RE: [PATCH V2 01/13] pm80xx : Increase request sg length.

From: <Deepak.Ukey@microchip.com>
To: <jinpu.wang@cloud.ionos.com>
Cc: <linux-scsi@vger.kernel.org>,
	<Vasanthalakshmi.Tharmarajan@microchip.com>,
	<Viswas.G@microchip.com>, <jinpu.wang@profitbricks.com>,
	<martin.petersen@oracle.com>, <yuuzheng@google.com>,
	<auradkar@google.com>, <vishakhavc@google.com>,
	<bjashnani@google.com>, <radha@google.com>,
	<akshatzen@google.com>
Subject: RE: [PATCH V2 01/13] pm80xx : Increase request sg length.
Date: Fri, 17 Jan 2020 15:50:11 +0000	[thread overview]
Message-ID: <MN2PR11MB3550076952E7D79D68385AF8EF310@MN2PR11MB3550.namprd11.prod.outlook.com> (raw)
In-Reply-To: <CAMGffEmP+UfQS+9dUEjQvnJ0HZLRD-X_xJufRK_iQ8JTPQzb5A@mail.gmail.com>

On Fri, Jan 17, 2020 at 8:10 AM Deepak Ukey <deepak.ukey@microchip.com> wrote:
>
> From: Peter Chang <dpf@google.com>
>
> Increasing the per-request size maximum (max_sectors_kb) runs into the 
> per-device dma scatter gather list limit (max_segments) for users of 
> the io vector system calls (eg, readv and writev). This is because the 
> kernel combines io vectors into dma segments when possible, but it 
> doesn't work for our user because the vectors in the buffer cache get 
> scrambled.
> This change bumps the advertised max scatter gather length to 528 to 
> cover 2M w/ x86's 4k pages and some extra for the user checksum.
> It trims the size of some of the tables we don't care about and 
> exposes all of the command slots upstream to the scsi layer
>
> Signed-off-by: Peter Chang <dpf@google.com>
> Signed-off-by: Deepak Ukey <deepak.ukey@microchip.com>
> Signed-off-by: Viswas G <Viswas.G@microchip.com>
> Signed-off-by: Radha Ramachandran <radha@google.com>
> Reported-by: kbuild test robot <lkp@intel.com>
> ---
>  drivers/scsi/pm8001/pm8001_defs.h | 5 +++--  
> drivers/scsi/pm8001/pm8001_init.c | 2 +-
>  2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/scsi/pm8001/pm8001_defs.h 
> b/drivers/scsi/pm8001/pm8001_defs.h
> index 48e0624ecc68..1c7f15fd69ce 100644
> --- a/drivers/scsi/pm8001/pm8001_defs.h
> +++ b/drivers/scsi/pm8001/pm8001_defs.h
> @@ -75,7 +75,7 @@ enum port_type {
>  };
>
>  /* driver compile-time configuration */
> -#define        PM8001_MAX_CCB           512    /* max ccbs supported */
> +#define        PM8001_MAX_CCB           256    /* max ccbs supported */
Hi Deepack,

Why do you reduce PM8001_MAX_CCB?
--- PM8001 driver has a memory limit in the machine. If we increase the sg length, we need to trade-off it by decreasing PM8001_MAX_CCB. PM8001_MAX_CCB = 256 does not have any influence on normal use.

The patch itself looks fine.