linux-kernel - Re: [PATCH 3/3] cxl: Avoid to create dax regions for type2 accelerators

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Message-ID: <20240804173813.00001018@Huawei.com>
Date: Sun, 4 Aug 2024 17:38:13 +0100
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Huang Ying <ying.huang@...el.com>
CC: Dan Williams <dan.j.williams@...el.com>, Dave Jiang
	<dave.jiang@...el.com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, "Alejandro
 Lucero" <alucerop@....com>
Subject: Re: [PATCH 3/3] cxl: Avoid to create dax regions for type2
 accelerators

On Mon, 29 Jul 2024 16:46:11 +0800
Huang Ying <ying.huang@...el.com> wrote:

> The memory range of a type2 accelerator should be managed by the type2
> accelerator specific driver instead of the common dax region drivers,
> as discussed in [1].
> 
> [1] https://lore.kernel.org/linux-cxl/66469ff1b8fbc_2c2629427@dwillia2-xfh.jf.intel.com.notmuch/
> 
> So, in this patch, we skip dax regions creation for type2 accelerator
> device memory regions.
> 
> Based on: https://lore.kernel.org/linux-cxl/168592159835.1948938.1647215579839222774.stgit@dwillia2-xfh.jf.intel.com/
> 
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Co-developed-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: Jonathan Cameron <jonathan.cameron@...wei.com>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Alison Schofield <alison.schofield@...el.com>
> Cc: Vishal Verma <vishal.l.verma@...el.com>
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Alejandro Lucero <alucerop@....com>
> ---
>  drivers/cxl/core/region.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 9a483c8a32fd..b37e12bb4a35 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3435,6 +3435,14 @@ static int cxl_region_probe(struct device *dev)
>  					p->res->start, p->res->end, cxlr,
>  					is_system_ram) > 0)
>  			return 0;
> +		/*
> +		 * HDM-D[B] (device-memory) regions have accelerator
> +		 * specific usage, skip device-dax registration.
> +		 */
> +		if (cxlr->type == CXL_DECODER_DEVMEM)
> +			return 0;

As in previous need to be careful as that may not mean it's
an accelerator.

However, we do need to deal with BI setup for HDM-DB type 3 devices
etc and to check the HDM Decoder capability registers to make sure
Supported Coherence model is appropriate. (e.g. 11 for host only or
device coherency - HDM-H/HDM-DB)

> +
> +		/* HDM-H routes to device-dax */
>  		return devm_cxl_add_dax_region(cxlr);
>  	default:
>  		dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",