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Subject: Re: [PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask
To: Christoph Hellwig <hch@lst.de>, =?UTF-8?Q?C=c3=a9dric_Le_Goater?=
 <clg@kaod.org>, Michael Ellerman <mpe@ellerman.id.au>,
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From: Alexey Kardashevskiy <aik@ozlabs.ru>
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Date: Wed, 9 Sep 2020 19:36:04 +1000
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On 09/09/2020 17:58, Christoph Hellwig wrote:
> On Tue, Sep 08, 2020 at 11:10:03PM +1000, Alexey Kardashevskiy wrote:
>>>> a-ha, this makes more sense, thanks. Then I guess we need to revert that
>>>> one bit from yours f1565c24b596, do not we?
>>>
>>> Why?  The was the original intent of the API, but now we also use
>>> internally to check the addressing capabilities.
>>
>> The bigger mask the better, no? As it is now, it's limited by the window 
>> size which happens to be bigger than 4GB but smaller then full 64bit (48bit 
>> on my system)
> 
> Yes, the bigger mask is better.  But I don't see why you'd want to
> revert the dma bypass code for that entirely.
> 

I want dma_get_required_mask() to return the bigger mask always.

Now it depends on (in dma_alloc_direct()):
1. dev->dma_ops_bypass: set via pci_set_(coherent_)dma_mask();
2. dev->coherent_dma_mask - the same;
3. dev->bus_dma_limit - usually not set at all.

So until we set the mask, dma_get_required_mask() returns smaller mask.
So aacraid and likes (which calls dma_get_required_mask() before setting
it) will remain prone for breaks.


[forgot to cc: other folks last time, fixed now]

-- 
Alexey