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

From: Christoph Hellwig <[email-masked]>
Date: Wed, 30 Sep 2020 18:52:12 +0200

On Wed, Sep 30, 2020 at 06:45:02PM +0200, Paul Cercueil wrote:
>> We don't have such a thing in the Linux API at all.
>
> dma_pgprot(dev, vma->vm_page_prot, DMA_ATTR_NON_CONSISTENT);
>
> That was giving me non-coherent cached memory, and now I don't have an 
> alternative.

Looking at Linux 5.9-rc dma_pgprot is defined as:

pgprot_t dma_pgprot(struct device *dev, pgprot_t prot, unsigned long attrs)
{
	if (force_dma_unencrypted(dev))
		prot = pgprot_decrypted(prot);
	if (dev_is_dma_coherent(dev) ||
	    (IS_ENABLED(CONFIG_DMA_NONCOHERENT_CACHE_SYNC) &&
             (attrs & DMA_ATTR_NON_CONSISTENT)))
		return prot;
#ifdef CONFIG_ARCH_HAS_DMA_WRITE_COMBINE
	if (attrs & DMA_ATTR_WRITE_COMBINE)
		return pgprot_writecombine(prot);
#endif
	return pgprot_dmacoherent(prot);
}

so it doesn't change vma->vm_page_prot at all.

The only place that uses _CACHE_CACHABLE_NONCOHERENT is the MIPS specific
kmap_noncoherent which ha sa single caller that doesn't leak anywhere
into driver code.