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

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

On 27/10/2022 23:25, Damien Le Moal wrote:
>> So we have this overall flow:
>>
>> ata_exec_internal_sg():
>>    -> alloc request
>>    -> blk_execute_rq_nowait()
>>        ... -> scsi_queue_rq()
>> 		-> sht->reserved_queuecommd()
>> 			-> ata_internal_queuecommand()
>>
>> And then we have ata_internal_queuecommand() -> ata_sas_queuecmd() ->
>> ata_scsi_queue_internal() -> ata_qc_issue().
>>
>> Hope it makes sense.
> OK. Got it.
> However, ata_exec_internal_sg() being used only from EH context with the
> queue quiesced, will blk_execute_rq_nowait() work ? Is there an exception
> for internal reserved tags ?
> 

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.

Thanks,
John