linux-kernel - Re: [RFC PATCH 1/3] dma-mapping: make overriding GFP

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Date:   Fri, 27 Sep 2019 23:21:04 +0200
From:   Christoph Hellwig <hch@....de>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>, linux-s390@...r.kernel.org,
        Janosch Frank <frankja@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        linux-kernel@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        iommu@...ts.linux-foundation.org,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Subject: Re: [RFC PATCH 1/3] dma-mapping: make overriding GFP_* flags arch
 customizable

On Fri, Sep 27, 2019 at 02:33:14AM +0200, Halil Pasic wrote:
> Thank you for your feedback. Just to be sure we are on the same pager, I
> read commit a0be1db4304f like this:
> 1) virtio_pci_legacy needs to allocate the virtqueues so that the base
> address fits 44 bits
> 2) if 64 bit dma is possible they set coherent_dma_mask to
>   DMA_BIT_MASK(44) and dma_mask to DMA_BIT_MASK(64)
> 3) since the queues get allocated with coherent allocations 1) is
> satisfied
> 4) when the streaming mappings see a buffer that is beyond
>   DMA_BIT_MASK(44) then it has to treat it as not coherent memory
>   and do the syncing magic (which isn't actually required, just
>   a side effect of the workaround.

1-3 is correct, 4 is not.  The coherent mask is a little misnamed and
doesn't have to anything with coherency.  It is the mask for DMA
allocations, while the dma mask is for streaming mappings.

> I've already implemented a patch (see after the scissors line) that
> takes a similar route as commit a0be1db4304f, but I consider that a
> workaround at best. But if that is what the community wants... I have to
> get the job done one way or the other.

That patch (minus the comments about being a workaround) is what you
should have done from the beginning.