linux-kernel - RE: [PATCH] sd: remove redundant check for BLK_DEF_MAX

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Date:	Wed, 8 Jun 2016 04:22:21 +0000
From:	Long Li <longli@...rosoft.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	Tom Yan <tom.ty89@...il.com>,
	"James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] sd: remove redundant check for BLK_DEF_MAX_SECTORS

Hi Martin,

Thanks for looking into this. The problem I'm trying to solve is that, I want to have lower layer driver to setup max_sectors bigger than BLK_DEF_MAX_SECTORS. In Hyper-v, we use 2MB max transfer I/O size, in future version the max transfer I/O size will increase to 8MB.

The implementation of sd.c limits the maximum value of max_sectors  to BLK_DEF_MAX_SECTORS.  Because sd_revalidate_disk is called late in the SCSI disk initialization process, there is no way for a lower layer driver to set this value to its "bigger" optimal size. 

The reason why I think it may not be necessary for sd.c to setup max_sectors, it's because this value may have already been setup twice before reaching the code in sd.c:
1. When this disk device is first scanned, or re-scanned (in scsi_scan.c), where it eventually calls __scsi_init_queue(), and use the max_sectors in the scsi_host_template.
2. in slave_configure of scsi_host_template, when the lower layer driver implements this function in its template and it can change this value there.

Long

> -----Original Message-----
> From: Martin K. Petersen [mailto:martin.petersen@...cle.com]
> Sent: Monday, June 6, 2016 8:42 PM
> To: Long Li <longli@...rosoft.com>
> Cc: Tom Yan <tom.ty89@...il.com>; James E.J. Bottomley
> <jejb@...ux.vnet.ibm.com>; Martin K. Petersen
> <martin.petersen@...cle.com>; linux-scsi@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH] sd: remove redundant check for
> BLK_DEF_MAX_SECTORS
> 
> >>>>> "Long" == Long Li <longli@...rosoft.com> writes:
> 
> Long,
> 
> Long> The reason is that, max_sectors already has value at this point,
> Long> the default value is SCSI_DEFAULT_MAX_SECTORS
> Long> (include/scsi/scsi_host.h). The lower layer host driver can change
> Long> this value in its template.
> 
> The LLD sets max_hw_sectors which indicates the capabilities of the
> controller DMA hardware. Whereas the max_sectors limit is set by sd to
> either follow advise by the device or--if not provided--use the block layer
> default. max_sectors governs the size of READ/WRITE requests and do not
> reflect the capabilities of the DMA hardware.
> 
> Long> I think the drivers care about this value have already set it. So
> Long> it's better not to change it again. If they want max_sectors to be
> Long> set by sd, they can use BLOCK LIMITS VPD to tell it to do so.
> 
> Most drivers don't have the luxury of being able to generate VPDs for their
> attached target devices :)
> 
> --
> Martin K. Petersen	Oracle Linux Engineering