Re: CONFIG_ARM_DMA_MEM_BUFFERABLE and readl/writel weirdness

Saravana Kannan
2011-03-09T04:58:23Z

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?

Thanks,
Saravana