linux-kernel - Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys

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Date:   Wed, 23 Oct 2019 11:53:41 +0000
From:   Laurentiu Tudor <laurentiu.tudor@....com>
To:     Robin Murphy <robin.murphy@....com>, "hch@....de" <hch@....de>,
        "joro@...tes.org" <joro@...tes.org>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ioana Ciornei <ioana.ciornei@....com>
CC:     Leo Li <leoyang.li@....com>,
        Diana Madalina Craciun <diana.craciun@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Madalin-cristian Bucur <madalin.bucur@....com>
Subject: Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api
 dma_addr_to_phys_addr()

Hi Robin,

On 22.10.2019 16:25, Robin Murphy wrote:
> On 22/10/2019 13:55, Laurentiu Tudor wrote:
>> From: Laurentiu Tudor <laurentiu.tudor@....com>
>>
>> Introduce a new dma map op called dma_addr_to_phys_addr() that converts
>> a dma address to the physical address backing it up and add wrapper for
>> it.
> 
> I'd really love it if there was a name which could encapsulate that this 
> is *only* for extreme special cases of constrained descriptors/pagetable 
> entries/etc. where there's simply no practical way to keep track of a 
> CPU address alongside the DMA address, and the only option is this 
> potentially-arbitrarily-complex operation (I mean, on some systems it 
> may end up taking locks and poking hardware).
> 
> Either way it's tricky - much as I don't like adding an interface which 
> is ripe for drivers to misuse, I also really don't want hacks like 
> bdf95923086f shoved into other APIs to compensate, so on balance I'd 
> probably consider this proposal ever so slightly the lesser evil.

We had an internal discussion over these points you are raising and 
Madalin (cc-ed) came up with another idea: instead of adding this prone 
to misuse api how about experimenting with a new dma unmap and dma sync 
variants that would return the physical address by calling the newly 
introduced dma map op. Something along these lines:
  * phys_addr_t dma_unmap_page_ret_phys(...)
  * phys_addr_t dma_unmap_single_ret_phys(...)
  * phys_addr_t dma_sync_single_for_cpu_ret_phys(...)
I'm thinking that this proposal should reduce the risks opened by the 
initial variant.
Please let me know what you think.

---
Thanks & Best Regards, Laurentiu