: ad3c7b18c5b3 ("arm: use swiotlb for bounce buffering on LPAE
configs")

So it gets picked for stable kernels as well?

>  	if (IS_ENABLED(CONFIG_ZONE_DMA))
>  		min_mask = DMA_BIT_MASK(zone_dma_bits);
>  	else
> 

Thank you,
- PÃter

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