linux-kernel - Re: [PATCH] drm/nouveau/nvkm/dp: Add hack to fix DP 1.3+ DPCD issues

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Message-ID: <CACO55tvfTDu8XcKowWXcSRqp8OMLb8Q4jnPG_Fn5y=yJy-Dqbw@mail.gmail.com>
Date:   Sun, 9 Jul 2023 01:42:10 +0200
From:   Karol Herbst <kherbst@...hat.com>
To:     Lyude Paul <lyude@...hat.com>
Cc:     nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Airlie <airlied@...hat.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/nouveau/nvkm/dp: Add hack to fix DP 1.3+ DPCD issues

On Fri, Jul 7, 2023 at 11:58 PM Lyude Paul <lyude@...hat.com> wrote:
>
> Currently we use the drm_dp_dpcd_read_caps() helper in the DRM side of
> nouveau in order to read the DPCD of a DP connector, which makes sure we do
> the right thing and also check for extended DPCD caps. However, it turns
> out we're not currently doing this on the nvkm side since we don't have
> access to the drm_dp_aux structure there - which means that the DRM side of
> the driver and the NVKM side can end up with different DPCD capabilities
> for the same connector.
>
> Ideally to fix this, we want to start setting up the drm_dp_aux device in
> NVKM before we've made contact with the DRM side - which should be pretty
> easy to accomplish (I'm already working on it!). Until then however, let's
> workaround this problem by porting a copy of drm_dp_read_dpcd_caps() into
> NVKM - which should fix this issue.
>
> Issue: https://gitlab.freedesktop.org/drm/nouveau/-/issues/211

Should a Fixes: or Cc: stable tag be added so it gets backported?

> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c | 48 ++++++++++++++++++-
>  1 file changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c b/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
> index 40c8ea43c42f..b8ac66b4a2c4 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
> @@ -26,6 +26,8 @@
>  #include "head.h"
>  #include "ior.h"
>
> +#include <drm/display/drm_dp.h>
> +
>  #include <subdev/bios.h>
>  #include <subdev/bios/init.h>
>  #include <subdev/gpio.h>
> @@ -634,6 +636,50 @@ nvkm_dp_enable_supported_link_rates(struct nvkm_outp *outp)
>         return outp->dp.rates != 0;
>  }
>
> +/* XXX: This is a big fat hack, and this is just drm_dp_read_dpcd_caps()

Well.. maybe we should rephrase that _if_ we want to see it
backported. But is this code really that bad? It kinda looks
reasonable enough.

> + * converted to work inside nvkm. This is a temporary holdover until we start
> + * passing the drm_dp_aux device through NVKM
> + */
> +static int
> +nvkm_dp_read_dpcd_caps(struct nvkm_outp *outp)
> +{
> +       struct nvkm_i2c_aux *aux = outp->dp.aux;
> +       u8 dpcd_ext[DP_RECEIVER_CAP_SIZE];
> +       int ret;
> +
> +       ret = nvkm_rdaux(aux, DPCD_RC00_DPCD_REV, outp->dp.dpcd, DP_RECEIVER_CAP_SIZE);
> +       if (ret < 0)
> +               return ret;
> +
> +       /*
> +        * Prior to DP1.3 the bit represented by
> +        * DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT was reserved.
> +        * If it is set DP_DPCD_REV at 0000h could be at a value less than
> +        * the true capability of the panel. The only way to check is to
> +        * then compare 0000h and 2200h.
> +        */
> +       if (!(outp->dp.dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> +             DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT))
> +               return 0;
> +
> +       ret = nvkm_rdaux(aux, DP_DP13_DPCD_REV, dpcd_ext, sizeof(dpcd_ext));
> +       if (ret < 0)
> +               return ret;
> +
> +       if (outp->dp.dpcd[DP_DPCD_REV] > dpcd_ext[DP_DPCD_REV]) {
> +               OUTP_DBG(outp, "Extended DPCD rev less than base DPCD rev (%d > %d)\n",
> +                        outp->dp.dpcd[DP_DPCD_REV], dpcd_ext[DP_DPCD_REV]);
> +               return 0;
> +       }
> +
> +       if (!memcmp(outp->dp.dpcd, dpcd_ext, sizeof(dpcd_ext)))
> +               return 0;
> +
> +       memcpy(outp->dp.dpcd, dpcd_ext, sizeof(dpcd_ext));
> +
> +       return 0;
> +}
> +
>  void
>  nvkm_dp_enable(struct nvkm_outp *outp, bool auxpwr)
>  {
> @@ -689,7 +735,7 @@ nvkm_dp_enable(struct nvkm_outp *outp, bool auxpwr)
>                         memset(outp->dp.lttpr, 0x00, sizeof(outp->dp.lttpr));
>                 }
>
> -               if (!nvkm_rdaux(aux, DPCD_RC00_DPCD_REV, outp->dp.dpcd, sizeof(outp->dp.dpcd))) {
> +               if (!nvkm_dp_read_dpcd_caps(outp)) {
>                         const u8 rates[] = { 0x1e, 0x14, 0x0a, 0x06, 0 };
>                         const u8 *rate;
>                         int rate_max;
> --
> 2.40.1
>