linux-kernel - Re: [PATCH v6 2/4] Add support for SCT Write Same

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Date:   Tue, 23 Aug 2016 05:56:26 -0500
From:   Shaun Tancheff <shaun.tancheff@...gate.com>
To:     Tom Yan <tom.ty89@...il.com>
Cc:     Shaun Tancheff <shaun@...cheff.com>, linux-ide@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Hellwig <hch@....de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Damien Le Moal <damien.lemoal@...t.com>,
        Hannes Reinecke <hare@...e.de>,
        Josh Bingaman <josh.bingaman@...gate.com>,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH v6 2/4] Add support for SCT Write Same

On Tue, Aug 23, 2016 at 5:37 AM, Tom Yan <tom.ty89@...il.com> wrote:
> On 22 August 2016 at 04:23, Shaun Tancheff <shaun@...cheff.com> wrote:
>> +/**
>> + * ata_format_dsm_trim_descr() - SATL Write Same to ATA SCT Write Same
>> + * @cmd: SCSI command being translated
>> + * @lba: Starting sector
>> + * @num: Number of logical sectors to be zero'd.
>> + *
>> + * Rewrite the WRITE SAME descriptor to be an SCT Write Same formatted
>> + * descriptor.
>> + * NOTE: Writes a pattern (0's) in the foreground.
>> + *       Large write-same requents can timeout.
>> + */
>> +static void ata_format_sct_write_same(struct scsi_cmnd *cmd, u64 lba, u64 num)
>> +{
>> +       u16 *sctpg;
>> +       unsigned long flags;
>> +
>> +       spin_lock_irqsave(&ata_scsi_rbuf_lock, flags);
>> +       sctpg = ((void *)ata_scsi_rbuf);
>
> Because ata_scsi_rbuf is of a fixed size of ATA_SCSI_RBUF_SIZE.
>
> #define ATA_SCSI_RBUF_SIZE      4096
> ...
> static u8 ata_scsi_rbuf[ATA_SCSI_RBUF_SIZE];
>
>> +
>> +       put_unaligned_le16(0x0002,  &sctpg[0]); /* SCT_ACT_WRITE_SAME */
>> +       put_unaligned_le16(0x0101,  &sctpg[1]); /* WRITE PTRN FG */
>> +       put_unaligned_le64(lba,     &sctpg[2]);
>> +       put_unaligned_le64(num,     &sctpg[6]);
>> +       put_unaligned_le32(0u,      &sctpg[10]);
>> +
>> +       sg_copy_from_buffer(scsi_sglist(cmd), scsi_sg_count(cmd), sctpg, 512);
>
> You have no reason to use 512 here instead of ATA_SCSI_RBUF_SIZE this time.

Ah .. because SCT Write Same is a fixed 512 byte transfer?
Ah .. because I only have 512 bytes to copy?

>> +       spin_unlock_irqrestore(&ata_scsi_rbuf_lock, flags);
>> +}