linux-kernel - Re: [PATCH] scsi: libsas: Reset num_scatter if libata mark qc as NODATA

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Message-ID: <CABGCNpDdK2+DNTJpzjig3hX3W_7JB8nEcRcuRnv8Z4oRSq2-dA@mail.gmail.com>
Date:   Wed, 17 Mar 2021 17:24:44 -0700
From:   Jolly Shah <jollys@...gle.com>
To:     John Garry <john.garry@...wei.com>
Cc:     jejb@...ux.ibm.com, martin.petersen@...cle.com,
        a.darwish@...utronix.de, Jason Yan <yanaijie@...wei.com>,
        luojiaxing@...wei.com, dan.carpenter@...cle.com,
        b.zolnierkie@...sung.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi: libsas: Reset num_scatter if libata mark qc as NODATA

Hi John,

Thanks for the review.


On Wed, Mar 17, 2021 at 4:44 AM John Garry <john.garry@...wei.com> wrote:
>
> On 16/03/2021 19:39, Jolly Shah wrote:
> > When the cache_type for the scsi device is changed, the scsi layer
> > issues a MODE_SELECT command. The caching mode details are communicated
> > via a request buffer associated with the scsi command with data
> > direction set as DMA_TO_DEVICE (scsi_mode_select). When this command
> > reaches the libata layer, as a part of generic initial setup, libata
> > layer sets up the scatterlist for the command using the scsi command
> > (ata_scsi_qc_new). This command is then translated by the libata layer
> > into ATA_CMD_SET_FEATURES (ata_scsi_mode_select_xlat). The libata layer
> > treats this as a non data command (ata_mselect_caching), since it only
> > needs an ata taskfile to pass the caching on/off information to the
> > device. It does not need the scatterlist that has been setup, so it does
> > not perform dma_map_sg on the scatterlist (ata_qc_issue).
>
> So if we don't perform the dma_map_sg() on the sgl at this point, then
> it seems to me that we should not perform for_each_sg() on it either,
> right? That is still what happens in sas_ata_qc_issue() in this case.
>

Yes that's right. To avoid that, I can add elseif block for
ATA_PROT_NODATA before for_each_sg() is performed. Currently there's
existing code block for ATA_PROT_NODATA after for_each_sg()  is
performed,
reused that to reset num_scatter. Please suggest.

> > Unfortunately,
> > when this command reaches the libsas layer(sas_ata_qc_issue), libsas
> > layer sees it as a non data command with a scatterlist. It cannot
> > extract the correct dma length, since the scatterlist has not been
> > mapped with dma_map_sg for a DMA operation. When this partially
> > constructed SAS task reaches pm80xx LLDD, it results in below warning.
> >
> > "pm80xx_chip_sata_req 6058: The sg list address
> > start_addr=0x0000000000000000 data_len=0x0end_addr_high=0xffffffff
> > end_addr_low=0xffffffff has crossed 4G boundary"
> >
> > This patch assigns appropriate value to  num_sectors for ata non data
> > commands.
> >
> > Signed-off-by: Jolly Shah <jollys@...gle.com>
> > ---
> >   drivers/scsi/libsas/sas_ata.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/scsi/libsas/sas_ata.c b/drivers/scsi/libsas/sas_ata.c
> > index 024e5a550759..94ec08cebbaa 100644
> > --- a/drivers/scsi/libsas/sas_ata.c
> > +++ b/drivers/scsi/libsas/sas_ata.c
> > @@ -209,10 +209,12 @@ static unsigned int sas_ata_qc_issue(struct ata_queued_cmd *qc)
> >               task->num_scatter = si;
> >       }
> >
> > -     if (qc->tf.protocol == ATA_PROT_NODATA)
> > +     if (qc->tf.protocol == ATA_PROT_NODATA) {
> >               task->data_dir = DMA_NONE;
> > -     else
> > +             task->num_scatter = 0;
>
> task->num_scatter has already been set in this function. Best not set it
> twice.
>

Sure. Please suggest if I should update patch to above suggested
approach. That will avoid setting num_scatter twice.

Thanks,
Jolly Shah

> Thanks,
> John
>
> > +     } else {
> >               task->data_dir = qc->dma_dir;
> > +     }
> >       task->scatter = qc->sg;
> >       task->ata_task.retry_count = 1;
> >       task->task_state_flags = SAS_TASK_STATE_PENDING;
> >
>