Hi, Arnd:
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:
>> Hi, Arnd:
>>
>> 2018-01-23 16:23 GMT+08:00 Greentime Hu <green.hu@xxxxxxxxx>:
>>> Hi, Arnd:
>>>
>>> 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:
>>>>> From: Greentime Hu <greentime@xxxxxxxxxxxxx>
>>>>>
>>>>> This patch adds support for the DMA mapping API. It uses dma_map_ops for
>>>>> flexibility.
>>>>>
>>>>> Signed-off-by: Vincent Chen <vincentc@xxxxxxxxxxxxx>
>>>>> Signed-off-by: Greentime Hu <greentime@xxxxxxxxxxxxx>
>>>>
>>>> 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?
Yes, we can config it to write-back or write-through.
> - can the cache be shared with another CPU or a device?
No, we don't support it.
> - if the cache is shared, is it always coherent, never coherent, or
> either of them?
We don't support SMP and the device will access memory through bus. I
think the cache is not shared.
> - could the same memory be visible at different physical addresses
> and have conflicting caches?
We currently don't have such kind of SoC memory map.
> - 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.
> - 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.
> - can there be an IOMMU?
No.
> - are there write-buffers in the CPU that might need to get flushed before
> flushing the cache?
Yes, there are write-buffers in front of CPU caches but it should be
transparent to SW. We don't need to flush it.
> - could cache lines be loaded speculatively or with read-ahead while
> a buffer is owned by a device?
No.
