Re: [PATCH] serio: PS2 gpio bit banging driver for the serio bus

[Linus Walleij <linus.walleij@linaro.org>]

On Thu, Aug 17, 2017 at 11:09 AM, Russell King - ARM Linux
<linux@armlinux.org.uk> wrote:
> On Fri, Aug 11, 2017 at 11:16:20AM +0200, Linus Walleij wrote:
>> writel() should be guaranteeing that the values hit the hardware, wmb() is
>> spelled out "write memory barrier" I don't see what you're after here.
>
> Incorrect.  writel() has a barrier which ensures that data written to
> memory (eg, dma coherent memory) is visible to the hardware prior to
> the write hitting the hardware.
>
> There is no barrier to ensure that the write hits the hardware in a
> timely manner - the write can be buffered by the buses, which will
> delay it before it hits its destination.
>
> PCI particularly buffers MMIO writes, and the requirement there has
> always been that if you need the write to hit the hardware in a timely
> fashion, you must perform a read-back to force the bus to deliver the
> write (since a read is not allowed to overlap a write.)
>
> The solution is never to use barrier() - barrier() is a _compiler_
> barrier and does nothing for posted writes on hardware buses.

Thanks Russell. I tend to forget things over time even though I have
some memory of this being spelled out to me in the past :(

Yours,
Linus Walleij