[openwrt/openwrt] oxnas: reduce size of ATA DMA descriptor space

[LEDE Commits]

dangole pushed a commit to openwrt/openwrt.git, branch openwrt-19.07:
https://git.openwrt.org/5a1e4a7fdb2d732a485afb7cdcfc359e15ac259e

commit 5a1e4a7fdb2d732a485afb7cdcfc359e15ac259e
Author: Daniel Golle <daniel at makrotopia.org>
AuthorDate: Mon Aug 10 23:33:00 2020 +0100

    oxnas: reduce size of ATA DMA descriptor space
    
    After years of trying to find the reason for random kernel crashes
    while both CPU and SATA are under load it has been found.
    Some odd commented-out #defines in kref's single-port driver [1] which
    were copied from the vendor driver made me develop a theory:
    The IO-mapped memory area for DMA descriptors apparetly got some holes
    just before the alignment boundaries.
    This feels like an off-by-one bug in the hardware or maybe those fields
    are used internally by the SATA controller's firmware.
    Whatever the cause is: they cannot be used and trying to use them
    results in reading back unexpected stuff and ends up with oopsing
    Unable to handle kernel paging request at virtual address d085c004
    
    Work around the issue by reducing the area used for bmdma descriptors.
    This reduces SATA performance (iops) quite a bit, but finally makes
    things work reliably. Possibly one could optimize this much more by
    really just skipping the holes in that memory area -- however, that
    seems to be non-trivial with the driver and libata in it's current form
    (suggestions are welcome).
    The 'proper' way to have good SATA performance would be to make use of
    the hardware RAID features (one can use the JBOD mode to access even
    just a single disc transparently through the RAID controller integrated
    in the SATA host instead of accessing the SATA ports 'raw' as we do
    now).
    
    [1]: https://github.com/kref/linux-oxnas/blob/master/drivers/ata/sata_oxnas.c#L25
    
    Signed-off-by: Daniel Golle <daniel at makrotopia.org>
    (cherry picked from commit 5793112f751ee3d9f841af4846d68e6b1ff1bff4,
    including fixup commit d75e75306301852a848824cf268d8b58eda28a8a)
---
 target/linux/oxnas/files/drivers/ata/sata_oxnas.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/linux/oxnas/files/drivers/ata/sata_oxnas.c b/target/linux/oxnas/files/drivers/ata/sata_oxnas.c
index 64afa728a1..ecfbf5f01a 100644
--- a/target/linux/oxnas/files/drivers/ata/sata_oxnas.c
+++ b/target/linux/oxnas/files/drivers/ata/sata_oxnas.c
@@ -70,7 +70,7 @@ struct sgdma_request {
 
 /* Controller information */
 enum {
-	SATA_OXNAS_MAX_PRD = 254,
+	SATA_OXNAS_MAX_PRD = 63,
 	SATA_OXNAS_DMA_SIZE = SATA_OXNAS_MAX_PRD *
 				sizeof(struct ata_bmdma_prd) +
 				sizeof(struct sgdma_request),