[GIT PULL v3] updates to qbman (soc drivers) to support arm/arm64

[Arnd Bergmann]

On Fri, Jun 23, 2017 at 8:58 PM, Roy Pledge <roy.pledge at nxp.com> wrote:
> On 6/23/2017 11:23 AM, Mark Rutland wrote:
>> On Fri, Jun 23, 2017 at 04:56:10PM +0200, Arnd Bergmann wrote:
>>> On Tue, Jun 20, 2017 at 7:27 PM, Leo Li <leoyang.li at nxp.com> wrote:
>>>
>>> 2. I know we have discussed the unusual way this driver accesses MMIO
>>> registers in the past, using ioremap_wc() to map them and the manually
>>> flushing the caches to store the cache contents into the MMIO registers.
>>> What I don't know is whether there was any conclusion on this topic whether
>>> this is actually allowed by the architecture or at least the chip, based on
>>> implementation-specific features that make it work even when the architecture
>>> doesn't guarantee it.
>> From prior discussions, my understanding was that the region in question
>> was memory reserved for the device, rather than MMIO registers.

Ok.

>> The prior discussion on that front were largely to do with teh
>> shareability of that memory, which is an orthogonal concern.
>>
>> If these are actually MMIO registers, a Device memory type must be used,
>> rather than a Normal memory type. There are a number of things that
>> could go wrong due to relaxations permitted for Normal memory, such as
>> speculative reads, the potential set of access sizes, memory
>> transactions that the endpoint might not understand, etc.
> The memory for this device (what we refer to as Software Portals) has 2
> regions. One region is MMIO registers and we access it using
> readl()/writel() APIs.

Ok, good.

> The second region is what we refer to as the cacheable area.  This is
> memory implemented as part of the QBMan device and the device accepts
> cacheline sized transactions from the interconnect. This is needed
> because the descriptors read and written by SW are fairly large (larger
> that 64 bits/less than a cacheline) and in order to meet the data rates
> of our high speed ethernet ports and other accelerators we need the CPU
> to be able to form the descriptor in a CPU cache and flush it safely
> when the device is read to consume it.  Myself and the system architect
> have had many discussions with our design counterparts in ARM to ensure
> that our interaction with the core/interconnect/device are safe for the
> set of CPU cores and interconnects we integrate into our products.
>
> I understand there are concerns regarding our shareablity proposal
> (which is not enabled in this patch set). We have been collecting some
> information and talking to ARM and I do intend to address these concerns
> but I was delaying confusing things more until this basic support gets
> accepted and merged.

Can you summarize what the constraints are that we have for mapping
this area? E.g. why can't we just have an uncached write-combining
mapping and skip the flushes?

>>> Can I have an Ack from the architecture maintainers (Russell, Catalin,
>>> Will) on the use of these architecture specific interfaces?
>>>
>>> static inline void dpaa_flush(void *p)
>>> {
>>> #ifdef CONFIG_PPC
>>>         flush_dcache_range((unsigned long)p, (unsigned long)p+64);
>>> #elif defined(CONFIG_ARM32)
>>>         __cpuc_flush_dcache_area(p, 64);
>>> #elif defined(CONFIG_ARM64)
>>>         __flush_dcache_area(p, 64);
>>> #endif
>>> }
>> Assuming this is memory, why can't the driver use the DMA APIs to handle
>> this without reaching into arch-internal APIs?
>
> I agree this isn't pretty - I think we could use
> dma_sync_single_for_device() here but I am concerned it will be
> expensive and hurt performance significantly. The DMA APIs have a lot of
> branches. At some point we were doing 'dc cvac' here and even switching
> to the above calls caused a measurable drop in throughput at high frame
> rates.

I'd suggest we start out by converting this to some standard API
first, regardless of performance, to get it working properly with code
that should be maintainable at least, and make progress with your
hardware enablement.

In parallel, we can discuss what kind of API we actually need and
how to fit that into the existing frameworks. This may take a while
as it depends on your other patch set, and perhaps input from
other parties that may have similar requirements. I could imagine
that e.g. Marvell, Cavium or Annapurna (Amazon) do something
similar in their hardware, so if we find that we need a new API,
we should define it in a way that works for everyone.

        Arnd