Re: [PATCH 2/5] cxl/hdm: Use 4-byte reads to retrieve HDM decoder base+limit

[Dan Williams <dan.j.williams@intel.com>]

Alison Schofield wrote:
> On Fri, Apr 14, 2023 at 11:54:00AM -0700, Dan Williams wrote:
> > The CXL specification mandates that 4-byte registers must be accessed
> > with 4-byte access cycles. CXL 3.0 8.2.3 "Component Register Layout and
> > Definition" states that the behavior is undefined if (2) 32-bit
> > registers are accessed as an 8-byte quantity. It turns out that at least
> > one hardware implementation is sensitive to this in practice. The @size
> > variable results in zero with:
> > 
> >     size = readq(hdm + CXL_HDM_DECODER0_SIZE_LOW_OFFSET(which));
> > 
> > ...and the correct size with:
> > 
> >     lo = readl(hdm + CXL_HDM_DECODER0_SIZE_LOW_OFFSET(which));
> >     hi = readl(hdm + CXL_HDM_DECODER0_SIZE_HIGH_OFFSET(which));
> >     size = (hi << 32) + lo;
> > 
> > Fixes: d17d0540a0db ("cxl/core/hdm: Add CXL standard decoder enumeration to the core")
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> 
> I see you got rid of ioread64_hi_lo(), so this can't be
> happening anywhere else. Are all the other readl, writel
> usages known to be OK, or do you need review help against
> the spec?

Good question. That's what I looked to answer in patch3. As far as I can
see all the other readq() usage in the driver is for registers defined
as 64-bit, so that patch ended up only being a deletion of unneeded
includes.