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    SubjectRe: [PATCH v2] fix sign extension with 1.5TB usb-storage LBD=y
    On 04/22/2009 01:00 AM, Dave Hansen wrote:
    > Here's a patch implementing Al's suggestion. Not quite as trivial as
    > Matthew's, but even nicer aesthetically. (Description stolen from
    > Matthew's patch). I have verified that this fixes my issue.
    >
    > --
    >
    > Shifting an unsigned char implicitly casts it to a signed int. This
    > caused 'lba' to sign-extend and Linux would then try READ CAPACITY 16
    > which was not supported by at least one drive. Using the
    > get_unaligned_be*() helpers keeps us from having to worry about how the
    > extension might occur.
    >
    > Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
    >
    > diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
    > index 3fcb64b..c50142b 100644
    > --- a/drivers/scsi/sd.c
    > +++ b/drivers/scsi/sd.c
    > @@ -50,6 +50,7 @@
    > #include <linux/string_helpers.h>
    > #include <linux/async.h>
    > #include <asm/uaccess.h>
    > +#include <asm/unaligned.h>
    >
    > #include <scsi/scsi.h>
    > #include <scsi/scsi_cmnd.h>
    > @@ -1344,12 +1345,8 @@ static int read_capacity_16(struct scsi_disk *sdkp, struct scsi_device *sdp,
    > return -EINVAL;
    > }
    >
    > - sector_size = (buffer[8] << 24) | (buffer[9] << 16) |
    > - (buffer[10] << 8) | buffer[11];
    > - lba = (((u64)buffer[0] << 56) | ((u64)buffer[1] << 48) |
    > - ((u64)buffer[2] << 40) | ((u64)buffer[3] << 32) |
    > - ((u64)buffer[4] << 24) | ((u64)buffer[5] << 16) |
    > - ((u64)buffer[6] << 8) | (u64)buffer[7]);
    > + sector_size = get_unaligned_be32(&buffer[8]);
    > + lba = get_unaligned_be64(&buffer[0]);

    These are actually aligned access it might be worth sacrificing a cast
    to be32/64 for sake of speed.

    >
    > sd_read_protection_type(sdkp, buffer);
    >
    > @@ -1400,10 +1397,8 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
    > return -EINVAL;
    > }
    >
    > - sector_size = (buffer[4] << 24) | (buffer[5] << 16) |
    > - (buffer[6] << 8) | buffer[7];
    > - lba = (buffer[0] << 24) | (buffer[1] << 16) |
    > - (buffer[2] << 8) | buffer[3];
    > + sector_size = get_unaligned_be32(&buffer[4]);
    > + lba = get_unaligned_be32(&buffer[0]);
    >

    Here too, both are actually aligned.

    > if ((sizeof(sdkp->capacity) == 4) && (lba == 0xffffffff)) {
    > sd_printk(KERN_ERR, sdkp, "Too big for this kernel. Use a "
    >
    >
    > -- Dave
    >

    I'm not 100% convinced just a thought.
    Boaz


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