Re: [PATCH RFC v3 16/22] ata: libata-scsi: Allocate sdev early in port probe

Hannes Reinecke
2022-10-27T09:51:41Z

On 10/27/22 11:16, Damien Le Moal wrote:
> On 10/27/22 17:11, Hannes Reinecke wrote:
>> On 10/27/22 03:34, Damien Le Moal wrote:
>>> On 10/25/22 19:18, John Garry wrote:
>>>> Currently the per-ata device sdev is allocated as part of the scsi
>>>> target
>>>> scan, which is after the ata port probe.
>>>>
>>>> However it is useful to have the sdev available in the port probe. As an
>>>> example of an advantage, if the request queue is available in the probe
>>>> (which it would be if the sdev is available), then it is possible to use
>>>> a SCSI cmnd for ATA internal commands. The benefit of this is then we
>>>> can
>>>> put the ATA qc structure in the SCSI cmnd private data. It will also be
>>>> useful if we want to send ATA internal commands as requests.
>>>>
>>>> Export scsi_target_reap() so that it can be used to put the extra
>>>> reference we get when allocating the sdev.
>>>>
>>>> Signed-off-by: John Garry <john.garry@huawei.com>
>>>> ---
>>>>   drivers/ata/libata-eh.c   |  1 +
>>>>   drivers/ata/libata-scsi.c | 23 +++++++++--------------
>>>>   drivers/scsi/scsi_scan.c  |  1 +
>>>>   3 files changed, 11 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
>>>> index 08e11bc312c2..1ed5b1b64792 100644
>>>> --- a/drivers/ata/libata-eh.c
>>>> +++ b/drivers/ata/libata-eh.c
>>>> @@ -3446,6 +3446,7 @@ static int ata_eh_schedule_probe(struct
>>>> ata_device *dev)
>>>>         ata_eh_detach_dev(dev);
>>>>       ata_dev_init(dev);
>>>> +    ata_scsi_setup_sdev(dev);
>>>>       ehc->did_probe_mask |= (1 << dev->devno);
>>>>       ehc->i.action |= ATA_EH_RESET;
>>>>       ehc->saved_xfer_mode[dev->devno] = 0;
>>>> diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
>>>> index efdba852e363..476e0ef4bd29 100644
>>>> --- a/drivers/ata/libata-scsi.c
>>>> +++ b/drivers/ata/libata-scsi.c
>>>> @@ -1109,7 +1109,12 @@ int ata_scsi_dev_config(struct scsi_device
>>>> *sdev, struct ata_device *dev)
>>>>       if (dev->flags & ATA_DFLAG_TRUSTED)
>>>>           sdev->security_supported = 1;
>>>>   -    dev->sdev = sdev;
>>>> +    /*
>>>> +     * Put extra reference which we get when allocating the starget
>>>> +     * initially
>>>> +     */
>>>> +    scsi_target_reap(scsi_target(sdev));
>>>> +
>>>>       return 0;
>>>>   }
>>>>   @@ -4289,26 +4294,16 @@ void ata_scsi_scan_host(struct ata_port
>>>> *ap, int sync)
>>>>    repeat:
>>>>       ata_for_each_link(link, ap, EDGE) {
>>>>           ata_for_each_dev(dev, link, ENABLED) {
>>>> -            struct scsi_device *sdev;
>>>> +            struct Scsi_Host *shost = ap->scsi_host;
>>>>               int channel = 0, id = 0;
>>>>   -            if (dev->sdev)
>>>> -                continue;
>>>> -
>>>>               if (ata_is_host_link(link))
>>>>                   id = dev->devno;
>>>>               else
>>>>                   channel = link->pmp;
>>>>   -            sdev = __scsi_add_device(ap->scsi_host, channel, id, 0,
>>>> -                         NULL);
>>>> -            if (!IS_ERR(sdev)) {
>>>> -                dev->sdev = sdev;
>>>> -                ata_scsi_assign_ofnode(dev, ap);
>>>
>>> Is there something equivalent to what this function does inside
>>> scsi_scan_target() ? I had a quick look but did not see anything...
>>>
>> Typically, the SCSI layer has two ways of scanning.
>> One it the old-style serial scanning (originating in the old SCSI
>> parallel model):
>> The scanning code will blindly scan _all_ devices up to max_luns, and
>> attach every device for which the scanning code returns 'OK'.
>> The other one is to issue REPORT_LUNS and scan all LUNs returned there.
>>
>> Mapped to libata we would need to figure out a stable SCSI enumeration,
>> given that we have PMP and slave devices to content with.
>> To my knowledge we have ATA ports, each can have either a 'master' and
>> 'slave' device, _or_ it be a PMP port when it can support up to 16
>> devices, right?
>
> yes
>
>> Point being, master/slave and PMP are exclusive, right?
>
> Never heard of pmp with ide cable :)
>
See?

>> So we can make the master as LUN 0, and the slave as LUN 1.
>
> Yes, but isn't that a little wrong ? That would assume that the ata port
> is the device and the ata devices the luns of that device. But beside
> the "link busy" stuff that needs to be dealt with, master and slave are
> independent devices, unlike LUNs. No ?
> Well; technically, yes.

But we already enumerate the ata ports (which is typically done by the
hardware/PCI layer etc), and if we were try to model slave devices as
independent ports we would either have to insert a numbering (awkward)
or add a number at the en (even more awkward).

And the one key takeaway from the 'multiple actuators' discussion is
that LUNs _are_ independent (cf all the hoops they had to jump through
to define a command spanning several LUNs ...)(which, incidentally, we
could leverage here, too ...), and the target port really only serves as
an enumeration thingie to address the LUNs.

So we _could_ map the master device on LUN 0 and the slave device on LUN
1 with no loss of functionality, _but_ enable a consistent SCSI enumeration.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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