Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys_addr()

[Laurentiu Tudor]

On 24.10.2019 14:04, Robin Murphy wrote:
> On 2019-10-24 8:49 am, Laurentiu Tudor wrote:
>>
>>
>> On 24.10.2019 05:01, hch@xxxxxx wrote:
>>> On Wed, Oct 23, 2019 at 11:53:41AM +0000, Laurentiu Tudor wrote:
>>>> 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:
>>>> ÂÂÂ * phys_addr_t dma_unmap_page_ret_phys(...)
>>>> ÂÂÂ * phys_addr_t dma_unmap_single_ret_phys(...)
>>>> ÂÂÂ * 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.
>>>
>>> I'm not sure what the ret is supposed to mean, but I generally like
>>> that idea better.
>>
>> It was supposed to be short for "return" but given that I'm not good at
>> naming stuff I'll just drop it.
> 
> Hmm, how about something like "dma_unmap_*_desc" for the context of the 
> mapped DMA address also being used as a descriptor token?

Alright.

>>> We also need to make sure there is an easy way
>>> to figure out if these APIs are available, as they generally aren't
>>> for any non-IOMMU API IOMMU drivers.
>>
>> I was really hoping to manage making them as generic as possible but
>> anyway, I'll start working on a PoC and see how it turns out. This will
>> probably happen sometime next next week as the following week I'll be
>> traveling to a conference.
> 
> AFAICS, even a full implementation of these APIs would have to be 
> capable of returning an indication that there is no valid physical 
> address - e.g. if unmap is called with a bogus DMA address that was 
> never mapped. At that point there'sseemingly no problem just 
> implementing the trivial case on top of any existing unmap/sync 
> callbacks for everyone. I'd imagine that drivers which want this aren't 
> likely to run on the older architectures where the weird IOMMUs live, so 
> they could probably just always treat failure as unexpected and fatal 
> either way.
> 
> In fact, I'm now wondering whether it's likely to be common that users 
> want the physical address specifically, or whether it would make sense 
> to return the original VA/page, both for symmetry with the corresponding 
> map calls and for the ease of being able to return NULL when necessary.

That's sounds wonderful as it should make the code leaner in the drivers.

---
Best Regards, Laurentiu