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[209.85.128.47]) by smtp.gmail.com with ESMTPSA id r16-20020aa7d150000000b0051e19bf66a4sm10371476edo.83.2023.07.17.11.22.42 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 17 Jul 2023 11:22:42 -0700 (PDT) Received: by mail-wm1-f47.google.com with SMTP id 5b1f17b1804b1-3fbd200d354so9515e9.1 for ; Mon, 17 Jul 2023 11:22:42 -0700 (PDT) X-Received: by 2002:a50:c242:0:b0:50b:c48c:8a25 with SMTP id t2-20020a50c242000000b0050bc48c8a25mr7850edf.6.1689617747640; Mon, 17 Jul 2023 11:15:47 -0700 (PDT) MIME-Version: 1.0 References: <20230607215224.2067679-1-dianders@chromium.org> <20230607144931.v2.8.Ib1a98309c455cd7e26b931c69993d4fba33bbe15@changeid> In-Reply-To: From: Doug Anderson Date: Mon, 17 Jul 2023 11:15:35 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 08/10] HID: i2c-hid: Support being a panel follower To: Benjamin Tissoires Cc: Jiri Kosina , Bjorn Andersson , Konrad Dybcio , Rob Herring , Frank Rowand , Krzysztof Kozlowski , Conor Dooley , Neil Armstrong , Sam Ravnborg , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , dri-devel@lists.freedesktop.org, Dmitry Torokhov , linux-input@vger.kernel.org, Daniel Vetter , linux-kernel@vger.kernel.org, hsinyi@google.com, cros-qcom-dts-watchers@chromium.org, devicetree@vger.kernel.org, yangcong5@huaqin.corp-partner.google.com, linux-arm-msm@vger.kernel.org, Chris Morgan Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Benjamin, On Mon, Jun 26, 2023 at 3:49=E2=80=AFPM Doug Anderson wrote: > > Benjamin, > > On Thu, Jun 8, 2023 at 8:37=E2=80=AFAM Benjamin Tissoires > wrote: > > > > > +static const struct drm_panel_follower_funcs i2c_hid_core_panel_foll= ower_funcs =3D { > > > + .panel_prepared =3D i2c_hid_core_panel_prepared, > > > + .panel_unpreparing =3D i2c_hid_core_panel_unpreparing, > > > +}; > > > > Can we make that above block at least behind a Kconfig? > > > > i2c-hid is often used for touchpads, and the notion of drm panel has > > nothing to do with them. So I'd be more confident if we could disable > > that code if not required. > > Now that other concerns are addressed, I started trying to write up a > v3 and I found myself writing this as the description of the Kconfig > entry: > > -- > config I2C_HID_SUPPORT_PANEL_FOLLOWER > bool "Support i2c-hid devices that must be power sequenced with a panel" > > Say Y here if you want support for i2c-hid devices that need to > coordinate power sequencing with a panel. This is typically important > when you have a panel and a touchscreen that share power rails or > reset GPIOs. If you say N here then the kernel will not try to honor > any shared power sequencing for your hardware. In the best case, > ignoring power sequencing when it's needed will draw extra power. In > the worst case this will prevent your hardware from functioning or > could even damage your hardware. > > If unsure, say Y. > > -- > > I can certainly go that way, but I just wanted to truly make sure > that's what we want. Specifically: > > 1. If we put the panel follower code behind a Kconfig then we actually > have no idea if a touchscreen was intended to be a panel follower. > Specifically the panel follower API is the one that detects the > connection between the panel and the i2c-hid device, so without being > able to call the panel follower API we have no idea that an i2c-hid > device was supposed to be a panel follower. > > 2. It is conceivable that power sequencing a device incorrectly could > truly cause hardware damage. > > Together, those points mean that if you turn off the Kconfig entry and > then try to boot on a device that needed that Kconfig setting that you > might damage hardware. I can code it up that way if you want, but it > worries me... > > > Alternatives that I can think of: > > a) I could change the panel follower API so that panel followers are > in charge of detecting the panel that they follow. Today, that looks > like: > > panel_np =3D of_parse_phandle(dev->of_node, "panel", 0); > if (panel_np) > /* It's a panel follower */ > of_node_put(panel_np); > > ...so we could put that code in each touchscreen driver and then fail > to probe i2c-hid if we detect that we're supposed to be a panel > follower but the Kconfig is turned off. The above doesn't seem > massively ideal since it duplicates code. Also, one reason why I put > that code in drm_panel_add_follower() is that I think this concept > will eventually be needed even for non-DT cases. I don't know how to > write the non-DT code right now, though... > > > b) I could open-code detect the panel follower case but leave the > actual linking to the panel follower API. AKA add to i2c-hid: > > if (of_property_read_bool(dev->of_node, "panel")) > /* It's a panel follower */ > > ...that's a smaller bit of code, but feels like an abstraction > violation. It also would need to be updated if/when we added support > for non-DT panel followers. > > > c) I could add a "static inline" implementation of b) to "drm_panel.h". > > That sounds great and I started doing it. ...but then realized that it > means adding to drm_panel.h: > > #include > #include > > ...because otherwise of_property_read_bool() isn't defined and "struct > device" can't be dereferenced. That might be OK, but it looks as if > folks have been working hard to avoid things like this in header > files. Presumably it would get uglier if/when we added the non-DT > case, as well. That being said, I can give it a shot... > > -- > > At this point, I'm hoping for some advice. How important is it for you > to have a Kconfig for "I2C_HID_SUPPORT_PANEL_FOLLOWER"? > > NOTE: even if I don't add the Kconfig, I could at least create a > function for registering the panel follower that would get most of the > panel follower logic out of the probe function. Would that be enough? I'd love to send a new version of this patch series, but I'm still stuck with the above issue. I'm hoping you might have a minute to provide your thoughts. If I don't hear anything, I'll try a v3 where I don't have the Kconfig for "I2C_HID_SUPPORT_PANEL_FOLLOWER" but just try to pull a little more of the code out of the probe function. Thanks for your time! -Doug