linux-kernel - Re: [PATCH v14 05/10] drm/mediatek: Add MT8195 Embedded DisplayPort driver

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Date:   Thu, 21 Jul 2022 14:24:58 +0800
From:   CK Hu <ck.hu@...iatek.com>
To:     Rex-BC Chen <rex-bc.chen@...iatek.com>,
        "chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "mripard@...nel.org" <mripard@...nel.org>,
        "tzimmermann@...e.de" <tzimmermann@...e.de>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "deller@....de" <deller@....de>,
        "airlied@...ux.ie" <airlied@...ux.ie>
CC:     "msp@...libre.com" <msp@...libre.com>,
        "granquet@...libre.com" <granquet@...libre.com>,
        "Jitao Shi (石记涛)" 
        <jitao.shi@...iatek.com>,
        "wenst@...omium.org" <wenst@...omium.org>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>,
        "LiangXu Xu (徐亮)" 
        <LiangXu.Xu@...iatek.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        Project_Global_Chrome_Upstream_Group 
        <Project_Global_Chrome_Upstream_Group@...iatek.com>
Subject: Re: [PATCH v14 05/10] drm/mediatek: Add MT8195 Embedded DisplayPort
 driver

Hi, Rex:

On Thu, 2022-07-21 at 10:38 +0800, Rex-BC Chen wrote:
> On Fri, 2022-07-15 at 16:51 +0800, CK Hu wrote:
> > Hi, Bo-Chen:
> > 
> > On Tue, 2022-07-12 at 19:12 +0800, Bo-Chen Chen wrote:
> > > From: Markus Schneider-Pargmann <msp@...libre.com>
> > > 
> > > This patch adds a embedded displayport driver for the MediaTek
> > > mt8195
> > > SoC.
> > > 
> > > It supports the MT8195, the embedded DisplayPort units. It offers
> > > DisplayPort 1.4 with up to 4 lanes.
> > > 
> > > The driver creates a child device for the phy. The child device
> > > will
> > > never exist without the parent being active. As they are sharing
> > > a
> > > register range, the parent passes a regmap pointer to the child
> > > so
> > > that
> > > both can work with the same register range. The phy driver sets
> > > device
> > > data that is read by the parent to get the phy device that can be
> > > used
> > > to control the phy properties.
> > > 
> > > This driver is based on an initial version by
> > > Jitao shi <jitao.shi@...iatek.com>
> > > 
> > > Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>
> > > Signed-off-by: Guillaume Ranquet <granquet@...libre.com>
> > > Signed-off-by: Bo-Chen Chen <rex-bc.chen@...iatek.com>
> > > ---
> > 
> > [snip]
> > 
> > > +static void mtk_dp_hpd_sink_event(struct mtk_dp *mtk_dp)
> > > +{
> > > +	ssize_t ret;
> > > +	u8 sink_count;
> > > +	u8 link_status[DP_LINK_STATUS_SIZE] = {};
> > > +	u32 sink_count_reg = DP_SINK_COUNT_ESI;
> > > +	u32 link_status_reg = DP_LANE0_1_STATUS;
> > > +
> > > +	ret = drm_dp_dpcd_readb(&mtk_dp->aux, sink_count_reg,
> > > &sink_count);
> > 
> > According to your last reply, if drm_dp_dpcd_readb() fail, we could
> > skip below statement. So this just for error checking? If so, the
> > next
> > drm_dp_dpcd_read() would also do the error checking, so the
> > checking
> > here is redundant.
> > 
> > Regards,
> > CK
> > 
> 
> Hello CK,
> 
> sorry, I don't get your point.
> We use "drm_dp_dpcd_readb(&mtk_dp->aux, sink_count_reg, &sink_count)"
> to get sink count and use "drm_dp_dpcd_read(&mtk_dp->aux,
> link_status_reg, link_status, sizeof(link_status));" to get link
> status.
> 
> If we don't get any sink count, we don't need to check the link
> status.
> Therefore, we just return if we are failed to get sink count.

I assume that when error happen, both read sink_count and read
link_status would fail, so you could directly read link_status. Do you
think that when error happen, only read sink_count would fail and read
link_status would success? It it is the later case, we should keep the
checking of sink_count.

Regards,
CK

> 
> BRs,
> Bo-Chen
> 
> > > +	if (ret < 1) {
> > > +		drm_err(mtk_dp->drm_dev, "Read sink count failed\n");
> > > +		return;
> > > +	}
> > > +
> > > +	drm_dbg(mtk_dp->drm_dev,
> > > +		"read sink count from dpcd: %d\n", sink_count);
> > > +
> > > +	ret = drm_dp_dpcd_read(&mtk_dp->aux, link_status_reg,
> > > link_status,
> > > +			       sizeof(link_status));
> > > +	if (!ret) {
> > > +		drm_err(mtk_dp->drm_dev, "Read link status failed\n");
> > > +		return;
> > > +	}
> > > +
> > > +	if (!drm_dp_channel_eq_ok(link_status, mtk_dp-
> > > > train_info.lane_count)) {
> > > 
> > > +		drm_err(mtk_dp->drm_dev, "Channel EQ failed\n");
> > > +		return;
> > > +	}
> > > +
> > > +	if (link_status[1] & DP_REMOTE_CONTROL_COMMAND_PENDING)
> > > +		drm_dp_dpcd_writeb(&mtk_dp->aux,
> > > DP_DEVICE_SERVICE_IRQ_VECTOR,
> > > +				   DP_REMOTE_CONTROL_COMMAND_PENDING);
> > > +}
> > > +
> > 
> > 
> 
>