Re: CONFIG_ARM_DMA_MEM_BUFFERABLE and readl/writel weirdness

Saravana Kannan
2011-03-09T09:32:39Z

On Wed, March 9, 2011 12:05 am, Russell King - ARM Linux wrote:
> On Tue, Mar 08, 2011 at 08:58:20PM -0800, Saravana Kannan wrote:
>> On 03/03/2011 02:24 AM, Russell King - ARM Linux wrote:
>>> On Wed, Mar 02, 2011 at 11:49:47PM -0800, Saravana Kannan wrote:
>>>>> I think you misunderstand what's going on. IO accesses are always
>>>>> ordered
>>>>> with respect to themselves. The barriers are there to ensure
>>>>> ordering
>>>>> between DMA coherent memory (normal non-cached memory) and IO
>>>>> accesses
>>>>> (device).
>>>>
>>>> Unfortunately this is not correct. The ARM spec doesn't guarantee that
>>>> all IO accesses should be ordered with respect to themselves. It only
>>>> requires that the ordering should be guaranteed at least within a 1KB
>>>> region.
>>>>
>>>> You can find this info in ARMv7 ARM spec[1] named
>>>> "DDI0406B_arm_architecture_reference_manual_errata_markup_8_0.pdf", on
>>>> page A3-45. There is a para that goes:
>>>>
>>>> "Accesses must arrive at any particular memory-mapped peripheral or
>>>> block of memory in program order, that is, A1 must arrive before A2.
>>>> There are no ordering restrictions about when accesses arrive at
>>>> different peripherals or blocks of memory, provided that the accesses
>>>> follow the general ordering rules given in this section."
>>>
>>> That is news to me. My DDI0406B does not have this paragraph, so it's
>>> something that ARM has sprung upon us without telling *anyone* about
>>> it.
>>> It's not unreasonable or even unexpected. That is exactly the same
>>> condition which applies on buses like PCI due to write posting on
>>> bridges
>>> downstream of the CPU, and issuing memory barriers will not help with
>>> that.
>>
>> While the PCI stuff is true, as you say, it's not related to mb()s. The
>> mb()s matter to the point of getting the writes to the intended
>> "devices addresses" in the program order. What happens after that is a
>> separate issue.
>>
>> So, going back to the discussion of mb()s and readl/writel (and
>> variations), I think we should no longer state the all IO accesses are
>> ordered wrt each other. Are we in agreement on this?
>
> No, because you clearly haven't understood the point I made.
>
> I still believe you are wrong on this point.

I'm not going to pretend to be a PCI expert, but I think the ARMv7 ARM
text I quoted makes it pretty clear that all IO accesses are not ordered
wrt each other. So, why do you still disagree on that point?

-Saravana