RE: [PATCH] cxl: check decoder count for end device

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

Dave Jiang wrote:
> CXL spec rev3.0 8.2.4.19.1 added definition for up to 32 decoders. It also
> indicates that for devices, only 10 decoders should be advertised. Add
> check on number of decoders greater than 10 for devices and reset to 10 to
> force spec compliance.
> 
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
>  drivers/cxl/core/hdm.c |   16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> index d1d2caea5c62..1919d99d157e 100644
> --- a/drivers/cxl/core/hdm.c
> +++ b/drivers/cxl/core/hdm.c
> @@ -71,9 +71,23 @@ EXPORT_SYMBOL_NS_GPL(devm_cxl_add_passthrough_decoder, CXL);
>  static void parse_hdm_decoder_caps(struct cxl_hdm *cxlhdm)
>  {
>  	u32 hdm_cap;
> +	int decoder_count;
> +	struct device *dev = &cxlhdm->port->dev;
>  
>  	hdm_cap = readl(cxlhdm->regs.hdm_decoder + CXL_HDM_DECODER_CAP_OFFSET);
> -	cxlhdm->decoder_count = cxl_hdm_decoder_count(hdm_cap);
> +	decoder_count = cxl_hdm_decoder_count(hdm_cap);
> +	/*
> +	 * CXL spec rev3.0 8.2.4.19.1 indicates CXL devices shall not advertise
> +	 * more than 10 decoders. Switches and Host Bridges may advertise up to
> +	 * 32 decoders. Set the decoders to 10 for devices if more than 10 are
> +	 * found.
> +	 */
> +	if (is_cxl_endpoint(cxlhdm->port) && decoder_count > 10) {
> +		dev_warn(dev, "Reset decoders count (%d) to 10, spec violation!\n",
> +			 decoder_count);
> +		decoder_count = 10;

I agree with the reporting, but not the limitation. The Robustness
Principle says if allowing it does no harm, then let it through.