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[2001:1c00:c32:7800:5bfa:a036:83f0:f9ec]) by smtp.gmail.com with ESMTPSA id d17-20020aa7ce11000000b00522572f323dsm8504731edv.16.2023.09.06.08.19.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Sep 2023 08:19:53 -0700 (PDT) Message-ID: Date: Wed, 6 Sep 2023 17:19:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v3 1/1] backlight: hid_bl: Add VESA VCP HID backlight driver Content-Language: en-US, nl To: Julius Zint , =?UTF-8?Q?Thomas_Wei=c3=9fschuh?= Cc: Lee Jones , Daniel Thompson , Jingoo Han , Jiri Kosina , Benjamin Tissoires , Helge Deller , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-input@vger.kernel.org, linux-fbdev@vger.kernel.org References: <20230820094118.20521-1-julius@zint.sh> <20230820094118.20521-2-julius@zint.sh> <9a5364de-28e1-1d4a-1d3a-d6dcedb7e659@zint.sh> From: Hans de Goede In-Reply-To: <9a5364de-28e1-1d4a-1d3a-d6dcedb7e659@zint.sh> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org Hi Julius, On 9/4/23 21:02, Julius Zint wrote: > > > On Mon, 4 Sep 2023, Thomas Weißschuh wrote: > >> +Cc Hans who ins involved with the backlight subsystem >> >> Hi Julius, >> >> today I stumbled upon a mail from Hans [0], which explains that the >> backlight subsystem is not actually a good fit (yet?) for external >> displays. >> >> It seems a new API is in the works that would better fit, but I'm not >> sure about the state of this API. Maybe Hans can clarify. >> >> This also ties back to my review question how userspace can figure out >> to which display a backlight devices applies. So far it can not. >> >> [0] https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@redhat.com/ >> > > Hi Thomas, > > thanks for the hint. I will make sure to give this a proper read and > see, if it fits my use case better then the current backlight subsystem. Note the actual proposal for the new usespace API for display brightness control is here: https://lore.kernel.org/dri-devel/b61d3eeb-6213-afac-2e70-7b9791c86d2e@redhat.com/ I have finished / stabilized the backlight code refactor which I landed in 6.1, which is a prerequisite for the above proposal. But I have not been able to make time to actually implement the above proposal; and I don't know when I will be able to make time for this. > Especially since I wasnt able to properly address your other review > comments for now. You are right that the name should align better with > the kernel module and also, that it is possible for multiple displays to > be attached. > > In its current state, this would mean that you could only control the > backlight for the first HID device (enough for me :-). > > The systemd-backlight@.service uses not only the file name, but also the > full bus path for storing/restoring backlights. I did not yet get around > to see how the desktops handle brightness control, but since the > systemd-backlight@.service already uses the name, its important to stay > the same over multiple boots. > > I would be able to get a handle on the underlying USB device and use the > serial to uniquely (and persistently) name the backlight. But it does > feel hacky doing it this way. So mutter (gnome-shell compositor library) has a similar issue when saving monitor layouts and I can tell you beforehand that monitor serial numbers by themselves are not unique enough. Some models just report 123456789 as serial and if you have a dual-monitor setup with 2 such monitors and name the backlight class device -vcp-hid or something like that you will still end up with 2 identical names. To avoid this when saving monitor layouts mutter saves both the port to which the monitor is attached (e.g. DP-1 DP-2) and the serialnumber and on startup / monitor hotplug when it checks to see if it has saved layout info for the monitor it checks the port+serialnr combination. So what I think you should do is figure out a way to map which VCP HID device maps to which drm-connector and then use the connector-name + serial-nr to generate the backlight device name. We will need the mapping the a drm-connector object anyway for the new brightness API proposal from above. Note this does NOT solve the fact that registering a new backlight class device for an external monitor on a laptop will hopelessly confuse userspace, see: https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@redhat.com/ Regards, Hans