linux-kernel - Re: [PATCH 4/6] MIPS: refactor the maybe coherent DMA indicators

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Date:   Wed, 10 Feb 2021 09:57:24 +0100
From:   Christoph Hellwig <hch@....de>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 4/6] MIPS: refactor the maybe coherent DMA indicators

On Tue, Feb 09, 2021 at 02:12:37PM +0100, Thomas Bogendoerfer wrote:
> > +#ifdef CONFIG_DMA_MAYBE_COHERENT
> > +extern bool dma_default_coherent;
> >  static inline bool dev_is_dma_coherent(struct device *dev)
> >  {
> > -	return coherentio == IO_COHERENCE_ENABLED ||
> > -		(coherentio == IO_COHERENCE_DEFAULT && hw_coherentio);
> > +	return dma_default_coherent;
> 
> this breaks overriding of coherentio via command line. plat_mem_setup/
> plat_setup_iocoherency is called before earlyparams are handled. So
> changing coherentio after that doesn't have any effect.

Hmm.  In that case a manual override does actually work for alchemy,
as that initializes coherentio from plat_mem_setup().  But the
elaborate sanity checking that malta performs in plat_setup_iocoherency
is rather pointless then, as coherentio will always be set to
IO_COHERENCE_DISABLED at this point.