Re: [PATCH v2] cxl/region: Refactor granularity select in cxl_port_setup_targets()

[Dave Jiang <dave.jiang@intel.com>]

On 8/22/23 11:09, alison.schofield@intel.com wrote:
> From: Alison Schofield <alison.schofield@intel.com>
> 
> In cxl_port_setup_targets() the region driver validates the
> configuration of auto-discovered region decoders, as well
> as decoders the driver is preparing to program.
> 
> The existing calculations use the encoded interleave granularity
> value to create an interleave granularity that properly fans out
> when routing an x1 interleave to a greater than x1 interleave.
> 
> That all worked well, until this config came along:
> Host Bridge: 2 way at 256 granularity
>      Switch Decoder_A:	1 way at 512
>          Endpoint_X:	2 way at 256
>      Switch Decoder_B:	1 way at 512
>          Endpoint_Y:	2 way at 256
> 
> When the Host Bridge interleave is greater than 1 and the root
> decoder interleave is exactly 1, the region driver needs to
> consider the number of targets in the region when calculating
> the expected granularity.
> 
> While examining the existing logic, and trying to cover the case
> above, a couple of simplifications appeared, hence this proposed
> refactoring.
> 
> The first simplification is to apply the logic to the nominal
> values and use the existing helper function granularity_to_eig() to
> translate the desired granularity to the encoded form. This means
> the comment and code regarding setting address bits is discarded.
> Although that logic is not wrong, it adds a level of complexity that
> is not required in the granularity selection. The eig and eiw are
> indeed part of the routing instructions programmed into the decoders.
> Up-level the discussion to nominal ways and granularity for clearer
> analysis.
> 
> The second simplification reduces the logic to a single granularity
> calculation that works for all cases. The new calculation doesn't
> care if parent_iw => 1 because parent_iw is used as a multiplier.
> 
> The refactor cleans up a useless assignment of eiw made after the
> iw is already calculated.
> 
> Regression testing included an examination of all of the ways and
> granularity selections made during a run of the cxl_test unit tests.
> There were no differences in selections before and after this patch.
> 
> Fixes: ("27b3f8d13830 cxl/region: Program target lists")
> Signed-off-by: Alison Schofield <alison.schofield@intel.com>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> ---
> 
> Changes in v2:
> - No changes to the code. Commit log fixups only.
> - Correct the commit log example. Endpoints are 2 * 256 (Jonathan)
> - Use 'encoded' and 'nominal' when referring to the interleave
>    granularity format (Dan, DaveJ)
> - Commit log grammar & spelling fixups (Dan, DaveJ)
> - Add Fixes Tag (Dan)
> - v1: https://lore.kernel.org/linux-cxl/20230804232726.1672782-1-alison.schofield@intel.com/
> 
>   drivers/cxl/core/region.c | 17 ++++++++---------
>   1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index e115ba382e04..5a1cc59cca99 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -1154,16 +1154,15 @@ static int cxl_port_setup_targets(struct cxl_port *port,
>   	}
>   
>   	/*
> -	 * If @parent_port is masking address bits, pick the next unused address
> -	 * bit to route @port's targets.
> +	 * Interleave granularity is a multiple of @parent_port granularity.
> +	 * Multiplier is the parent port interleave ways.
>   	 */
> -	if (parent_iw > 1 && cxl_rr->nr_targets > 1) {
> -		u32 address_bit = max(peig + peiw, eiw + peig);
> -
> -		eig = address_bit - eiw + 1;
> -	} else {
> -		eiw = peiw;
> -		eig = peig;
> +	rc = granularity_to_eig(parent_ig * parent_iw, &eig);
> +	if (rc) {
> +		dev_dbg(&cxlr->dev,
> +			"%s: invalid granularity calculation (%d * %d)\n",
> +			dev_name(&parent_port->dev), parent_ig, parent_iw);
> +		return rc;
>   	}
>   
>   	rc = eig_to_granularity(eig, &ig);
> 
> base-commit: fe77cc2e5a6a7c85f5c6ef8a39d7694ffc7f41c9