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Date:   Thu, 14 Jan 2021 17:25:31 +0000
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     Ben Widawsky <ben.widawsky@intel.com>
CC:     <linux-cxl@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-pci@vger.kernel.org>,
        "linux-acpi@vger.kernel.org, Ira Weiny" <ira.weiny@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "Vishal Verma" <vishal.l.verma@intel.com>,
        "Kelley, Sean V" <sean.v.kelley@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        "Bjorn Helgaas" <helgaas@kernel.org>,
        Jon Masters <jcm@jonmasters.org>,
        Chris Browy <cbrowy@avery-design.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Christoph Hellwig" <hch@infradead.org>,
        <daniel.lll@alibaba-inc.com>
Subject: Re: [RFC PATCH v3 13/16] cxl/mem: Create concept of enabled
 commands
Message-ID: <20210114172531.0000347a@Huawei.com>
In-Reply-To: <20210111225121.820014-15-ben.widawsky@intel.com>
References: <20210111225121.820014-1-ben.widawsky@intel.com>
        <20210111225121.820014-15-ben.widawsky@intel.com>
Organization: Huawei Technologies Research and Development (UK) Ltd.
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On Mon, 11 Jan 2021 14:51:18 -0800
Ben Widawsky <ben.widawsky@intel.com> wrote:

> CXL devices must implement the Device Command Interface (described in
> 8.2.9 of the CXL 2.0 spec). While the driver already maintains a list of
> commands it supports, there is still a need to be able to distinguish
> between commands that the driver knows about from commands that may not
> be supported by the hardware. No such commands currently are defined in
> the driver.
> 
> The implementation leaves the statically defined table of commands and
> supplements it with a bitmap to determine commands that are enabled.
> 
> ---
> 
> There are multiple approaches that can be taken, but this is nice for a
> few reasons.
> 
> Here are some of the other solutions:
> 
> Create a per instance table with only the supported commands.
> 1. Having a fixed command id -> command mapping is much easier to manage
>    for development and debugging.
> 2. Dealing with dynamic memory allocation for the table adds unnecessary
>    complexity.
> 3. Most tables for device types are likely to be quite similar.
> 4. Makes it difficult to implement helper macros like cxl_for_each_cmd()
> 
> If the per instance table did preserve ids, #1 above can be addressed.
> However, as "enable" is currently the only mutable state for the
> commands, it would yield a lot of overhead for not much gain.
> Additionally, the other issues remain.
> 
> If "enable" remains the only mutable state, I believe this to be the
> best solution. Once the number of mutable elements in a command grows,
> it probably makes sense to move to per device instance state with a
> fixed command ID mapping.
Agreed with the logic.   

However, patch wise, should either drop the --- above or move this below the
--- after your sign off.  Otherwise you run the risk of git dropping your
sign off and resulting complaints from anyone run validation scripts
of the kernel tree that check for that.

> 
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> ---
>  drivers/cxl/cxl.h |  4 ++++
>  drivers/cxl/mem.c | 38 +++++++++++++++++++++++++++++++++++++-
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 537ac4d8e6bd..963ba30cb200 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -17,6 +17,9 @@
>  
>  #define CXL_GET_FIELD(word, field) FIELD_GET(field##_MASK, word)
>  
> +/* XXX: Arbitrary max */
> +#define CXL_MAX_COMMANDS 32

If going this way, probably want a build time check that you don't
go beyond this value for a given command set.  I haven't actually
thought about how to construct that but should be easy enough.

> +
>  /* Device  (CXL 2.0 - 8.2.8.3) */
>  #define CXLDEV_CAP_ARRAY_REG 0x0
>  #define CXLDEV_CAP_ARRAY_CAP_ID 0
> @@ -88,6 +91,7 @@ struct cxl_mem {
>  	} ram;
>  
>  	char firmware_version[0x10];
> +	DECLARE_BITMAP(enabled_cmds, CXL_MAX_COMMANDS);
>  
>  	/* Cap 0001h - CXL_CAP_CAP_ID_DEVICE_STATUS */
>  	struct {
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index a824cfd4342a..20b26fa2c466 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -114,6 +114,8 @@ static struct {
>   *    would typically be used for deprecated commands.
>   *  * %CXL_CMD_FLAG_MANDATORY: Hardware must support this command. This flag is
>   *    only used internally by the driver for sanity checking.
> + *  * %CXL_CMD_INTERNAL_FLAG_PSEUDO: This is a pseudo command which doesn't have
> + *    a direct mapping to hardware. They are implicitly always enabled.
>   *
>   * The cxl_mem_command is the driver's internal representation of commands that
>   * are supported by the driver. Some of these commands may not be supported by
> @@ -129,6 +131,7 @@ struct cxl_mem_command {
>  #define CXL_CMD_INTERNAL_FLAG_NONE 0
>  #define CXL_CMD_INTERNAL_FLAG_HIDDEN BIT(0)
>  #define CXL_CMD_INTERNAL_FLAG_MANDATORY BIT(1)
> +#define CXL_CMD_INTERNAL_FLAG_PSEUDO BIT(2)
>  };
>  
>  /*
> @@ -140,7 +143,7 @@ struct cxl_mem_command {
>  static struct cxl_mem_command mem_commands[] = {
>  	CXL_CMD(INVALID, KERNEL, 0, 0, HIDDEN),
>  	CXL_CMD(IDENTIFY, NONE, 0, 0x43, MANDATORY),
> -	CXL_CMD(RAW, NONE, ~0, ~0, MANDATORY),
> +	CXL_CMD(RAW, NONE, ~0, ~0, PSEUDO),
>  };
>  
>  #define cxl_for_each_cmd(cmd)                                                  \
> @@ -618,6 +621,10 @@ static int cxl_validate_cmd_from_user(struct cxl_mem *cxlm,
>  	c = &mem_commands[send_cmd->id];
>  	info = &c->info;
>  
> +	/* Check that the command is enabled for hardware */
> +	if (!test_bit(cxl_cmd_index(c), cxlm->enabled_cmds))
> +		return -ENOTTY;
> +
>  	if (info->flags & CXL_MEM_COMMAND_FLAG_KERNEL)
>  		return -EPERM;
>  
> @@ -1029,6 +1036,31 @@ static int cxl_mem_add_memdev(struct cxl_mem *cxlm)
>  	return rc;
>  }
>  
> +/**
> + * cxl_mem_enumerate_cmds() - Enumerate commands for a device.
> + * @cxlm: The device.
> + *
> + * Returns 0 if enumerate completed successfully.
> + *
> + * CXL devices have optional support for certain commands. This function will
> + * determine the set of supported commands for the hardware and update the
> + * enabled_cmds bitmap in the @cxlm.
> + */
> +static int cxl_mem_enumerate_cmds(struct cxl_mem *cxlm)
> +{
> +	struct cxl_mem_command *c;
> +
> +	/* All commands are considered enabled for now (except INVALID). */
> +	cxl_for_each_cmd(c) {
> +		if (c->flags & CXL_CMD_INTERNAL_FLAG_HIDDEN)
> +			continue;
> +
> +		set_bit(cxl_cmd_index(c), cxlm->enabled_cmds);
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * cxl_mem_identify() - Send the IDENTIFY command to the device.
>   * @cxlm: The device to identify.
> @@ -1147,6 +1179,10 @@ static int cxl_mem_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>  	if (rc)
>  		goto err;
>  
> +	rc = cxl_mem_enumerate_cmds(cxlm);
> +	if (rc)
> +		return rc;
> +
>  	rc = cxl_mem_identify(cxlm);
>  	if (rc)
>  		goto err;