Re: [PATCH 00/18] ARM: Add minimal Raspberry Pi 4 support

[Nicolas Saenz Julienne <nsaenzjulienne@suse.de>]

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On Tue, 2019-07-23 at 11:34 +0200, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 08:10:17PM +0200, Stefan Wahren wrote:
> > i rebased this series also and got this only on the RPi 4.
> > 
> > After reverting the following:
> > 
> > 79a986721de dma-mapping: remove dma_max_pfn
> > 7559d612dff0 mmc: core: let the dma map ops handle bouncing
> > 
> > This crash disappear, but wifi seems to be still broken.
> > 
> > Would be nice, if you can investigate further.
> 
> That means dma addressing on this system doesn't just work for some
> memory, and the mmc bounce buffering was papering over that just for
> mmc.  Do you have highmem on this system? 
> 
> You might want to try this series, which has been submitted upstream:
> 
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/arm-swiotlb

Hi Christoph,
I tried your series on top of Stefan's, it has no effect. I guess it's no
surprise as with mult_v7_defconfig, you get SWIOTLB=n & LPAE=n.

FYI DMA addressing constraints for RPi4 are the following: devices can only
access the first GB of ram even though the board might have up to 4GB of ram.
The DMA addresses are aliased with a 0xc0000000 offset. So 0x00000000 phys is
aliased to 0xc0000000 in DMA. This is the same as for an RFC you commented last
week trying to fix similar issues for arm64.

You state in "arm: use swiotlb for bounce buffer on LPAE configs" that "The DMA
API requires that 32-bit DMA masks are always supported". If I understand it
correctly this device breaks that assumption. Which implies we need a bounce
buffer system in place for any straming DMA user.

It seems we're unable to use dma-direct/swiotlb, so I enabled arm's dmabounce
on all devices hooked into RPi's limited interconnect, which fixes this issue.
Any thoughts on this?

diff --git a/arch/arm/mach-bcm/Kconfig b/arch/arm/mach-bcm/Kconfig
index 5e5f1fabc3d4..3db8deed83a6 100644
--- a/arch/arm/mach-bcm/Kconfig
+++ b/arch/arm/mach-bcm/Kconfig
@@ -168,6 +168,7 @@ config ARCH_BCM2835
        select PINCTRL
        select PINCTRL_BCM2835
        select MFD_CORE
+       select DMABOUNCE
        help
          This enables support for the Broadcom BCM2835 and BCM2836 SoCs.
          This SoC is used in the Raspberry Pi and Roku 2 devices.
diff --git a/arch/arm/mach-bcm/board_bcm2835.c b/arch/arm/mach-bcm/board_bcm2835.c
index c09cf25596af..be788849c4bb 100644
--- a/arch/arm/mach-bcm/board_bcm2835.c
+++ b/arch/arm/mach-bcm/board_bcm2835.c
@@ -3,6 +3,8 @@
  * Copyright (C) 2010 Broadcom
  */
 
+#include <linux/device.h>
+#include <linux/dma-mapping.h>
 #include <linux/init.h>
 #include <linux/irqchip.h>
 #include <linux/of_address.h>
@@ -24,8 +26,37 @@ static const char * const bcm2835_compat[] = {
        NULL
 };
 
+static int bcm2835_needs_bounce(struct device *dev, dma_addr_t dma_addr, size_t size)
+{
+       /*
+        * The accepted dma addresses are [0xc0000000, 0xffffffff] which map to
+        * ram's [0x00000000, 0x3fffffff].
+        */
+       return dma_addr < 3ULL * SZ_1G;
+}
+
+/*
+ * Setup DMA mask to 1GB on devices hanging from soc interconnect
+ */
+static int bcm2835_platform_notify(struct device *dev)
+{
+       if (dev->parent && !strcmp("soc", dev_name(dev->parent))) {
+               dev->dma_mask = &dev->coherent_dma_mask;
+               dev->coherent_dma_mask = DMA_BIT_MASK(30); /* 1GB */
+               dmabounce_register_dev(dev, 2048, 4096, bcm2835_needs_bounce);
+       }
+
+       return 0;
+}
+
+void __init bcm2835_init_early(void)
+{
+       platform_notify = bcm2835_platform_notify;
+}
+
 DT_MACHINE_START(BCM2835, "BCM2835")
        .dma_zone_size  = SZ_1G,
        .dt_compat = bcm2835_compat,
        .smp = smp_ops(bcm2836_smp_ops),
+       .init_early = bcm2835_init_early,
 MACHINE_END

 Regards,
 Nicolas


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