Re: [PATCH 0/3] RFC: addition to DMA API

[Ming Lei <ming.lei@canonical.com>]

On Thu, Sep 1, 2011 at 4:45 PM, Will Deacon <will.deacon@arm.com> wrote:
> On Thu, Sep 01, 2011 at 04:41:46AM +0100, Ming Lei wrote:
>> Hi,
>>
>> On Thu, Sep 1, 2011 at 11:09 AM, Alan Stern <stern@rowland.harvard.edu> wrote:
>> >
>> > No, this is completely wrong.
>> >
>> > Firstly, you are forgetting about other architectures, ones in which
>> > writes to coherent memory aren't buffered.  On those architectures
>> > there's no way to prevent the DMA bus master from seeing an
>> > intermediate state of the data structures.  Therefore the driver has to
>> > be written so that even when this happens, everything will work
>> > correctly.
>> >
>> > Secondly, even when write flushes are used, you can't guarantee that
>> > the DMA bus master will see an atomic update.  It might turn out that
>> > the hardware occasionally flushes some writes very quickly, before the
>> > data-structure updates are complete.
>> >
>> > Thirdly, you are mixing up memory barriers with write flushes.  The
>> > barriers are used to make sure that writes are done in the correct
>> > order, whereas the flushes are used to make sure that writes are done
>> > reasonably quickly.  One has nothing to do with the other, even if by
>> > coincidence on ARM a memory barrier causes a write flush.  On other
>> > architectures this might not be true.
>>
>> I agree all about above, but what I described is from another view.
>> I post out the example before explaining my idea further:
>>
>>
>>       CPU                     device
>>       A=1;
>>       wmb
>>       B=2;
>>                                       read B
>>                                       read A
>>
>> one wmb is used to order 'A=1' and 'B=2', which will make the two write
>> operations reach to physical memory as the order: 'A=1' first, 'B=2' second.
>> Then the device can observe the two write events as the order above,
>> so if device has seen 'B==2', then device will surely see 'A==1'.
>>
>> Suppose writing to A is operation to update dma descriptor, the above example
>> can make device always see a atomic update of descriptor, can't it?
>>
>> My idea is that the memory access patterns are to be considered for
>> writer of device driver. For example, many memory access patterns on
>> EHCI hardware are described in detail.  Of course, device driver should
>> make full use of the background info, below is a example from ehci driver:
>>
>> qh_link_async():
>>
>>       /*prepare qh descriptor*/
>>       qh->qh_next = head->qh_next;
>>       qh->hw->hw_next = head->hw->hw_next;
>>       wmb ();
>>
>>       /*link the qh descriptor into hardware queue*/
>>       head->qh_next.qh = qh;
>>       head->hw->hw_next = dma;
>>
>> so once EHCI fetches a qh with the address of 'dma', it will always see
>> consistent content of qh descriptor, which could not be updated partially.
>
> I'm struggling to see what you're getting at here. The proposal has
> *absolutely nothing* to do with memory barriers. All of the existing
> barriers will remain - they are needed for correctness. What changes is the
> addition of an /optional/ flush operation in order to guarantee some sort of
> immediacy for writes to the coherent buffer.

First, most of barriers have made this kind of flush not necessary, as
explained int the example above. Even for ehci driver, the flush to be
added is only __one__ special case, so could you list other cases in
which the flush operation is a must and memory barrier can't do it?

If one can understand dma master access order on shared memory
in the context, it is not difficult to use memory barrier to avoid the
flush operation.

Second, as I said before, the flush operation is very easy to cause dma
descriptor updated in partial, as qtd descriptor updated in the ehci case.
It is one of the side effects of the flush to be introduced.  Fortunately, EHCI
can handle this correctly, but can other hardwares always handle this correctly?

>
>> >> 3, The new DMA API for the purpose to be introduced is much easier to
>> >> understand, and much easier to use than memory barrier, so it is very
>> >> possible to make device driver guys misuse or abuse it instead of using
>> >> memory barrier first to handle the case.
>> >
>> > That criticism could apply to almost any new feature.  We shouldn't be
>> > afraid to adopt something new merely because it's so easy to use that
>> > it might be misused.
>>
>> This point depends on the #1 and #2.
>
> Huh? I don't see the connection. If your worry is that people will start
> littering their code with flush calls, I don't think that's especially
> likely. The usual problem (from what I've seen) is that barriers tend to be
> missing rather than overused so I don't see why this would be different for
> a what has been proposed.

Barrier is often missed because it is very difficult to use and one have to
understand the dma master access order on shared memory by . The
flush to be introduced is very easy to understand, so it is very likely to be
abused, isn't it?


thanks,
--
Ming Lei