linux-kernel - Re: [RFC PATCH v5 6/6] drm: exynos: mixer: Add interconnect support

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Date:   Mon, 1 Jun 2020 16:58:18 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Sylwester Nawrocki <s.nawrocki@...sung.com>,
        georgi.djakov@...aro.org, krzk@...nel.org
Cc:     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-samsung-soc@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v5 6/6] drm: exynos: mixer: Add interconnect support

Hi Sylwester,

On 5/30/20 1:32 AM, Sylwester Nawrocki wrote:
> From: Marek Szyprowski <m.szyprowski@...sung.com>
> 
> 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 bandiwidth value is calculated from selected DRM mode, i.e.
> video plane width, height, refresh rate and pixel format.
> 
> Co-developed-by: Artur Świgoń <a.swigon@...sung.com>
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> [s.nawrocki: renamed soc_path variable to icc_path, edited commit desc.]
> Signed-off-by: Sylwester Nawrocki <s.nawrocki@...sung.com>
> ---
> Changes for v5:
>  - renamed soc_path variable to icc_path
> ---
>  drivers/gpu/drm/exynos/exynos_mixer.c | 73 ++++++++++++++++++++++++++++++++---
>  1 file changed, 68 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/exynos/exynos_mixer.c b/drivers/gpu/drm/exynos/exynos_mixer.c
> index 21b726b..bdae683 100644
> --- a/drivers/gpu/drm/exynos/exynos_mixer.c
> +++ b/drivers/gpu/drm/exynos/exynos_mixer.c
> @@ -13,6 +13,7 @@
>  #include <linux/component.h>
>  #include <linux/delay.h>
>  #include <linux/i2c.h>
> +#include <linux/interconnect.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
>  #include <linux/kernel.h>
> @@ -98,6 +99,7 @@ struct mixer_context {
>  	struct exynos_drm_crtc	*crtc;
>  	struct exynos_drm_plane	planes[MIXER_WIN_NR];
>  	unsigned long		flags;
> +	struct icc_path		*icc_path;
>  
>  	int			irq;
>  	void __iomem		*mixer_regs;
> @@ -934,6 +936,42 @@ static void mixer_disable_vblank(struct exynos_drm_crtc *crtc)
>  	mixer_reg_writemask(mixer_ctx, MXR_INT_EN, 0, MXR_INT_EN_VSYNC);
>  }
>  
> +static void mixer_set_memory_bandwidth(struct exynos_drm_crtc *crtc)
> +{
> +	struct drm_display_mode *mode = &crtc->base.state->adjusted_mode;
> +	struct mixer_context *ctx = crtc->ctx;
> +	unsigned long bw, bandwidth = 0;
> +	u32 avg_bw, peak_bw;
> +	int i, j, sub;
> +
> +	if (!ctx->icc_path)
> +		return;
> +
> +	for (i = 0; i < MIXER_WIN_NR; i++) {
> +		struct drm_plane *plane = &ctx->planes[i].base;
> +		const struct drm_format_info *format;
> +
> +		if (plane->state && plane->state->crtc && plane->state->fb) {
> +			format = plane->state->fb->format;
> +			bw = mode->hdisplay * mode->vdisplay *
> +							drm_mode_vrefresh(mode);
> +			if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> +				bw /= 2;
> +			for (j = 0; j < format->num_planes; j++) {
> +				sub = j ? (format->vsub * format->hsub) : 1;
> +				bandwidth += format->cpp[j] * bw / sub;

First of all, I agree this approach.

Could you please add more detailed comments for understadning
about this calculation? As you commented, it seems that
the final bandwidth contains the width/height/refresh rate
and pixel format. If you add one real example, it will be very helpful.


(snip)

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics