[PATCH v2 0/5] Fix broken DMAFLUSHP on Rockchips platform

Sonny Rao sonnyrao at chromium.org
Mon Aug 31 14:40:10 PDT 2015

On Thu, Aug 27, 2015 at 5:36 PM, Shawn Lin <shawn.lin at rock-chips.com> wrote:
> The purpose of the DMAFLUSHP instruction:
> - Tell the peripheral to clear its status and control registers.
> - Send a message to the peripheral to resend its level status.
> There are 3 timings described in PL330 Technical Reference Manual:
> - Timing 1: Burst request, can work well without DMAFLUSHP.
> - Timing 2: Single and burst request, DMAC will ignore the single
>             transfer request. This timing happens if there are single
>             and burst request.
> - Timing 3: Single transfers for a burst request, DMAC should signals
>             datype to request the peripheral to flush the contents of
>             any control registers. This timing happens if there is
>             not enough MFIFO to places the burst data.
> A peripheral may signal a DMA request during the execution of
> DMAFLUSHP instruction, that cause DMA request being ignored by DMAC.
> But DMAC and all peripherals on RK3X SoCs DO NOT support DMAFLUSHP.
> It can't send a message to the peripheral to resend DMA request,
> and the peripheral can't acknowledge a flush request from DMAC.
> So all DMA requests should NOT be ignored by DMAC, and DMAC will not
> notify the peripheral to flush.
> To fix this problem, we need:
> - Do NOT execute DMAFLUSHP instruction.
> - Timing 2 and timing 3 should not happen.
> Because on RK3X SoCs, there are 6 or below  channels and 32 MFIFO depth
> for DMAC_BUS, and 8 channels and 64 MFIFO depth for DMAC_PERI, it is
> impossible to hit the timing 3 if burst length is equal or less than 4.

Fixing this issue also requires changes to drivers, so it would be
nice if you put those changes into the same patchset.
Otherwise someone may apply this series and expect things to work but
they will still be broken. Specifically the peripherals should be
setting their burst sizes for their DMA requests low enough to avoid
needing the working DMAFLUSHP instruction.

Also, I remember we ran into an issue when we tried using burst length
of 4 with the i2s device on RK3288 because we could get requests that
either weren't aligned or a multiple of 4 sizes and some transfers
would just fail, so we ended up using a burst size of 1.  I recommend
if we aren't sure about size or alignment for a particular peripheral,
a burst size of 1 is safest.  For something like a block device, I
think we can use the larger size bursts.  That's another reason to
include the driver fixes in the series, just so we get it right,

> Since the request type signal by the peripheral can only be set by
> software. We can set Rockchip Soc's GRF_PERIDMAC_CON0[2:1] to select single
> or burst request, if it is set b01,  all of the peripharals will signal a brust
> request. So the timing 2 will not happen, too.
> So DMAC on RK3X can support single or burst transfer, but can't support
> mixed transfer.
> Because burst transfer is more efficient than single transfer, this is
> confirmed by our ASIC team, who strongly suggest to use burst transfer.
> And this is confirmed by Addy's test on RK3288-Pink2 board, the speed of
> spi flash burst transfer will increase about two times than single transfer.
> Also, I have tested dw_mmc with pl330 on RK3188 platform to double confirm
> the result. That means burst transfer is reansonable.
> So we need a quirk not to execute DMAFLUSHP instruction and to use burst
> transfer.
> Note:
> - The Rockchip Soc default value of GRF_PERIDMAC_CON0[2:1] is b01. To
>   support brust transfer, these bits should not be changed in bootloader.
> Changes in v2:
> - amend the author
> - reorder the patches suggested by Doug
> - add Reviewed-by: Doug Anderson <dianders at chromium.org> for
>   rk3288.dtsi patch and arm-pl330.txt patch
> - amend Olof's mail address
> Changes in v1:
> - rename broken-no-flushp to "arm,pl330-broken-no-flushp" suggested
>   by Krzysztof.
> - add From original author.
> - remove Sunny's tag
> Addy Ke (2):
>   DMA: pl330: add quirk for broken no flushp
>   ARM: dts: Add arm,pl330-broken-no-flushp quirk for rk3288 platform
> Boojin Kim (1):
>   DMA: pl330: support burst mode for dev-to-mem and mem-to-dev transmit
> Shawn Lin (2):
>   Documentation: arm-pl330: add description of
>     arm,pl330-broken-no-flushp
>   ARM: dts: Add arm,pl330-broken-no-flushp quirk for rk3xxx platform
>  .../devicetree/bindings/dma/arm-pl330.txt          |   1 +
>  arch/arm/boot/dts/rk3288.dtsi                      |   3 +
>  arch/arm/boot/dts/rk3xxx.dtsi                      |   3 +
>  drivers/dma/pl330.c                                | 101 +++++++++++++++------
>  4 files changed, 79 insertions(+), 29 deletions(-)
> --
> 2.3.7
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