On Wed, Mar 04, 2015 at 05:47:40PM +0000, Andre Przywara wrote:

Philip,

sorry for the late reply, that was stuck in my Drafts folder :-(

On 02/17/2015 04:16 PM, Dave P Martin wrote:

quoted

On Tue, Feb 17, 2015 at 10:55:35AM -0500, Philip Elcan wrote:

[...]

quoted

quoted

I'm a little late to address this patchset, but the SBSA defines all
the Generic UART registers 32-bit wide. However, the amba-pl011 driver
uses 16-bit accessors. How will you be handling that? Can the ARM PL011
hardware handle 32-bit access?

Interesting question.  The PL011 TRM [1] specifies only a 16-bit-wide
APB bus interface, but does not say that 32-bit accesses won't work.

I suspect that 32-bit accesses will work on all or most PL011s -- if
that looks too risky or we can't find enough test platforms to be sure
of this, then we could maybe abstract the register access size as a
quirk.

Andre may already have an answer on this.

I am not sure if the 32-bit register _width_ mentioned in the spec
really mandates 32-bit accesses, also the width may be just a spec bug.
Since the SBSA states that an ARM PL011r1p5 is a valid SBSA-UART
implementation, I wonder how this goes together. Also the highest
non-reserved bit in the registers is bit 15 in PL011 and bit 11 in the
SBSA subset.

I fear the actual bus connection is an implementation detail. Given the
fact that all existing PL011 hardware so far works with the 16bit
accesses, I don't dare to change this.

So I'd suggest to keep it as readw/writew for now and the revisit this
topic if some SBSA UART users complain. That makes it easier to justify
the rather invasive change to all MMIO accessors (which I already tried
on one machine for now without problems, btw).

+1

As I understand it, the PL011 bus interface does not take access size
into account at all, so providing "large-enough" accesses are used to
cover all the bits meaningful to the UART, things are likely to work
fine -- and this seems to be the case for all platforms currently
supported by the amba-pl011 driver.

Cheers
---Dave