On Wed, Apr 27, 2022 at 05:23:22PM +0300, Jani Nikula wrote: > On Wed, 27 Apr 2022, Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Thu, Apr 14, 2022 at 01:24:30PM +0300, Jani Nikula wrote: > >> On Mon, 11 Apr 2022, Alex Deucher <alexdeucher@xxxxxxxxx> wrote: > >> > On Mon, Apr 11, 2022 at 6:18 AM Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > >> >> > >> >> Hi, > >> >> > >> >> On 4/8/22 17:11, Alex Deucher wrote: > >> >> > On Fri, Apr 8, 2022 at 10:56 AM Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > >> >> >> > >> >> >> Hi, > >> >> >> > >> >> >> On 4/8/22 16:08, Alex Deucher wrote: > >> >> >>> On Fri, Apr 8, 2022 at 4:07 AM Daniel Vetter <daniel@xxxxxxxx> wrote: > >> >> >>>> > >> >> >>>> On Thu, Apr 07, 2022 at 05:05:52PM -0400, Alex Deucher wrote: > >> >> >>>>> On Thu, Apr 7, 2022 at 1:43 PM Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > >> >> >>>>>> > >> >> >>>>>> Hi Simon, > >> >> >>>>>> > >> >> >>>>>> On 4/7/22 18:51, Simon Ser wrote: > >> >> >>>>>>> Very nice plan! Big +1 for the overall approach. > >> >> >>>>>> > >> >> >>>>>> Thanks. > >> >> >>>>>> > >> >> >>>>>>> On Thursday, April 7th, 2022 at 17:38, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > >> >> >>>>>>> > >> >> >>>>>>>> The drm_connector brightness properties > >> >> >>>>>>>> ======================================= > >> >> >>>>>>>> > >> >> >>>>>>>> bl_brightness: rw 0-int32_max property controlling the brightness setting > >> >> >>>>>>>> of the connected display. The actual maximum of this will be less then > >> >> >>>>>>>> int32_max and is given in bl_brightness_max. > >> >> >>>>>>> > >> >> >>>>>>> Do we need to split this up into two props for sw/hw state? The privacy screen > >> >> >>>>>>> stuff needed this, but you're pretty familiar with that. :) > >> >> >>>>>> > >> >> >>>>>> Luckily that won't be necessary, since the privacy-screen is a security > >> >> >>>>>> feature the firmware/embedded-controller may refuse our requests > >> >> >>>>>> (may temporarily lock-out changes) and/or may make changes without > >> >> >>>>>> us requesting them itself. Neither is really the case with the > >> >> >>>>>> brightness setting of displays. > >> >> >>>>>> > >> >> >>>>>>>> bl_brightness_max: ro 0-int32_max property giving the actual maximum > >> >> >>>>>>>> of the display's brightness setting. This will report 0 when brightness > >> >> >>>>>>>> control is not available (yet). > >> >> >>>>>>> > >> >> >>>>>>> I don't think we actually need that one. Integer KMS props all have a > >> >> >>>>>>> range which can be fetched via drmModeGetProperty. The max can be > >> >> >>>>>>> exposed via this range. Example with the existing alpha prop: > >> >> >>>>>>> > >> >> >>>>>>> "alpha": range [0, UINT16_MAX] = 65535 > >> >> >>>>>> > >> >> >>>>>> Right, I already knew that, which is why I explicitly added a range > >> >> >>>>>> to the props already. The problem is that the range must be set > >> >> >>>>>> before registering the connector and when the backlight driver > >> >> >>>>>> only shows up (much) later during boot then we don't know the > >> >> >>>>>> range when registering the connector. I guess we could "patch-up" > >> >> >>>>>> the range later. But AFAIK that would be a bit of abuse of the > >> >> >>>>>> property API as the range is intended to never change, not > >> >> >>>>>> even after hotplug uevents. At least atm there is no infra > >> >> >>>>>> in the kernel to change the range later. > >> >> >>>>>> > >> >> >>>>>> Which is why I added an explicit bl_brightness_max property > >> >> >>>>>> of which the value gives the actual effective maximum of the > >> >> >>>>>> brightness. > >> >> >>>> > >> >> >>>> Uh ... I'm not a huge fan tbh. The thing is, if we allow hotplugging > >> >> >>>> brightness control later on then we just perpetuate the nonsense we have > >> >> >>>> right now, forever. > >> >> >>>> > >> >> >>>> Imo we should support two kinds of drivers: > >> >> >>>> > >> >> >>>> - drivers which are non-crap, and make sure their backlight driver is > >> >> >>>> loaded before they register the drm_device (or at least the > >> >> >>>> drm_connector). For those we want the drm_connector->backlight pointer > >> >> >>>> to bit static over the lifetime of the connector, and then we can also > >> >> >>>> set up the brightness range correctly. > >> >> >>>> > >> >> >>>> - funny drivers which implement the glorious fallback dance which > >> >> >>>> libbacklight implements currently in userspace. Imo for these drivers we > >> >> >>>> should have a libbacklight_heuristics_backlight, which normalizes or > >> >> >>>> whatever, and is also ways there. And then internally handles the > >> >> >>>> fallback mess to the "right" backlight driver. > >> >> >>>> > >> >> >>>> We might have some gaps on acpi systems to make sure the drm driver can > >> >> >>>> wait for the backlight driver to show up, but that's about it. > >> >> >>>> > >> >> >>>> Hotplugging random pieces later on is really not how drivers work nowadays > >> >> >>>> with deferred probe and component framework and all that. > >> >> >>>> > >> >> >>>>>> I did consider using the range for this and updating it > >> >> >>>>>> on the fly I think nothing is really preventing us from > >> >> >>>>>> doing so, but it very much feels like abusing the generic > >> >> >>>>>> properties API. > >> >> >>>>>> > >> >> >>>>>>>> bl_brightness_0_is_min_brightness: ro, boolean > >> >> >>>>>>>> When this is set to true then it is safe to set brightness to 0 > >> >> >>>>>>>> without worrying that this completely turns the backlight off causing > >> >> >>>>>>>> the screen to become unreadable. When this is false setting brightness > >> >> >>>>>>>> to 0 may turn the backlight off, but this is not guaranteed. > >> >> >>>>>>>> This will e.g. be true when directly driving a PWM and the video-BIOS > >> >> >>>>>>>> has provided a minimum (non 0) duty-cycle below which the driver will > >> >> >>>>>>>> never go. > >> >> >>>>>>> > >> >> >>>>>>> Hm. It's quite unfortunate that it's impossible to have strong guarantees > >> >> >>>>>>> here. > >> >> >>>>>>> > >> >> >>>>>>> Is there any way we can avoid this prop? > >> >> >>>>>> > >> >> >>>>>> Not really, the problem is that we really don't know if 0 is off > >> >> >>>>>> or min-brightness. In the given example where we actually never go > >> >> >>>>>> down to a duty-cycle of 0% because the video BIOS tables tell us > >> >> >>>>>> not to, we can be certain that setting the brightness prop to 0 > >> >> >>>>>> will not turn of the backlight, since we then set the duty-cycle > >> >> >>>>>> to the VBT provided minimum. Note the intend here is to only set > >> >> >>>>>> the boolean to true if the VBT provided minimum is _not_ 0, 0 > >> >> >>>>>> just means the vendor did not bother to provide a minimum. > >> >> >>>>>> > >> >> >>>>>> Currently e.g. GNOME never goes lower then something like 5% > >> >> >>>>>> of brightness_max to avoid accidentally turning the screen off. > >> >> >>>>>> > >> >> >>>>>> Turning the screen off is quite bad to do on e.g. tablets where > >> >> >>>>>> the GUI is the only way to undo the brightness change and now > >> >> >>>>>> the user can no longer see the GUI. > >> >> >>>>>> > >> >> >>>>>> The idea behind this boolean is to give e.g. GNOME a way to > >> >> >>>>>> know that it is safe to go down to 0% and for it to use > >> >> >>>>>> the entire range. > >> >> >>>>> > >> >> >>>>> Why not just make it policy that 0 is defined as minimum brightness, > >> >> >>>>> not off, and have all drivers conform to that? > >> >> >>>> > >> >> >>>> Because the backlight subsystem isn't as consistent on this, and it's been > >> >> >>>> an epic source of confusion since forever. > >> >> >>>> > >> >> >>>> What's worse, there's both userspace out there which assumes brightness = > >> >> >>>> 0 is a really fast dpms off _and_ userspace that assumes that brightness = > >> >> >>>> 0 is the lowest setting. Of course on different sets of machines. > >> >> >>>> > >> >> >>>> So yeah we're screwed. I have no idea how to get out of this. > >> >> >>> > >> >> >>> Yes, but this is a new API. So can't we do better? Sure the old > >> >> >>> backlight interface is broken, but why carry around clunky workarounds > >> >> >>> for new interfaces? > >> >> >> > >> >> >> Right we certainly need to define the behavior of the new API > >> >> >> clearly, so that userspace does not misuse / misinterpret it. > >> >> >> > >> >> >> The intend is for brightness=0 to mean minimum brightness > >> >> >> to still be able to see what is on the screen. But the problem > >> >> >> is that in many cases the GPU driver directly controls a PWM > >> >> >> output, no minimum PWM value is given in the video BIOS tables > >> >> >> and actually setting the PWM to 0% dutycycle turns off the > >> >> >> screen. > >> >> > > >> >> > Sure. So have the GPU driver map 0 to some valid minimum if that is > >> >> > the case or might be the case. If bugs come up, we can add quirks in > >> >> > the GPU drivers. > >> >> > >> >> The problem is that when 0% duty-cycle is not off, but minimum > >> >> brightness because there is some smart backlight-controller involved > >> >> downstream of the pwm, then of we limit it to say min 5% then we > >> >> have just limited the range of the brightness. GNOME already does > >> >> this in userspace now and it is already receiving bug-reports > >> >> from users that GNOME does not allow the brightness to go as low > >> >> as they like to have it in a dark(ish) room. > >> >> > >> >> And in some cases 5% is likely not enough for the backlight to > >> >> actually turn on. So it will be wrong in one direction on some > >> >> devices and wrong in the other direction in other devices. > >> >> > >> >> Which means that to satisfy everyone here we will need a ton > >> >> of quirks, much too many to maintain in the kernel IMHO. > >> >> > >> >> > >> >> >> So we can only promise a best-effort to make brightness=0 > >> >> >> mean minimum brightness, combined with documenting that it > >> >> >> may turn off the backlight and that userspace _must_ never > >> >> >> depend on it turning off the backlight. > >> >> >> > >> >> >> Also note that setting a direct PWM output to duty-cycle 0% > >> >> >> does not guarantee that the backlight goes off, this may be > >> >> >> an input for a special backlight LED driver IC like the > >> >> >> TI LP855x series which can have an internal lookup > >> >> >> table causing it to actually output a minimum brightness > >> >> >> when its PWM input is at 0% dutycycle. So this is a case > >> >> >> where we just don't get enough info from the fw/hw to be able > >> >> >> to offer the guarantees which we would like to guarantee. > >> >> > > >> >> > So set it to a level we can guarantee can call it 0. If we have the > >> >> > flag we are just punting on the problem in my opinion. > >> >> > >> >> Right this an impossible problem to solve so the intent is indeed > >> >> to punt this to userspace, which IMHO is the best thing we can do > >> >> here. The idea behind the "bl_brightness_0_is_min_brightness: > >> >> ro, boolean" property is to provide a hint to userspace to help > >> >> userspace deal with this (and if userspace ends up using e.g. > >> >> systemd's hwdb for this to avoid unnecessary entries in hwdb). > >> >> > >> >> > The kernel > >> >> > driver would seem to have a better idea what is a valid minimum than > >> >> > some arbitrary userspace application. > >> >> > >> >> If the kernel driver knows the valid minimum then it can make 0 > >> >> actually be that valid minimum as you suggest and it can set the > >> >> hint flag to let userspace know this. OTOH there are many cases > >> >> where the kernel's guess is just as bad as userspace's guess and > >> >> there are too many laptops where this is the case to quirk > >> >> ourselves out of this situation. > >> >> > >> >> > Plus then if we need a > >> >> > workaround for what is the minimum valid brightness, we can fix it one > >> >> > place rather than letting every application try and fix it. > >> >> > >> >> I wish we could solve this in the kernel, but at least on > >> >> laptops with Intel integrated gfx many vendors don't bother > >> >> to put a non 0 value in the minimum duty-cycle field of the > >> >> VBT, so there really is no good way to solve this. > >> > > >> > We have similar issues with AMD platforms. Some platforms don't > >> > populate the min value tables, but in the driver we set the minimum > >> > safe value as the default min value when that happens. It may not > >> > always go as low as the platform may be capable of, but at least we > >> > have consistent behavior and it's all controlled in one place. > >> > > >> >> > >> >> If the userspace folks ever want to do a database for this, > >> >> I would expect them to do something with hwdb + udev which > >> >> can then be shared between the different desktop-environments. > >> > > >> > So why not do it in the kernel? At least that way everyone will get > >> > it the fixes as they happen. A big user database may or may not > >> > happen and behavior will be inconsistent across desktop environments > >> > until that does. I don't really see any value in having the flag. > >> > There will be cases where the flag is wrong and will require kernel > >> > fixes anyway (e.g., OEM switches panel due to supply chain issues and > >> > forgets to update the vbios, etc.), so why not just define 0 as > >> > minimum safe backlight value? If it's too low and flickers or turns > >> > the backlight off, we quirk it. If a particular platform can go > >> > lower, we can quirk it. If we add the flag then we need to not only > >> > add quirks for the minimum value, but we also have to deal with quirks > >> > for when the flag is set wrong. So now we are maintaining two quirks > >> > instead of one. > >> > >> Just chiming in, there are certainly plenty of panels and designs where > >> 0 PWM duty cycle is physically not possible, and thus 0 brightness > >> simply can't universally mean off. > >> > >> Daniel referred to a case where 0 brightness was used as fast mini dpms > >> off, and I think it's fundamentally a broken use case. We can't > >> guarantee to be able to support that. I think the appeal was partly in > >> being able to do it without access to kms api, quick and dirty via > >> sysfs. > >> > >> Please let's just make 0 the minimum but not off. If you want off, you > >> do modeset, and the driver can follow panel timings etc. > >> > >> I think that's also something the kernel can actually guarantee, while > >> we can't guarantee 0 is off. > > > > Yes. > > > > The trouble is that we have platforms where it works like this, and so > > retroactively redefining what brightness 0 means would be a regression. I > > guess just another reason for why we should roll this out step by step, > > with latest platforms first. > > > > Or we shrug and decide to break things like that and redefine the > > backlight semantics a bit. Or well define them properly to begin with :-) > > I say it's a new interface, and does not have to follow old interface > semantics. When userspace switches over, it has to adapt. Just shrug off > any "regression" reports where the comparison is against the old > interface. Not sure we can fix the new interface without changing the old one. Like when we specifiy that 0 means lowest setting, people will be annoyed when it's actually off, and then fixing the backlight driver would break the old stuff. I'm leaning towards breaking the old stuff a bit and making this clean :-) The regression is just power usage when you close the lid and stuff like that, should be ok-ish for these older machines with funny userspace that doesn't do a dpms off. -Daniel > > BR, > Jani. > > > > -Daniel > > > >> > >> > >> BR, > >> Jani. > >> > >> > >> > >> > >> -- > >> Jani Nikula, Intel Open Source Graphics Center > > -- > Jani Nikula, Intel Open Source Graphics Center -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
