cxl/region: Disallow region granularity != window granularity

The endpoint decode granularity must be <= the window granularity
otherwise capacity in the endpoints is lost in the decode. Consider an
attempt to have a region granularity of 512 with 4 devices within a
window that maps 2 host bridges at a granularity of 256 bytes:

HPA	DPA Offset	HB	Port	EP
0x0	0x0		0	0	0
0x100	0x0		1	0	2
0x200	0x100		0	0	0
0x300	0x100		1	0	2
0x400	0x200		0	1	1
0x500	0x200		1	1	3
0x600	0x300		0	1	1
0x700	0x300		1	1	3
0x800	0x400		0	0	0
0x900	0x400		1	0	2
0xA00	0x500		0	0	0
0xB00	0x500		1	0	2

Notice how endpoint0 maps HPA 0x0 and 0x200 correctly, but then at HPA
0x800 it results in DPA 0x200-0x400 on being skipped.

Fix this by restricing the region granularity to be equal to the window
granularity resulting in the following for a x4 region under a x2 window
at a granularity of 256.

HPA	DPA Offset	HB	Port	EP
0x0	0x0		0	0	0
0x100	0x0		1	0	2
0x200	0x0		0	1	1
0x300	0x0		1	1	3
0x400	0x100		0	0	0
0x500	0x100		1	0	2
0x600	0x100		0	1	1
0x700	0x100		1	1	3

Not that it ever made practical sense to support region granularity >
window granularity. The window rotates host bridges causing endpoints to
never see a consecutive stream of requests at the desired granularity
without breaks to issue cycles to the other host bridge.

Fixes: 80d10a6cee05 ("cxl/region: Add interleave geometry attributes")
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/165973127171.1526540.9923273539049172976.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 641bc63..4011480 100644 (file)
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -395,13 +395,14 @@ static ssize_t interleave_granularity_store(struct device *dev,
                return rc;
 
        /*
-        * Disallow region granularity less than root granularity to
-        * simplify the implementation. Otherwise, region's with a
-        * granularity less than the root interleave result in needing
-        * multiple endpoints to support a single slot in the
-        * interleave.
+        * When the host-bridge is interleaved, disallow region granularity !=
+        * root granularity. Regions with a granularity less than the root
+        * interleave result in needing multiple endpoints to support a single
+        * slot in the interleave (possible to suport in the future). Regions
+        * with a granularity greater than the root interleave result in invalid
+        * DPA translations (invalid to support).
         */
-       if (val < cxld->interleave_granularity)
+       if (cxld->interleave_ways > 1 && val != cxld->interleave_granularity)
                return -EINVAL;
 
        rc = down_write_killable(&cxl_region_rwsem);