The patch isn't about how the request from the block layer will be
processed (to form the SCSI commands).
What it addresses is blk_queue_max_write_same_sectors() and
blk_queue_max_discard_sectors() that are called in the SCSI disk
driver. You can see that they are called with an input of the Maximum
Write Same Length times (logical_block_size >> 9), which is to convert
the number of sectors to an appropriate value for the block layer
(512-byte block based). On 4Kn drives, the multiplier will be 4096 >>
9, which is 8. So if the reported Maximum Write Same Length is
4194240, when the value is passed onto the block layer to set the
limits for a 4Kn drive, it will be 4194240 * 8. (And when this value
is represented in bytes, it will be further multiplied by 512 and over
32-bit.)
`logical_block_size >> 9` is pretty much the same thing as
``logical_block_size / 512`. I should have probably used the bit shift
way instead.
On 11 August 2016 at 17:04, Shaun Tancheff <sh...@tancheff.com> wrote:
> On Thu, Aug 11, 2016 at 3:26 AM, <tom.t...@gmail.com> wrote:
>> From: Tom Yan <tom.t...@gmail.com>
>>
>> Currently we advertise Maximum Write Same Length based on the
>> maximum number of sectors that one-block TRIM payload can cover.
>> The field are used to derived discard_max_bytes and
>> write_same_max_bytes limits in the block layer, which currently can
>> at max be 0xffffffff (32-bit).
>>
>> However, with a AF 4Kn drive, the derived limits would be 65535 *
>> 64 * 4096 = 0x3fffc0000 (34-bit). Therefore, we now devide
>> ATA_MAX_TRIM_RNUM with (logical sector size / 512), so that the
>> derived limits will not overflow.
>>
>> The limits are now also consistent among drives with different
>> logical sector sizes. (Although that may or may not be what we
>> want ultimately when the SCSI / block layer allows larger
>> representation in the future.)
>>
>> Although 4Kn ATA SSDs may not be a thing on the market yet, this
>> patch is necessary for forthcoming SCT Write Same translation
>> support, which could be available on traditional HDDs where 4Kn is
>> already a thing. Also it should not change the current behavior on
>> drives with 512-byte logical sectors.
>>
>> Note: this patch is not about AF 512e drives.
>> Signed-off-by: Tom Yan <tom.t...@gmail.com>
>>
>> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
>> index be9c76c..dcadcaf 100644
>> --- a/drivers/ata/libata-scsi.c
>> +++ b/drivers/ata/libata-scsi.c
>> @@ -2295,6 +2295,7 @@ static unsigned int ata_scsiop_inq_89(struct
>> ata_scsi_args *args, u8 *rbuf)
>> static unsigned int ata_scsiop_inq_b0(struct ata_scsi_args *args, u8 *rbuf)
>> {
>> u16 min_io_sectors;
>> + u32 sector_size;
>>
>> rbuf[1] = 0xb0;
>> rbuf[3] = 0x3c; /* required VPD size with unmap support */
>> @@ -2309,17 +2310,27 @@ static unsigned int ata_scsiop_inq_b0(struct
>> ata_scsi_args *args, u8 *rbuf)
>> min_io_sectors = 1 << ata_id_log2_per_physical_sector(args->id);
>> put_unaligned_be16(min_io_sectors, &rbuf[6]);
>>
>> - /*
>> - * Optimal unmap granularity.
>> - *
>> - * The ATA spec doesn't even know about a granularity or alignment
>> - * for the TRIM command. We can leave away most of the unmap related
>> - * VPD page entries, but we have specifify a granularity to signal
>> - * that we support some form of unmap - in thise case via WRITE SAME
>> - * with the unmap bit set.
>> - */
>> + sector_size = ata_id_logical_sector_size(args->id);
>> if (ata_id_has_trim(args->id)) {
>> - put_unaligned_be64(65535 * ATA_MAX_TRIM_RNUM, &rbuf[36]);
>> + /*
>> + * Maximum write same length.
>> + *
>> + * Avoid overflow in discard_max_bytes and
>> write_same_max_bytes
>> + * with AF 4Kn drives. Also make them consistent among drives
>> + * with different logical sector sizes.
>> + */
>> + put_unaligned_be64(65535 * ATA_MAX_TRIM_RNUM /
>> + (sector_size / 512), &rbuf[36]);
>
> I think the existing fixups in sd_setup_discard_cmnd() and
> sd_setup_write_same_cmnd()
> are 'doing the right thing'.
>
> If I understand the stack correctly:
>
> libata-scsi.c (and sd.c) both report a maximum in terms of 512 byte sectors.
> The upper layer stack works (mostly) on a mix of bytes and 512 byte sectors
> agnostic of the underlying hardware ... mostly. There are some bits in the
> files systems and block layer that are honoring the logical block size being
> larger 512 bytes as all I/O being generated are multiples of the logical block
> size as per block device's request_queue / queue_limits.
>
> So regardless of a 4Kn device being able to handle an 8x larger I/O as per
> the logical sector being bigger that's basically ignored, for convenience.
>
> In the scsi upper layer as the command are being setup the shift from
> 512 to 'sector_size' is handled to the number of device sectors is
> matched up to the request:
>
> sector >>= ilog2(sdp->sector_size) - 9;
> nr_sectors >>= ilog2(sdp->sector_size) - 9;
>
> So if you correctly report number of logical sectors here you break
> the 'fix' in sd.c
>
> At least that is my understanding.
>
>> +
>> + /*
>> + * Optimal unmap granularity.
>> + *
>> + * The ATA spec doesn't even know about a granularity or
>> alignment
>> + * for the TRIM command. We can leave away most of the
>> unmap related
>> + * VPD page entries, but we have specifify a granularity to
>> signal
>> + * that we support some form of unmap - in thise case via
>> WRITE SAME
>> + * with the unmap bit set.
>> + */
>> put_unaligned_be32(1, &rbuf[28]);
>> }
>>
>> --
>> 2.9.2
>>
>
> Regards,
> Shaun
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