scsi: pm80xx: Increase request sg length

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.  Also reduced the PM8001_MAX_CCB to 256 as
pm8001 driver has memory limit depend on machine capability. 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

Link: https://lore.kernel.org/r/20200316074906.9119-2-deepak.ukey@microchip.comReported-by: kbuild test robot <lkp@intel.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
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>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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 */
 #define PM8001_MPI_QUEUE         1024   /* maximum mpi queue entries */
 #define	PM8001_MAX_INB_NUM	 1
 #define	PM8001_MAX_OUTB_NUM	 1
@@ -99,7 +99,8 @@ enum port_type {
 #define OB			(CI + PM8001_MAX_SPCV_INB_NUM)
 #define PI			(OB + PM8001_MAX_SPCV_OUTB_NUM)
 #define USI_MAX_MEMCNT		(PI + PM8001_MAX_SPCV_OUTB_NUM)
-#define PM8001_MAX_DMA_SG	SG_ALL
+#define	CONFIG_SCSI_PM8001_MAX_DMA_SG	528
+#define PM8001_MAX_DMA_SG	CONFIG_SCSI_PM8001_MAX_DMA_SG
 enum memory_region_num {
 	AAP1 = 0x0, /* application acceleration processor */
 	IOP,	    /* IO processor */

drivers/scsi/pm8001/pm8001_init.c

@@ -95,7 +95,7 @@ static struct scsi_host_template pm8001_sht = {
 	.bios_param		= sas_bios_param,
 	.can_queue		= 1,
 	.this_id		= -1,
-	.sg_tablesize		= SG_ALL,
+	.sg_tablesize		= PM8001_MAX_DMA_SG,
 	.max_sectors		= SCSI_DEFAULT_MAX_SECTORS,
 	.eh_device_reset_handler = sas_eh_device_reset_handler,
 	.eh_target_reset_handler = sas_eh_target_reset_handler,