Re: [PATCH RFC v3 2/7] ata: libata-scsi: Add ata_internal_queuecommand()

[John Garry <john.garry@huawei.com>]

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