linux-kernel - amd-iommu: get_highest_supported_ivhd

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Message-ID: <e8a87c2b-a29a-ccf9-49c6-3cfceaa208bb@gmail.com>
Date:   Mon, 15 May 2023 21:36:05 +0700
From:   Bui Quang Minh <minhquangbui99@...il.com>
To:     Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        Wan Zongshun <vincent.wan@....com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: amd-iommu: get_highest_supported_ivhd_type question

Hello everyone,

Since this commit 8c7142f56fedfc6824b5bca56fee1f443e01746b (iommu/amd: 
Use the most comprehensive IVHD type that the driver can support), 
amd_iommu driver can support IVHD type 0x11 and 0x40 beside old type 
0x10. This commit introduces a new function to determine the appropriate 
IVHD type

	/**
	 * get_highest_supported_ivhd_type - Look up the appropriate IVHD type
	 * @ivrs          Pointer to the IVRS header
	 *
	 * This function search through all IVDB of the maximum supported IVHD
	 */
	static u8 get_highest_supported_ivhd_type(struct acpi_table_header *ivrs)
	{
		u8 *base = (u8 *)ivrs;
		struct ivhd_header *ivhd = (struct ivhd_header *)
						(base + IVRS_HEADER_LENGTH);
		u8 last_type = ivhd->type;
		u16 devid = ivhd->devid;

		while (((u8 *)ivhd - base < ivrs->length) &&
		       (ivhd->type <= ACPI_IVHD_TYPE_MAX_SUPPORTED)) {
			u8 *p = (u8 *) ivhd;

			if (ivhd->devid == devid)
				last_type = ivhd->type;
			ivhd = (struct ivhd_header *)(p + ivhd->length);
		}

		return last_type;
	}

As the name and comment suggest, the driver is intended to use the 
highest IVHD type provided in ACPI table. However, looking at the 
implementation, I see that it chooses to use the last IVHD type of the 
first devid appears in the ACPI table. Are there any reasons behind this 
implementation?

Thank you,
Quang Minh.