linux-kernel - Re: [PATCH 0/3] mmc: meson-gx: add ddr-access-quirk support

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Date:   Mon, 13 May 2019 11:58:43 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Neil Armstrong <narmstrong@...libre.com>, ulf.hansson@...aro.org,
        khilman@...libre.com
Cc:     baylibre-upstreaming@...ups.io, linux-mmc@...r.kernel.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] mmc: meson-gx: add ddr-access-quirk support

On Mon, 2019-05-13 at 11:15 +0200, Neil Armstrong wrote:
> On the Amlogic G12A SoC family, (only) the SDIO controller fails to access
> the data from DDR, leading to a broken controller.
> 
> Add the amlogic,ddr-access-quirk property so signal this particular
> controller has this bug and needs a quirk to work properly.
> 
> But each MMC controller has 1,5KiB of SRAM after the registers, that can
> be used as bounce buffer to avoid direct DDR access from the integrated
> DMAs (this SRAM may be used by the boot ROM when DDR is not yet initialized).
> 
> The quirk is to disable the chained descriptor for this controller, and
> use this SRAM memory zone as buffer for the bounce buffer fallback mode.
> 
> The performance hit hasn't been evaluated, but the fix has been tested
> using a WiFi AP6398S SDIO module, and the iperf3 Bandwidth measurement gave
> 55.2 Mbits/sec over a 63 Hours long test, with the SDIO ios set as High-Speed
> at 50MHz clock. It gave around 170 Mbits/sec as SDR104 and 200MHz clock.

These numbers looks to be limited by the MMC bandwidth of the related modes.
So, if the SRAM quirk introduce a penalty for the controller, it does not appear
to be a limiting factor, AFAICT.

> 
> Neil Armstrong (3):
>   dt-bindings: mmc: meson-gx: add ddr-access-quirk property
>   mmc: meson-gx: add ddr-access-quirk
>   arm64: dts: meson-g12a: add ddr-access-quirk property to SDIO
>     controller
> 
>  .../bindings/mmc/amlogic,meson-gx.txt         |  4 ++
>  arch/arm64/boot/dts/amlogic/meson-g12a.dtsi   |  1 +
>  drivers/mmc/host/meson-gx-mmc.c               | 65 +++++++++++++++----
>  3 files changed, 57 insertions(+), 13 deletions(-)
>