Re: [PATCH v6 16/36] nds32: DMA mapping API

[Greentime Hu]

Hi, Arnd:

2018-01-25 18:42 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
> On Thu, Jan 25, 2018 at 4:45 AM, Greentime Hu <green.hu@xxxxxxxxx> wrote:
>> 2018-01-24 19:36 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
>>> On Tue, Jan 23, 2018 at 12:52 PM, Greentime Hu <green.hu@xxxxxxxxx> wrote:
>>>> 2018-01-23 16:23 GMT+08:00 Greentime Hu <green.hu@xxxxxxxxx>:
>>>>> 2018-01-18 18:26 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
>>>>>> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@xxxxxxxxx> wrote:
>>>
>>> That looks reasonable enough, but it does depend on a number of factors,
>>> and the dma-mapping.h implementation is not just about cache flushes.
>>>
>>> As I don't know the microarchitecture, can you answer these questions:
>>>
>>> - are caches always write-back, or could they be write-through?
>> Yes, we can config it to write-back or write-through.
>
> Ok. If a WT-cache is common enough, you could optimize for that
> case by skipping the explicit writeback here and just doing a synchronizing
> instruction. Usually if the cache is configurable, one would pick the
> writeback option though, so it's probably not important.

Thank you for this suggestion.
We have optimized in cpu_dcache_wb_range() and it will be called from
cpu_dma_wb_range().
It will do nothing if it is a write-through config cache.

>>> - is the CPU physical address always the same as the address visible to the
>>>   device?
>> Yes, it is always the same unless the CPU uses local memory. The
>> physical address of local memory will overlap the original bus
>> address.
>> I think the local memory case can be ignored because we don't use it for now.
>
> Ok, makes sense.
>
>>> - are there devices that can only see a subset of the physical memory?
>> All devices are able to see the whole physical memory in our current
>> SoC, but I think other SoC may support such kind of HW behavior.
>
> This is one area that might need a more complex implementation then,
> depending on what devices are used in other SoCs. For network or
> storage devices, it's usually sufficient to configure a DMA mask
> from the "dma-ranges" property of the parent bus in the device tree,
> the kernel code will then use bounce buffers.
>
> For other types of drivers, using the streaming DMA interfaces
> can require using the swiotlb helper that performs the bounce
> buffering at in place of the cache operations. With a bit of luck,
> you won't ever need to worry about it, just mentioning it here in
> case you run into that problem later.
>
> The consistent_sync() implementaiton you showed earlier should be
> good enough then.  With that change,
>
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

Thank you. :)