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

[Arnd Bergmann]

On Tue, Jun 27, 2017 at 10:17 AM, Russell King - ARM Linux
<linux at armlinux.org.uk> wrote:
> On Tue, Jun 27, 2017 at 09:17:48AM +0200, Arnd Bergmann wrote:
>> 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.
>
> I think Roy is rather confused right now about what the driver does
> and doesn't do.
>
> With his patches, two areas are mapped on ARM:
>
> 1. The addr.ci area is mapped using ioremap().  This is a device mapping,
>    and is used by the "Cache-inhibited register access." functions.
>
> 2. The addr.ce area is mapped using ioremap_wc(), and all other stuff
>    including the verb accesses go through this region.  This is a
>    memory, non-cacheable mapping.
>
> The addr.ce region is the area that has a mixture of memset(), MMIO
> accessors, direct CPU accesses, cache flushes and prefetches used on
> it.
>
> As it is marked non-cacheable, the cache flushes are a pure waste of
> CPU cycles.  Prefetching to a non-cacheable area doesn't make much
> sense either.
>
> So, I think for ARM: just kill the cache flushing - its doing nothing
> useful.  The prefetching could also be removed as well to recover a
> few more CPU cycles.

Right, that sounds good. I wondered about the ioremap_wc() earlier
and thought I must have missed something as it didn't make any sense
to me, but apparently that was because the code really doesn't make
sense ;-).

It would be good to get consistent __iomem annotations on it too,
so we can see exactly where the MMIO interfaces are used incorrectly,
and find a fix for that.

I wonder if doing memcpy_toio on the write-combining mapping
will do what Roy wants and and up sending a single bus transaction
for one descriptor most of the time, while fitting in with the ioremap_wc()
interface and its __iomem pointers.

       Arnd