linux-kernel - Re: [PATCH RFC v3 0/7] drm/display: dp: add new DPCD access functions

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Message-ID: <Z8r01VDfSykLv5JX@phenom.ffwll.local>
Date: Fri, 7 Mar 2025 14:29:57 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: Dmitry Baryshkov <lumag@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Abhinav Kumar <quic_abhinavk@...cinc.com>,
	Sean Paul <sean@...rly.run>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	Andrzej Hajda <andrzej.hajda@...el.com>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Robert Foss <rfoss@...nel.org>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Jonas Karlman <jonas@...boo.se>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Xinliang Liu <xinliang.liu@...aro.org>,
	Tian Tao <tiantao6@...ilicon.com>,
	Xinwei Kong <kong.kongxinwei@...ilicon.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Yongqin Liu <yongqin.liu@...aro.org>,
	John Stultz <jstultz@...gle.com>, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	freedreno@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
	intel-xe@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
	Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH RFC v3 0/7] drm/display: dp: add new DPCD access functions

On Fri, Mar 07, 2025 at 06:34:42AM +0200, Dmitry Baryshkov wrote:
> Existing DPCD access functions return an error code or the number of
> bytes being read / write in case of partial access. However a lot of
> drivers either (incorrectly) ignore partial access or mishandle error
> codes. In other cases this results in a boilerplate code which compares
> returned value with the size.
> 
> As suggested by Jani implement new set of DPCD access helpers, which
> ignore partial access, always return 0 or an error code. Implement
> new helpers using existing functions to ensure backwards compatibility
> and to assess necessity to handle incomplete reads on a global scale.
> Currently only one possible place has been identified, dp-aux-dev, which
> needs to handle possible holes in DPCD.
> 
> This series targets only the DRM helpers code. If the approach is found
> to be acceptable, each of the drivers should be converted on its own.
> 
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

Just wanted to drop my "I like this" on your series here. Short
read/writes come from unix pipes, and they're everywhere, and yes ime
everyone gets them wrong. So ack or whatever that means :-)
-Sima

> ---
> Changes in v3:
> - Fixed cover letter (Jani)
> - Added intel-gfx and intel-xe to get the series CI-tested (Jani)
> - Link to v2: https://lore.kernel.org/r/20250301-drm-rework-dpcd-access-v2-0-4d92602fc7cd@linaro.org
> 
> Changes in v2:
> - Reimplemented new helpers using old ones (Lyude)
> - Reworked the drm_dp_dpcd_read_link_status() patch (Lyude)
> - Dropped the dp-aux-dev patch (Jani)
> - Link to v1: https://lore.kernel.org/r/20250117-drm-rework-dpcd-access-v1-0-7fc020e04dbc@linaro.org
> 
> ---
> Dmitry Baryshkov (7):
>       drm/display: dp: implement new access helpers
>       drm/display: dp: change drm_dp_dpcd_read_link_status() return value
>       drm/display: dp: use new DCPD access helpers
>       drm/display: dp-aux-dev: use new DCPD access helpers
>       drm/display: dp-cec: use new DCPD access helpers
>       drm/display: dp-mst-topology: use new DCPD access helpers
>       drm/display: dp-tunnel: use new DCPD access helpers
> 
>  drivers/gpu/drm/amd/amdgpu/atombios_dp.c           |   8 +-
>  .../gpu/drm/bridge/cadence/cdns-mhdp8546-core.c    |   2 +-
>  drivers/gpu/drm/display/drm_dp_aux_dev.c           |  12 +-
>  drivers/gpu/drm/display/drm_dp_cec.c               |  37 ++-
>  drivers/gpu/drm/display/drm_dp_helper.c            | 307 +++++++++------------
>  drivers/gpu/drm/display/drm_dp_mst_topology.c      | 105 ++++---
>  drivers/gpu/drm/display/drm_dp_tunnel.c            |  20 +-
>  drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c       |   4 +-
>  drivers/gpu/drm/msm/dp/dp_ctrl.c                   |  24 +-
>  drivers/gpu/drm/msm/dp/dp_link.c                   |  18 +-
>  drivers/gpu/drm/radeon/atombios_dp.c               |   8 +-
>  include/drm/display/drm_dp_helper.h                |  92 +++++-
>  12 files changed, 322 insertions(+), 315 deletions(-)
> ---
> base-commit: 565351ae7e0cee80e9b5ed84452a5b13644ffc4d
> change-id: 20241231-drm-rework-dpcd-access-b0fc2e47d613
> 
> Best regards,
> -- 
> Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch