Re: [PATCH 3/5] cxl/acpi: Minimize granularity for x1 interleaves

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

Jonathan Cameron wrote:
> On Tue, 2 Aug 2022 10:33:19 -0700
> Dan Williams <dan.j.williams@intel.com> wrote:
> 
> > Jonathan Cameron wrote:
> > > On Fri, 22 Jul 2022 17:56:09 -0700
> > > Dan Williams <dan.j.williams@intel.com> wrote:
> > >   
> > > > The kernel enforces that region granularity is >= to the top-level
> > > > interleave-granularity for the given CXL window. However, when the CXL
> > > > window interleave is x1, i.e. non-interleaved at the host bridge level,
> > > > then the specified granularity does not matter. Override the window
> > > > specified granularity to the CXL minimum so that any valid region
> > > > granularity is >= to the root granularity.
> > > > 
> > > > Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>  
> > > 
> > > Hi Dan,
> > > 
> > > Debugging exactly why this is failing (from cxl.git/preview) for my test setup...
> > > (1 hb, 8 rp, 8 direct connected devices)
> > > 
> > > If I set the interleave granularity of a region to 256, I end
> > > up with 256 for the CFMWS which is fine, then 512 for the HB which
> > > is not - EP interleave granularity is expected 256.
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/cxl/cxl.git/tree/drivers/cxl/core/region.c?h=preview#n1070
> > > 
> > > Calculates the eig as address_bit - eiw + 1
> > > 
> > > iw = 8
> > > eiw = 3
> > > peig = 0 (pig = 256)
> > > peiw = 0 (piw = 1)
> > > (all as expected I think...)
> > > 
> > > So address_bit = s max(peig + peiw, eiw + peig) = max(0, 3)
> > > and eig = 3 - 3 + 1 = 1 (ig = 512) which is wrong.
> > > 
> > > I'm not 100% sure on the logic behind this maths, but would expect eig = 0 as the output for this
> > > setup..  
> > 
> > Yeah, the "+ 1" is not required when routing from the x1 HB level. I can
> > reproduce this config with cxl_test to validate.
> 
> Other than this off by one, with the other fixes you posted everything now works for me
> with the particular test case above.

Oh, you are hitting this path in your test? I misread the condition when
this triggers. It should be skipping the granularity increment when the
port's parent interleave is x1. I.e. in this case with devices directly
connected to a host-bridge where the host-bridge is not interleaved then
there is no need for an extra address bit to route the decode.