drivers/edac: mod race fix i82875p

If ERRSTS indicates that there's no error then we don't need to bother reading
the other registers.

In addition to making the common case faster, this actually fixes a small race
where we don't see an error but we clear the error bits anyway, potentially
wiping away info on an error that happened in the interim (or where a CE
arrives between the first and second read of ERRSTS, causing us to falsely
claim "UE overwrote CE").

Signed-off-by: Jason Uhlenkott <juhlenko@akamai.com>
Signed-off-by: Douglas Thompson <dougthompson@xmission.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/edac/i82875p_edac.c

@@ -198,27 +198,28 @@ static void i82875p_get_error_info(struct mem_ctl_info *mci,
 	 * overwritten by UE.
 	 */
 	pci_read_config_word(pdev, I82875P_ERRSTS, &info->errsts);
+
+	if (!(info->errsts & 0x0081))
+		return;
+
 	pci_read_config_dword(pdev, I82875P_EAP, &info->eap);
 	pci_read_config_byte(pdev, I82875P_DES, &info->des);
 	pci_read_config_byte(pdev, I82875P_DERRSYN, &info->derrsyn);
 	pci_read_config_word(pdev, I82875P_ERRSTS, &info->errsts2);
 
-	pci_write_bits16(pdev, I82875P_ERRSTS, 0x0081, 0x0081);
-
 	/*
 	 * If the error is the same then we can for both reads then
 	 * the first set of reads is valid.  If there is a change then
 	 * there is a CE no info and the second set of reads is valid
 	 * and should be UE info.
 	 */
-	if (!(info->errsts2 & 0x0081))
-		return;
-
 	if ((info->errsts ^ info->errsts2) & 0x0081) {
 		pci_read_config_dword(pdev, I82875P_EAP, &info->eap);
 		pci_read_config_byte(pdev, I82875P_DES, &info->des);
 		pci_read_config_byte(pdev, I82875P_DERRSYN, &info->derrsyn);
 	}
+
+	pci_write_bits16(pdev, I82875P_ERRSTS, 0x0081, 0x0081);
 }
 
 static int i82875p_process_error_info(struct mem_ctl_info *mci,
@@ -229,7 +230,7 @@ static int i82875p_process_error_info(struct mem_ctl_info *mci,
 
 	multi_chan = mci->csrows[0].nr_channels - 1;
 
-	if (!(info->errsts2 & 0x0081))
+	if (!(info->errsts & 0x0081))
 		return 0;
 
 	if (!handle_errors)