Re: [PATCH v4 08/23] cxl: Add support for _DSM Function for retrieving QTG ID

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

On Wed, 19 Apr 2023 13:21:49 -0700
Dave Jiang <dave.jiang@intel.com> wrote:

> CXL spec v3.0 9.17.3 CXL Root Device Specific Methods (_DSM)
> 
> Add support to retrieve QTG ID via ACPI _DSM call. The _DSM call requires
> an input of an ACPI package with 4 dwords (read latency, write latency,
> read bandwidth, write bandwidth). The call returns a package with 1 WORD
> that provides the max supported QTG ID and a package that may contain 0 or
> more WORDs as the recommended QTG IDs in the recommended order.
> 
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> 

A few minor comments inline. 


> +/**
> + * cxl_acpi_evaluate_qtg_dsm - Retrieve QTG ids via ACPI _DSM
> + * @handle: ACPI handle
> + * @input: bandwidth and latency data
> + *
> + * Issue QTG _DSM with accompanied bandwidth and latency data in order to get
> + * the QTG IDs that falls within the performance data.
Falls within is a little vague.  Perhaps something like

the QTG IDs that are suitable for the performance point in order of most suitable
to least suitable.

> + */
> +struct qtg_dsm_output *cxl_acpi_evaluate_qtg_dsm(acpi_handle handle,
> +						 struct qtg_dsm_input *input)
> +{
> +	union acpi_object *out_obj, *out_buf, *pkg;
> +	union acpi_object in_buf = {
> +		.buffer = {
> +			.type = ACPI_TYPE_BUFFER,
> +			.pointer = (u8 *)input,
> +			.length = sizeof(u32) * 4,

sizeof(*input)?

Also, ACPI structures are always little endian. Do we need to be careful of that
here?

> +		},
> +	};
> +	union acpi_object in_obj = {
> +		.package = {
> +			.type = ACPI_TYPE_PACKAGE,
> +			.count = 1,
> +			.elements = &in_buf
> +		},
> +	};
> +	struct qtg_dsm_output *output = NULL;
> +	int len, rc, i;
> +	u16 *max_qtg;
> +
> +	out_obj = acpi_evaluate_dsm(handle, &acpi_cxl_qtg_id_guid, 1, 1, &in_obj);
> +	if (!out_obj)
> +		return ERR_PTR(-ENXIO);
> +
> +	if (out_obj->type != ACPI_TYPE_PACKAGE) {
> +		rc = -ENXIO;
> +		goto err;
> +	}
> +
> +	/* Check Max QTG ID */
> +	pkg = &out_obj->package.elements[0];
> +	if (pkg->type != ACPI_TYPE_BUFFER) {
> +		rc = -ENXIO;
> +		goto err;
> +	}
> +
> +	if (pkg->buffer.length != sizeof(u16)) {
> +		rc = -ENXIO;
> +		goto err;
> +	}
> +	max_qtg = (u16 *)pkg->buffer.pointer;
> +
> +	/* Retrieve QTG IDs package */
> +	pkg = &out_obj->package.elements[1];
> +	if (pkg->type != ACPI_TYPE_PACKAGE) {
> +		rc = -ENXIO;
> +		goto err;
> +	}
> +
> +	out_buf = &pkg->package.elements[0];
> +	if (out_buf->type != ACPI_TYPE_BUFFER) {
> +		rc = -ENXIO;
> +		goto err;
> +	}
> +
> +	len = out_buf->buffer.length;
> +
> +	/* It's legal to have 0 QTG entries */
> +	if (len == 0)
> +		goto out;
> +
> +	/* Malformed package, not multiple of WORD size */
> +	if (len % sizeof(u16)) {
> +		rc = -ENXIO;
> +		goto out;
> +	}
> +
> +	output = kmalloc(len + sizeof(*output), GFP_KERNEL);
> +	if (!output) {
> +		rc = -ENOMEM;
> +		goto err;
> +	}
> +
> +	output->nr = len / sizeof(u16);
> +	memcpy(output->qtg_ids, out_buf->buffer.pointer, len);
> +
> +	for (i = 0; i < output->nr; i++) {
> +		if (output->qtg_ids[i] > *max_qtg)
> +			pr_warn("QTG ID %u greater than MAX %u\n",
> +				output->qtg_ids[i], *max_qtg);
> +	}
> +
> +out:
> +	ACPI_FREE(out_obj);
> +	return output;
> +
> +err:
> +	ACPI_FREE(out_obj);
> +	return ERR_PTR(rc);

Why not combine these with something like

	return IS_ERR(rc) ? ERR_PTR(rc) : output;

I'm fine with leaving as it is, if this is common style for these
sorts of ACPI functions.

> +}
> +EXPORT_SYMBOL_NS_GPL(cxl_acpi_evaluate_qtg_dsm, CXL);