From: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
Right now the dma engine officially supports dma channel 11 and 12.
But the use of dma channel 12 immediately stalls the system.
The reason is that the interrupt assigned right now to dma channel 11
is actually a shared interrupt for dma channels 11 to 12.
So whenever the dma channel 12, 13 or 14 is used, it triggers the interrupt
for dma 11 which only clears the interrupt flag for dma channel 11.
This results in the interrupt 11 is triggered all the time.
On top the requesting dma channel12 (and the corresponding irq) triggers
an immeditate stall of the system, because for the 12th interrupt
any dma channel will trigger also this interrupt - even if the dma channel
is controlled by the firmware.
As the description in the default device-tree is (for the above reasons)
wrong a new means had to get devised that allows correct representation
of the mapping between interrupts and dma channels.
For this the existing (but unused) device-tree property "interrupt-names"
is used to define the mapping.
Implementing shared interrupts is another necessity when using dma channel
11 to 14, so it is a part of this patch.
This patchset requires: [PATCH 0/8 v4] bcm2835 DMA slave support
to be applied.
Tested playing BigBuckBunny on:
* fb-tft device (fb_st7735r)
- via spi-bcm2835 using slave_sg dma
* I2S Hifiberry DAC (snd_soc_hifiberry_dac)
- via bcm2835-i2s using cyclic dma
(with a few pending patches to make i2s work propperly)
Interrupt statistics after playing Big Buck Bunny:
root@raspcm:~# cat /proc/interrupts
CPU0
17: 0 ARMCTRL-level 1 Edge 2000b880.mailbox
27: 47685 ARMCTRL-level 35 Edge timer
33: 6042130 ARMCTRL-level 41 Edge 20980000.usb, dwc2_hsotg:usb1
40: 13250 ARMCTRL-level 48 Edge DMA IRQ
42: 13250 ARMCTRL-level 50 Edge DMA IRQ
44: 19087 ARMCTRL-level 52 Edge DMA IRQ
45: 0 ARMCTRL-level 53 Edge DMA IRQ
73: 0 ARMCTRL-level 81 Edge 20200000.gpio:bank0
74: 0 ARMCTRL-level 82 Edge 20200000.gpio:bank1
77: 0 ARMCTRL-level 85 Edge 20205000.i2c, 20804000.i2c, 20805000.i2c
78: 0 ARMCTRL-level 86 Edge 20204000.spi
81: 708 ARMCTRL-level 89 Edge uart-pl011
86: 277629 ARMCTRL-level 94 Edge mmc0
Err: 0
Tested also with the following property set:
brcm,dma-channel-mask = <0x7800>;
which results in only dma channels 11-14 being available
and thus all using the single shared irq.
Interrupt statistics for this case:
CPU0
17: 0 ARMCTRL-level 1 Edge 2000b880.mailbox
27: 46843 ARMCTRL-level 35 Edge timer
33: 5412801 ARMCTRL-level 41 Edge 20980000.usb, dwc2_hsotg:usb1
51: 45502 ARMCTRL-level 59 Edge DMA IRQ, DMA IRQ, DMA IRQ, DMA IRQ
73: 0 ARMCTRL-level 81 Edge 20200000.gpio:bank0
74: 0 ARMCTRL-level 82 Edge 20200000.gpio:bank1
77: 0 ARMCTRL-level 85 Edge 20205000.i2c, 20804000.i2c, 20805000.i2c
78: 0 ARMCTRL-level 86 Edge 20204000.spi
81: 706 ARMCTRL-level 89 Edge uart-pl011
86: 297235 ARMCTRL-level 94 Edge mmc0
Err: 0
Note that support for more interrupts than irq 0 to 14
is not implemented (and this value is also hard-coded).
Allowing dma channel 15 to get controlled well would require
further extensive changes to the device-tree (separate register region).
Also as mentioned above dma-channel 15 is used by the firmware
and is - for all practical purposes - not available to
the kernel.
Only when this situation changes a valid effort can get made.
One major concern here is the fact that we have no dedicated interrupt
for dma channel 15. There is only the "catch all" interrupt, which - as
mentioned above - also triggers for dma channels used by the firmware,
which leads to an unnecessary amount of interrupts and can result
in system lockups...
The only potential usage would be for dma transfers where we do
not need an interrupt, but that may require further changes
to the dmaengine and more to support correctly.
One such "candidate" for the use of dma channel 15 could be
the tx-dma of spi-bcm2835, as for this specifically we only need
to get the interrupt when the rx-dma finishes, but that is a
very special case.
Avoiding the interrupt for the spi-tx-dma is still something that
could get investigated in a different patch-set.
Changelog:
V1 -> V2: added explicit documentation for "dma-shared-all"
to the dt-bindings documentation
Martin Sperl (3):
dt/bindings: bcm2835: add interrupt-names property
dmaengine: bcm2835: use platform_get_irq_byname
ARM: bcm2835: add interrupt-names and apply correct mapping
.../devicetree/bindings/dma/brcm,bcm2835-dma.txt | 26 ++++++++
arch/arm/boot/dts/bcm283x.dtsi | 22 ++++++-
drivers/dma/bcm2835-dma.c | 77 ++++++++++++++++++----
3 files changed, 110 insertions(+), 15 deletions(-)
--
2.1.4
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