From: John Garry <john.garry@huawei.com>
To: Bart Van Assche <bvanassche@acm.org>, <jejb@linux.ibm.com>,
<martin.petersen@oracle.com>
Cc: <linux-scsi@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<ming.lei@redhat.com>
Subject: Re: [PATCH] scsi: core: Cap shost cmd_per_lun at can_queue
Date: Thu, 20 May 2021 18:24:02 +0100 [thread overview]
Message-ID: <457d23a9-deb0-4ee1-fe7f-5a63605d9686@huawei.com> (raw)
In-Reply-To: <439c6fb8-3799-bfae-7f44-9f8c26a7bf79@acm.org>
On 20/05/2021 17:57, Bart Van Assche wrote:
>> not be limited to 16b?
> Maybe I'm missing something but it is not clear to me why different
> structures in the SCSI headers use different data types for can_queue
> and cmd_per_lun?
For cmd_per_lun, is it related to SCSI task tag limit? SAM-3 says upto
64b for task tag, but then SAS uses 16b for TMF tag, so not sure.
Someone with more SCSI spec knowledge than we can clarify this.
>
> $ git grep -nHEw '(cmd_per_lun|can_queue);' include/scsi
> include/scsi/scsi_device.h:318: unsigned int can_queue;
> include/scsi/scsi_host.h:372: int can_queue;
> include/scsi/scsi_host.h:425: short cmd_per_lun;
> include/scsi/scsi_host.h:612: int can_queue;
> include/scsi/scsi_host.h:613: short cmd_per_lun;
>
>> It seems intentional that can_queue is int and cmd_per_lun is short.
> Intentional? It is not clear to me why? Even high-performance drivers
> like iSER and SRP set can_queue by default to a value that fits well in
> a 16-bit variable (512 and 64 respectively). The highest value that I
> found after a quick search is the following:
>
> #define ISCSI_TOTAL_CMDS_MAX 4096
I guess int was used for can_queue as an arbitrarily big number.
And if we try to use 16b for can_queue, reducing size of
variables/structure members sometimes breaks things, from my experience.
Thanks,
John
next prev parent reply other threads:[~2021-05-20 17:25 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-19 14:31 [PATCH] scsi: core: Cap shost cmd_per_lun at can_queue John Garry
2021-05-20 1:24 ` Ming Lei
2021-05-20 15:57 ` Bart Van Assche
2021-05-20 16:41 ` John Garry
2021-05-20 16:57 ` Bart Van Assche
2021-05-20 17:24 ` John Garry [this message]
2021-05-22 4:28 ` Martin K. Petersen
2021-05-22 4:35 ` Martin K. Petersen
2021-05-26 4:07 ` Martin K. Petersen
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