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

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Message-ID: <CAGnHSEm-EjtmtT+VDOdJqNMq_bZ-n8Qe_oMLcorZGJT3+550Fg@mail.gmail.com>
Date:   Thu, 25 Aug 2016 14:23:24 +0800
From:   Tom Yan <tom.ty89@...il.com>
To:     Shaun Tancheff <shaun@...cheff.com>
Cc:     Shaun Tancheff <shaun.tancheff@...gate.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

You only fill the bytes that you want to to set explicitly:

+       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]);

What I doubted is, if you don't memset (zero-fill) the buffer first,
will other bytes have indeterministic value that causes random
unexpected behavior?

On 25 August 2016 at 06:04, Shaun Tancheff <shaun@...cheff.com> wrote:
> On Wed, Aug 24, 2016 at 1:10 AM, Tom Yan <tom.ty89@...il.com> wrote:
>> Btw, I wonder if you need to memset your buffer with 0 first, like
>> what is done in ata_scsi_rbuf_get.
>
> It is not necessary as the defined buffer is completely filled out here.
>
> Are you thinking as a sort of future proofing?
> Ex: In the unlikely event that the SCT Write Same command
> descriptor is expanded in a future ACS?
>
> It is more likely to see the command deprecated and replaced with a
> new SCT feature.
>
> Regardless of how unlikely I would consider a memset here to clear
> the remainder of the payload.