Re: [PATCH v6 11/26] drm/bridge: samsung-dsim: use devm_drm_of_get_bridge[_by_node] to find the out_bridge

Maxime Ripard
2025-02-10T19:40:56Z

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On Thu, Feb 06, 2025 at 07:14:26PM +0100, Luca Ceresoli wrote:
> In order to support panels described either via graph links or via a
> subnode (e.g. "panel@0"), this driver uses low-level deprecated functions
> to find the next bridge. The resulting logic is complex and duplicates code
> already present in the DRM bridge core. Switch to the new APIs in DRM
> bridge core that allow to do the same in a much cleaner way.
>
> Note there are two slight changes in the new logic intended to improve the
> final result:
>
> * the old code looks for a subnode with any name except "port" or "ports",
> while the new code uses the node passed as a parameter
>
> * the old code looks for a subnode first and falls back to a graph link,
> while the new code uses the reverse order because graph links are the
> recommended device tree representation now
>
> The first change makes the code more robust by avoiding the risk of using
> an unrelated node which is not describing a panel and not names "port" or
> "ports".
>
> The second change is not expected to expose regressions because, in the
> cases where both a subnode and a graph link are used to describe a panel,
> the graph link should point to the subnode itself, such as in
> arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts
>
> As a further cleanup, use a temporary variable to assign dsi->out_bridge
> only on success. This avoids the risk of leaving a non-NULL value in
> dsi->out_bridge when samsung_dsim_host_attach() fails.
>
> Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
>
> ---
>
> This patch was added in v6.
> ---
> drivers/gpu/drm/bridge/samsung-dsim.c | 55 ++++++-----------------------------
> 1 file changed, 9 insertions(+), 46 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index f8b4fb8357659018ec0db65374ee5d05330639ae..bbd0a4f5a3f52b61bf48f10d6e8ca741bffa5e46 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -1704,55 +1704,16 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
> const struct samsung_dsim_plat_data *pdata = dsi->plat_data;
> struct device *dev = dsi->dev;
> struct device_node *np = dev->of_node;
> - struct device_node *remote;
> - struct drm_panel *panel;
> + struct drm_bridge *out_bridge;
> int ret;
>
> - /*
> - * Devices can also be child nodes when we also control that device
> - * through the upstream device (ie, MIPI-DCS for a MIPI-DSI device).
> - *
> - * Lookup for a child node of the given parent that isn't either port
> - * or ports.
> - */
> - for_each_available_child_of_node(np, remote) {
> - if (of_node_name_eq(remote, "port") ||
> - of_node_name_eq(remote, "ports"))
> - continue;
> + out_bridge = devm_drm_of_get_bridge(dev, np, 1, 0);
> + if (IS_ERR(out_bridge) && PTR_ERR(out_bridge) != -EPROBE_DEFER)
> + out_bridge = devm_drm_of_get_bridge_by_node(dev, device->dev.of_node);
>

For the same reason I mentioned earlier, this is inherently unsafe if
the bridge device goes away but the DRM device doesn't.

Maxime

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