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

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Date:   Wed, 24 Jan 2018 12:36:17 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Greentime Hu <green.hu@...il.com>
Cc:     Greentime <greentime@...estech.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Vincent Chen <deanbo422@...il.com>,
        DTML <devicetree@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-serial@...r.kernel.org,
        Geert Uytterhoeven <geert.uytterhoeven@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Rutland <mark.rutland@....com>, Greg KH <greg@...ah.com>,
        Guo Ren <ren_guo@...ky.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        David Miller <davem@...emloft.net>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v6 16/36] nds32: DMA mapping API

On Tue, Jan 23, 2018 at 12:52 PM, Greentime Hu <green.hu@...il.com> wrote:
> Hi, Arnd:
>
> 2018-01-23 16:23 GMT+08:00 Greentime Hu <green.hu@...il.com>:
>> Hi, Arnd:
>>
>> 2018-01-18 18:26 GMT+08:00 Arnd Bergmann <arnd@...db.de>:
>>> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@...il.com> wrote:
>>>> From: Greentime Hu <greentime@...estech.com>
>>>>
>>>> This patch adds support for the DMA mapping API. It uses dma_map_ops for
>>>> flexibility.
>>>>
>>>> Signed-off-by: Vincent Chen <vincentc@...estech.com>
>>>> Signed-off-by: Greentime Hu <greentime@...estech.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