linux-kernel - [PATCH] dmi_scan: Add comments on dmi_present() and the loop in dmi_scan

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Message-ID: <1373466723.24446.10.camel@deadeye.wl.decadent.org.uk>
Date:	Wed, 10 Jul 2013 15:32:03 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org
Cc:	Zhenzhong Duan <zhenzhong.duan@...cle.com>,
	Jean Delvare <jdelvare@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] dmi_scan: Add comments on dmi_present() and the loop in
 dmi_scan_machine()

My previous refactoring in commit 79bae42d51a5 resulted in slightly
tricky code (though I think it's more elegant).  Explain what it's
doing.

Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/firmware/dmi_scan.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
index b95159b..4fdb377 100644
--- a/drivers/firmware/dmi_scan.c
+++ b/drivers/firmware/dmi_scan.c
@@ -419,6 +419,13 @@ static void __init dmi_format_ids(char *buf, size_t len)
 			    dmi_get_system_info(DMI_BIOS_DATE));
 }
 
+/*
+ * Check for DMI/SMBIOS headers in the system firmware image.  Any
+ * SMBIOS header must start 16 bytes before the DMI header, so take a
+ * 32 byte buffer and check for DMI at offset 16 and SMBIOS at offset
+ * 0.  If the DMI header is present, set dmi_ver accordingly (SMBIOS
+ * takes precedence) and return 0.  Otherwise return 1.
+ */
 static int __init dmi_present(const u8 *buf)
 {
 	int smbios_ver;
@@ -506,6 +513,13 @@ void __init dmi_scan_machine(void)
 		if (p == NULL)
 			goto error;
 
+		/*
+		 * Iterate over all possible DMI header addresses q.
+		 * Maintain the 32 bytes around q in buf.  On the
+		 * first iteration, substitute zero for the
+		 * out-of-range bytes so there is no chance of falsely
+		 * detecting an SMBIOS header.
+		 */
 		memset(buf, 0, 16);
 		for (q = p; q < p + 0x10000; q += 16) {
 			memcpy_fromio(buf + 16, q, 16);


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