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

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Tue, 22 Aug 2023 11:09:28 -0700
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

Arguably doesn't matter what the granularity is for a 1 way
decode. Probably just drop that or does it matter for the
calculations somewhere?

      Switch Decoder_A: 1 way

Feel free to ignore the suggestion below.


>         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>
> ---
> 
> 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;

This threw me briefly.   eiw isn't used before this patch.
You 'could' pull that out as a precursor to make this logic a tiny bit
more obvious but meh it may not be worth doing so.

I've not checked the maths (As interleave gives me headaches far too quickly
so will rely on the you and the other reviewers to ensure that was right :)



> -		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