[PATCH] ARM: OMAP2+: Fix oops with LPAE and more than 2GB of memory

[Arnd Bergmann]

On Tuesday 13 October 2015 16:13:20 Tony Lindgren wrote:
> On boards with more than 2GB of RAM booting goes wrong with things not working
> and we're getting lots of l3 warnings:
> 
> WARNING: CPU: 0 PID: 1 at drivers/bus/omap_l3_noc.c:147 l3_interrupt_handler+0x260/0x384()
> 44000000.ocp:L3 Custom Error: MASTER MMC6 TARGET DMM1 (Idle): Data Access in User mode during Functional access
> ...
> [<c044e158>] (scsi_add_host_with_dma) from [<c04705c8>] (ata_scsi_add_hosts+0x5c/0x18c)
> [<c04705c8>] (ata_scsi_add_hosts) from [<c046b13c>] (ata_host_register+0x150/0x2cc)
> [<c046b13c>] (ata_host_register) from [<c046b38c>] (ata_host_activate+0xd4/0x124)
> [<c046b38c>] (ata_host_activate) from [<c047f42c>] (ahci_host_activate+0x5c/0x194)
> [<c047f42c>] (ahci_host_activate) from [<c0480854>] (ahci_platform_init_host+0x1f0/0x3f0)
> [<c0480854>] (ahci_platform_init_host) from [<c047c9dc>] (ahci_probe+0x70/0x98)
> [<c047c9dc>] (ahci_probe) from [<c04220cc>] (platform_drv_probe+0x54/0xb4)
> 
> Let's fix the issue by enabling ZONE_DMA for LPAE.
> 
> Signed-off-by: Tony Lindgren <tony at atomide.com>
> 

I suspect this is not the correct fix, even if it works around the problem.

Am I right that the AHCI device can access the first 4GB of address space
using 32-bit DMA, and that any RAM beyond 2GB is above that limit?

Does the ZONE_DMA have the same size? If not, you get more bounce buffers
than you want, which is bad for performance/

Another problem here is that it only works with the SCSI and net subsystems
that have a hack in there to create manual bounce buffers, but other drivers
that can do DMA to high addresses will not know about this.

The right solution would be to force the use of an IOMMU, and if that not
works, add support for SWIOTLB on ARM.

	Arnd