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Date:   Sat, 31 Oct 2020 13:47:17 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Sylwester Nawrocki <s.nawrocki@...sung.com>
Cc:     georgi.djakov@...aro.org, cw00.choi@...sung.com,
        devicetree@...r.kernel.org, robh+dt@...nel.org,
        a.swigon@...sung.com, myungjoo.ham@...sung.com,
        inki.dae@...sung.com, sw0312.kim@...sung.com,
        b.zolnierkie@...sung.com, m.szyprowski@...sung.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v7 6/6] drm: exynos: mixer: Add interconnect support

On Fri, Oct 30, 2020 at 01:51:49PM +0100, Sylwester Nawrocki wrote:
> This patch adds interconnect support to exynos-mixer. The mixer works
> the same as before when CONFIG_INTERCONNECT is 'n'.
> 
> For proper operation of the video mixer block we need to ensure the
> interconnect busses like DMC or LEFTBUS provide enough bandwidth so
> as to avoid DMA buffer underruns in the mixer block. I.e we need to
> prevent those busses from operating in low perfomance OPPs when
> the mixer is running.
> In this patch the bus bandwidth request is done through the interconnect
> API, the bandwidth value is calculated from selected DRM mode, i.e.
> video plane width, height, refresh rate and pixel format.
> 
> The bandwidth setting is synchronized with VSYNC when we are switching
> to lower bandwidth. This is required to ensure enough bandwidth for
> the device since new settings are normally being applied in the hardware
> synchronously with VSYNC.
> 
> Co-developed-by: Artur Świgoń <a.swigon@...sung.com>
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> Signed-off-by: Sylwester Nawrocki <s.nawrocki@...sung.com>
> ---
> Changes for v7:
>  - fixed incorrect setting of the ICC bandwidth when the mixer is
>    disabled, now the bandwidth is set explicitly to 0 in such case.
> 
> Changes for v6:
>  - the icc_set_bw() call is now only done when calculated value for
>    a crtc changes, this avoids unnecessary calls per each video frame
>  - added synchronization of the interconnect bandwidth setting with
>    the mixer VSYNC in order to avoid buffer underflow when we lower
>    the interconnect bandwidth when the hardware still operates with
>    previous mode settings that require higher bandwidth. This fixed
>    IOMMU faults observed e.g. during switching from two planes to
>    a single plane operation.
> 
> Changes for v5:
>  - renamed soc_path variable to icc_path
> ---
>  drivers/gpu/drm/exynos/exynos_mixer.c | 146 ++++++++++++++++++++++++++++++++--
>  1 file changed, 138 insertions(+), 8 deletions(-)


[...]

> @@ -1223,19 +1330,33 @@ static int mixer_probe(struct platform_device *pdev)
>  	struct device *dev = &pdev->dev;
>  	const struct mixer_drv_data *drv;
>  	struct mixer_context *ctx;
> +	struct icc_path *path;
>  	int ret;
>  
> +	/*
> +	 * Returns NULL if CONFIG_INTERCONNECT is disabled.

You could add here:
or if "interconnects" property does not exist.

Best regards,
Krzysztof

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