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

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Jan 23, 2018 at 12:52 PM, Greentime Hu <green.hu@gmail.com> wrote:
> Hi, Arnd:
>
> 2018-01-23 16:23 GMT+08:00 Greentime Hu <green.hu@gmail.com>:
>> Hi, Arnd:
>>
>> 2018-01-18 18:26 GMT+08:00 Arnd Bergmann <arnd@arndb.de>:
>>> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@gmail.com> wrote:
>>>> From: Greentime Hu <greentime@andestech.com>
>>>>
>>>> This patch adds support for the DMA mapping API. It uses dma_map_ops for
>>>> flexibility.
>>>>
>>>> Signed-off-by: Vincent Chen <vincentc@andestech.com>
>>>> Signed-off-by: Greentime Hu <greentime@andestech.com>
>>>
>>> I'm still unhappy about the way the cache flushes are done here as discussed
>>> before. It's not a show-stopped, but no Ack from me.
>>
>> How about this implementation?

> I am not sure if I understand it correctly.
> I list all the combinations.
>
> RAM to DEVICE
>     before DMA => writeback cache
>     after DMA => nop
>
> DEVICE to RAM
>     before DMA => nop
>     after DMA => invalidate cache
>
> static void consistent_sync(void *vaddr, size_t size, int direction, int master)
> {
>         unsigned long start = (unsigned long)vaddr;
>         unsigned long end = start + size;
>
>         if (master == FOR_CPU) {
>                 switch (direction) {
>                 case DMA_TO_DEVICE:
>                         break;
>                 case DMA_FROM_DEVICE:
>                 case DMA_BIDIRECTIONAL:
>                         cpu_dma_inval_range(start, end);
>                         break;
>                 default:
>                         BUG();
>                 }
>         } else {
>                 /* FOR_DEVICE */
>                 switch (direction) {
>                 case DMA_FROM_DEVICE:
>                         break;
>                 case DMA_TO_DEVICE:
>                 case DMA_BIDIRECTIONAL:
>                         cpu_dma_wb_range(start, end);
>                         break;
>                 default:
>                         BUG();
>                 }
>         }
> }

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?
- can the cache be shared with another CPU or a device?
- if the cache is shared, is it always coherent, never coherent, or
either of them?
- could the same memory be visible at different physical addresses
  and have conflicting caches?
- is the CPU physical address always the same as the address visible to the
  device?
- are there devices that can only see a subset of the physical memory?
- can there be an IOMMU?
- are there write-buffers in the CPU that might need to get flushed before
  flushing the cache?
- could cache lines be loaded speculatively or with read-ahead while
  a buffer is owned by a device?

        Arnd