James Bottomley wrote:
> On Thu, 2008-07-24 at 11:15 +0100, Alan Jenkins wrote:
>
>> 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 <alan-jenkins@xxxxxxxxxxxxxx>
>> Tested-by: Alan Jenkins <alan-jenkins@xxxxxxxxxxxxxx>
>>
>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
>> index 01cefbb..2415a1b 100644
>> --- a/drivers/scsi/sd.c
>> +++ b/drivers/scsi/sd.c
>> @@ -406,13 +406,23 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq)
>> }
>>
>> /*
>> - * Some devices (some sdcards for one) don't like it if the
>> - * last sector gets read in a larger then 1 sector read.
>> + * Some sd cardreaders can't handle multisector accesses which touch
>> + * the last one or two hardware sectors. There are likely to be even
>> + * buggier devices, so apply a workaround to the last eight sectors.
>> */
>> - if (unlikely(sdp->last_sector_bug &&
>> - rq->nr_sectors > sdp->sector_size / 512 &&
>> - block + this_count == get_capacity(disk)))
>> - this_count -= sdp->sector_size / 512;
>> + if (sdp->last_sector_bug) {
>>
>
> This should be unlikely() as the previous one was
>
Fine. I'll change it back to a single combined unlikely() test.
threshold = ...
if (unlikely(sdp->last_sector_bug && block + this_count > threshold)) {
>> + unsigned threshold = get_capacity(disk) - 8 * (sdp->sector_size / 512);
>>
>
> This can't be an unsigned, it has to be sector_t otherwise it could
> overflow on large capacity drives.
>
Ok.
> I can also see that someone will find a drive that needs the last 16 or
> something sectors, so perhaps the bare 8 should be a nice #define in
> sd.h (comment above would need altering too).
>
>
Good idea.
>
>> + if (block + this_count <= threshold) {
>>
>
> This should be likely() as well.
>
>
(see above)
>
>> + ; /* Okay as is */
>> + } else if (block < threshold) {
>> + /* Access up to the threshold but not beyond */
>> + this_count = threshold - block;
>> + } else {
>> + /* Access only a single hardware sector */
>> + this_count = sdp->sector_size / 512;
>> + }
>> + }
>>
>> SCSI_LOG_HLQUEUE(2, scmd_printk(KERN_INFO, SCpnt, "block=%llu\n",
>> (unsigned long long)block));
>> diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
>> index f6a9fe0..0d8d9c1 100644
>> --- a/include/scsi/scsi_device.h
>> +++ b/include/scsi/scsi_device.h
>> @@ -139,7 +139,8 @@ struct scsi_device {
>> unsigned fix_capacity:1; /* READ_CAPACITY is too high by 1 */
>> unsigned guess_capacity:1; /* READ_CAPACITY might be too high by 1 */
>> unsigned retry_hwerror:1; /* Retry HARDWARE_ERROR */
>> - unsigned last_sector_bug:1; /* Always read last sector in a 1 sector read */
>> + unsigned last_sector_bug:1; /* do not use multisector accesses on
>> + the last 8 hardware sectors */
>>
>> DECLARE_BITMAP(supported_events, SDEV_EVT_MAXBITS); /* supported events */
>> struct list_head event_list; /* asserted events */
>>
>
> Everything else looks fine ... do a quick turn around and I'll slide it
> under the merge window.
>
> Thanks,
>
> James
--
To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
