linux-kernel - Re: linux-next: build failure after merge of the drm tree

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Message-Id: <IXCHHQ.XHZEBJ42HOEJ1@crapouillou.net>
Date:   Wed, 30 Sep 2020 18:39:18 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Christoph Hellwig <hch@....de>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Dave Airlie <airlied@...ux.ie>,
        DRI <dri-devel@...ts.freedesktop.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the drm tree



Le mer. 30 sept. 2020 à 18:11, Christoph Hellwig <hch@....de> a écrit 
:
> On Wed, Sep 30, 2020 at 03:33:13PM +0200, Paul Cercueil wrote:
>>  One thing missing for remap_pfn_range(), I have no alternative for 
>> this:
>> 
>>  vma->vm_page_prot = dma_pgprot(dev, vma->vm_page_prot,
>>  DMA_ATTR_NON_CONSISTENT);
>> 
>>  So I have to do:
>> 
>>  vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>>  pgprot_val(vma->vm_page_prot) &= ~_CACHE_MASK;
>>  pgprot_val(vma->vm_page_prot) |= _CACHE_CACHABLE_NONCOHERENT;
>> 
>>  And that will only compile on MIPS, because these _CACHE_* macros 
>> are only
>>  defined there.
>> 
>>  I would need something like a pgprot_noncoherent(), I think.
> 
> dma_alloc_pages gives you cached memory, so you can't just use an
> uncached protection for the userspace mmap here.  If you want uncached
> memory you need to use dma_alloc_coherent paired with 
> dma_mmap_coherent.
> Or dma_alloc_wc for a slightly different flavor of uncached. (both
> of the map to dma_alloc_attrs / dma_mmap_attrs eventually).

I don't want uncached memory, I want non-coherent cached memory.

-Paul