Re: [PATCH] cxl/hdm: Fix skip allocations vs multiple pmem allocations

[Ira Weiny <ira.weiny@intel.com>]

On Fri, Aug 05, 2022 at 03:12:27PM -0700, Dan Williams wrote:
> Vishal notes that when attempting to define a second pmem region on a
> device the DPA allocation fails with a message of the form:
> 
>     decoder11.1: failed to reserve skipped space
> 
> Recall that the skip setting is used when there is a pmem allocation in
> the presence of free ram DPA space. The first pmem allocation skips over
> the free ram and subsequent pmem allocations do not require a skip. The
> bug is that a skip is still attempted and the DPA reservation code
> flags the double skip allocation conflict.
> 
> Fixes: cf880423b6a0 ("cxl/hdm: Add support for allocating DPA to an endpoint decoder")
> Reported-by: Vishal Verma <vishal.l.verma@intel.com>
> Tested-by: Vishal Verma <vishal.l.verma@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> ---
>  drivers/cxl/core/hdm.c |   11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index e096f74e19df..d1d2caea5c62 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -445,7 +445,16 @@ int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, unsigned long long size)
>  		start = free_pmem_start;
>  		avail = cxlds->pmem_res.end - start + 1;
>  		skip_start = free_ram_start;
> -		skip_end = start - 1;
> +
> +		/*
> +		 * If some pmem is already allocated, then that allocation
> +		 * already handled the skip.
> +		 */
> +		if (cxlds->pmem_res.child &&
> +		    skip_start == cxlds->pmem_res.child->start)
> +			skip_end = skip_start - 1;
> +		else
> +			skip_end = start - 1;
>  		skip = skip_end - skip_start + 1;
>  	} else {
>  		dev_dbg(dev, "mode not set\n");
>