On Thu, Jul 22, 2021 at 09:02:04PM +0200, Marek Vasut wrote: > On 7/22/21 1:28 PM, Daniel Thompson wrote: > > On Wed, Jul 21, 2021 at 08:46:42PM +0200, Marek Vasut wrote: > > > On 7/21/21 6:12 PM, Daniel Thompson wrote: > > > > On Wed, Jul 21, 2021 at 05:09:57PM +0200, Marek Vasut wrote: > > > > > On 7/21/21 12:49 PM, Daniel Thompson wrote: > > > > [...] > > > > This sails very close to the > > > > edge of what is in-scope for DT (at least it does it we can read > > > > the inherited state directly from the hardware). > > > > > > The problem with reading it out of hardware is that the hardware might be in > > > undefined state and expects Linux to define that state, so that does not > > > always work. Hence my initial suggestion to add a DT property to define the > > > state up front, instead of using these fragile heuristics. > > > > To achieve a flicker-free boot we must know the initial state of the > > backlight (not just the enable pin). > > The backlight hardware might be in uninitialized state and then Linux should > set the state, likely based on something in DT, because there is no previous > state to read. There is always a previous state. The kernel doesn't care whether that previous state was imposed by a power-on reset, the bootloader or a kexec. For the driver to come up flicker-free in all the different cases we need to know whether the backlight is currently emitting light or not and, if it is emitting light, then we need to know what the duty cycle is (currently we inherit require the PWM driver to correctly inherit the duty cycle from the hardware). So far, the previous state has been observable by the lower level drivers (GPIO, PWM, regulator). I remain reluctant to provide workarounds for cases where it is not observable without motivating hardware. I certainly wouldn't want to make such bindings mandatory since observable hardware registers are a far more reliable source of truth than what the DT tells us about what it thinks the bootloader (or power-on reset) actually did ;-). > > > > Overall I have fairly grave concerns that this simply moves > > > > fragility into the bootloader rather then reducing it. > > > > > > Wait a minute, I think we disconnected somewhere. I would rather prefer to > > > remove the fragility and bootloader dependency altogether, exactly to avoid > > > depending on the state the bootloader left the hardware in. > > > > The two fully flicker-free cases we support in pwm_bl are: > > > > 1. Backlight off after bootloader has completed. Backlight must be > > off after the probe completes (and never flicker on/off during the > > probe). This allows the display to put a splash image on the screen > > before lighting up the backlight (this avoids a flicker if there are > > a few frames between backlight coming on and the splash image being > > drawn). Naturally this requires help from the display system (and > > that the display system is aware of the backlight to be able to start > > it). > > > > 2. Backlight on with a splash image after bootloader has completed. > > Backlight must be on after the probe completes (and never flicker > > off/on during the probe). This also requires that the display system > > can take over the frame buffer without a flicker but that is > > completely independent of backlight. > > > > There is also a simpler case which is not "flicker-free" since the > > backlight may change level during the boot and may light up before > > there is an image on the screen (although we'd still to minimise > > flicker by ensuring there is only one change of backlight state/level > > during the probe (something your work will see fixed?): > > Actually no, my usecase is the backlight is not initialized by the > bootloader at all, the pins are just strapped to default to the right > values, the init is left to the kernel to do. It doesn't matter to us who established the initial state. In this case the backlight is off at handover. That means you are either case #1 (display system will unblank" the backlight automatically when the reset of the display unblanks) or #3 (BL driver will "unblank" the backlight during the probe). Your changes should result in a fix to both these cases! > > 3. Backlight is on after the probe completes. This is default if > > we don't know the display system will activate the backlight. > > This is an important legacy case since few userspaces know how > > to change the backlight power-state at boot. > > > > One oddity here is that #3 *also* needs to know the state of the > > backlight (on/off) to turn the backlight on without flickering > > (so it can figure out how to handle power_pwm_on_delay correctly) > > even though the final state is unconditionally on. That is the main > > reason I proposed an alternative to your patch (since this is > > currently broken). > > > > The other oddity is that the difference between #1 and #3 is due to > > *software* (which driver ends up responsible for unmuting the display) > > meaning that the bootloader/DT has no business discriminating between > > these two cases. > > > > Thus pwm_bl.c is based on making #2/#3 (which are similar) the default > > and switching to case #1 if there is a display driver to do the unblank > > (software) *and* that the backlight is currently off (currently read > > from hardware). Note that this is intentionally designed to that > > if the logic does go wrong we should get a small bug (a flicker) rather > > than a big one (a black screen). > > > > Wow! That is *way* longer than I intended when I started writing it. > > Anyhow I suspect any disconnect comes about due to the difference in > > backlight state *after* probe being, in part, to software structure > > rather than purely a hardware property. > > Maybe this should be added to documentation. I'll see what I can do. Daniel.
