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

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Date:   Thu, 26 Sep 2019 14:04:13 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Halil Pasic <pasic@...ux.ibm.com>, Christoph Hellwig <hch@....de>
Cc:     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 26/09/2019 13:37, Halil Pasic wrote:
> On Mon, 23 Sep 2019 17:21:17 +0200
> Christoph Hellwig <hch@....de> wrote:
> 
>> On Mon, Sep 23, 2019 at 02:34:16PM +0200, Halil Pasic wrote:
>>> Before commit 57bf5a8963f8 ("dma-mapping: clear harmful GFP_* flags in
>>> common code") tweaking the client code supplied GFP_* flags used to be
>>> an issue handled in the architecture specific code. The commit message
>>> suggests, that fixing the client code would actually be a better way
>>> of dealing with this.
>>>
>>> On s390 common I/O devices are generally capable of using the full 64
>>> bit address space for DMA I/O, but some chunks of the DMA memory need to
>>> be 31 bit addressable (in physical address space) because the
>>> instructions involved mandate it. Before switching to DMA API this used
>>> to be a non-issue, we used to allocate those chunks from ZONE_DMA.
>>> Currently our only option with the DMA API is to restrict the devices to
>>> (via dma_mask and dma_mask_coherent) to 31 bit, which is sub-optimal.
>>>
>>> Thus s390 we would benefit form having control over what flags are
>>> dropped.
>>
>> No way, sorry.  You need to express that using a dma mask instead of
>> overloading the GFP flags.
> 
> Thanks for your feedback and sorry for the delay. Can you help me figure
> out how can I express that using a dma mask?
> 
> IMHO what you ask from me is frankly impossible.
> 
> What I need is the ability to ask for  (considering the physical
> address) 31 bit addressable DMA memory if the chunk is supposed to host
> control-type data that needs to be 31 bit addressable because that is
> how the architecture is, without affecting the normal data-path. So
> normally 64 bit mask is fine but occasionally (control) we would need
> a 31 bit mask.

If it's possible to rework the "data" path to use streaming mappings 
instead of coherent allocations, you could potentially mimic what virtio 
does for a similar situation - see commit a0be1db4304f.

Robin.