From 2b142900784c6e38c8d39fa57d5f95ef08e735d8 Mon Sep 17 00:00:00 2001 From: Alan Jenkins Date: Sun, 27 Jul 2008 09:38:42 +0100 Subject: [SCSI] extend the last_sector_bug flag to cover more sectors The last_sector_bug flag was added to work around a bug in certain usb cardreaders, where they would crash if a multiple sector read included the last sector. The original implementation avoids this by e.g. splitting an 8 sector read which includes the last sector into a 7 sector read, and a single sector read for the last sector. The flag is enabled for all USB devices. This revealed a second bug in other usb cardreaders, which crash when they get a multiple sector read which stops 1 sector short of the last sector. Affected hardware includes the Kingston "MobileLite" external USB cardreader and the internal USB cardreader on the Asus EeePC. Extend the last_sector_bug workaround to ensure that any access which touches the last 8 hardware sectors of the device is a single sector long. Requests are shrunk as necessary to meet this constraint. This gives us a safety margin against potential unknown or future bugs affecting multi-sector access to the end of the device. The two known bugs only affect the last 2 sectors. However, they suggest that these devices are prone to fencepost errors and that multi-sector access to the end of the device is not well tested. Popular OS's use multi-sector accesses, but they rarely read the last few sectors. Linux (with udev & vol_id) automatically reads sectors from the end of the device on insertion. It is assumed that single sector accesses are more thoroughly tested during development. Signed-off-by: Alan Jenkins Tested-by: Alan Jenkins Signed-off-by: James Bottomley --- drivers/scsi/sd.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'drivers/scsi/sd.h') diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h index 550b2f70a1f..95b9f06534d 100644 --- a/drivers/scsi/sd.h +++ b/drivers/scsi/sd.h @@ -31,6 +31,12 @@ */ #define SD_BUF_SIZE 512 +/* + * Number of sectors at the end of the device to avoid multi-sector + * accesses to in the case of last_sector_bug + */ +#define SD_LAST_BUGGY_SECTORS 8 + struct scsi_disk { struct scsi_driver *driver; /* always &sd_template */ struct scsi_device *device; -- cgit v1.2.3-18-g5258