linux-kernel - Re: [PATCH v3 3/3] drm/bridge: parade-ps8640: Add support for AUX channel

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Message-ID: <CAD=FV=WtC3AQr44idgzVe9RCjb9i=+ekJ_wKKnKMcHRSQX7dfQ@mail.gmail.com>
Date:   Wed, 15 Sep 2021 14:27:40 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Philip Chen <philipchen@...omium.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v3 3/3] drm/bridge: parade-ps8640: Add support for AUX channel

Hi,

On Tue, Sep 14, 2021 at 5:57 PM Stephen Boyd <swboyd@...omium.org> wrote:
>
> Quoting Philip Chen (2021-09-14 16:28:45)
> > diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> > index 8d3e7a147170..dc349d729f5a 100644
> > --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> > +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> > @@ -117,6 +144,129 @@ static inline struct ps8640 *bridge_to_ps8640(struct drm_bridge *e)
> [...]
> > +       case DP_AUX_I2C_WRITE:
> > +       case DP_AUX_I2C_READ:
> > +               break;
> > +       default:
> > +               return -EINVAL;
> > +       }
> > +
> > +       ret = regmap_write(map, PAGE0_AUXCH_CFG3, AUXCH_CFG3_RESET);
> > +       if (ret) {
> > +               dev_err(dev, "failed to write PAGE0_AUXCH_CFG3: %d\n", ret);
>
> Can we use DRM_DEV_ERROR()?

I've never gotten clear guidance here. For instance, in some other
review I suggested using the DRM wrapper and got told "no" [1]. ;-)
The driver landed without the DRM_ERROR versions. I don't really care
lots so it's fine with me to use use DRM_DEV_ERROR, I just wish I
understood the rules...

[1] https://lore.kernel.org/all/49db7ef3-fa53-a274-7c69-c2d840b13058@denx.de/


> > +               return ret;
> > +       }
> > +
> > +       /* Assume it's good */
> > +       msg->reply = 0;
> > +
> > +       addr_len[0] = msg->address & 0xff;
> > +       addr_len[1] = (msg->address >> 8) & 0xff;
> > +       addr_len[2] = ((msg->request << 4) & SWAUX_CMD_MASK) |
> > +               ((msg->address >> 16) & SWAUX_ADDR_19_16_MASK);
>
> It really feels like this out to be possible with some sort of
> cpu_to_le32() API. We're shoving msg->address into 3 bytes and then
> adding in the request and some length. So we could do something like:
>
>         u32 addr_len;
>
>         addr_len = FIELD_PREP(SWAUX_ADDR_MASK, msg->address);
>         addr_len |= FIELD_PREP(SWAUX_CMD_MASK, msg->request);
>         if (len)
>                 addr_len |= FIELD_PREP(LEN_MASK, len - 1);
>         else
>                 addr_len |= FIELD_PREP(LEN_MASK, SWAUX_NO_PAYLOAD );
>
>         cpu_to_le32s(&addr_len);
>
>         regmap_bulk_write(map, PAGE0_SWAUX_ADDR_7_0, &addr_len, sizeof(addr_len));

You're arguing that your version of the code is more efficient? Easier
to understand? Something else? To me, Philip's initial version is
crystal clear and easy to map to the bridge datasheet but I need to
think more to confirm that your version is right. Thinking is hard and
I like to avoid it when possible.

In any case, it's definitely bikeshedding and I'll yield if everyone
likes the other version better. ;-)


> > +               return ret;
> > +       }
> > +
> > +       switch (data & SWAUX_STATUS_MASK) {
> > +       /* Ignore the DEFER cases as they are already handled in hardware */
> > +       case SWAUX_STATUS_NACK:
> > +       case SWAUX_STATUS_I2C_NACK:
> > +               /*
> > +                * The programming guide is not clear about whether a I2C NACK
> > +                * would trigger SWAUX_STATUS_NACK or SWAUX_STATUS_I2C_NACK. So
> > +                * we handle both cases together.
> > +                */
> > +               if (is_native_aux)
> > +                       msg->reply |= DP_AUX_NATIVE_REPLY_NACK;
> > +               else
> > +                       msg->reply |= DP_AUX_I2C_REPLY_NACK;
> > +
> > +               len = data & SWAUX_M_MASK;
> > +               return len;
>
> Why no 'return data & SWAUX_M_MASK;' and skip the assignment?

Actually, I think it's the "return" that's a bug, isn't it? If we're
doing a "read" and we're returning a positive number of bytes then we
need to actually _read_ them. Reading happens below, doesn't it?


-Doug