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Date:   Fri, 28 Oct 2022 09:33:19 +0100
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Subject: Re: [PATCH RFC v3 2/7] ata: libata-scsi: Add
 ata_internal_queuecommand()
To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        <jejb@linux.ibm.com>, <martin.petersen@oracle.com>, <hare@suse.de>,
        <bvanassche@acm.org>, <hch@lst.de>, <ming.lei@redhat.com>,
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        <linux-ide@vger.kernel.org>, <linux-scsi@vger.kernel.org>,
        <linuxarm@huawei.com>, <john.garry2@mail.dcu.ie>
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From:   John Garry <john.garry@huawei.com>
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On 28/10/2022 09:07, Damien Le Moal wrote:
>> Well, yeah. So if some error happens and EH kicks in, then full queue
>> depth of requests may be allocated. I have seen this for NCQ error. So
>> this is why I make in very first patch change allow us to allocate
>> reserved request from sdev request queue even when budget is fully
>> allocated.
>>
>> Please also note that for AHCI, I make reserved depth =1, while for SAS
>> controllers it is greater. This means that in theory we could alloc > 1x
>> reserved command for SATA disk, but I don't think it matters.
> Yes, 1 is enough. However, is 1 reserved out of 32 total, meaning that the
> user can only use 31 tags ? or is it 32+1 reserved ? which we can do since
> when using the reserved request, we will not use a hw tag (all reserved
> requests will be non-ncq).
> 
> The 32 + 1 scheme will work. 

Yes, 32 + 1 is what we want. I now think that there is a mistake in my 
code in this series for ahci.

So I think we need for ahci:

can_queue = 33
nr_reserved_cmds = 1

while I only have can_queue = 32

I need to check that again for ahci driver and AHCI SHT...

> But for CDL command completion handling, we
> will need a NCQ command to do a read log, to avoid forcing a queue drain.
> For that to reliably work, we'll need a 31+1+1 setup...
> 

So is your idea to permanently reserve 1 more command from 32 commands ? 
Or re-use 1 from 32 (and still also have 1 separate internal command)?

Thanks,
John