Hi all,
Pekka, Rajat,
Thank you for your input in this.
On 4/24/20 9:40 AM, Pekka Paalanen wrote:
On Thu, 23 Apr 2020 11:21:47 -0700 Rajat Jain rajatja@google.com wrote:
On Tue, Apr 21, 2020 at 7:46 AM Pekka Paalanen ppaalanen@gmail.com wrote:
On Tue, 21 Apr 2020 14:37:41 +0200 Hans de Goede hdegoede@redhat.com wrote:
TL;DR: Yes there will be races, because of both userspace + the firmware having; and potentially using r/w access to the privacy-screen state. But in practice I expect these to not really be an issue. Important here is that userspace only commits the property in a transaction to commit if it actually intends to change the property so as to not needlessly create a situation where we might hit the race.
As for 1 vs 2 properties for this I guess that in preparation for potential devices where the state is locked, having a r/w sw-state + a ro hw-state property makes sense.
So I suggest that we replace the current "privacy-screen" property from Rajat's patch-set with 2 props named:
"privacy-screen-sw-state" (r/w) "privacy-screen-hw-state" (ro)
Where for current gen hardware the privacy-screen-hw-state is just a mirror of the sw-state.
Just to make sure I understand the semantics correctly:
- The "privacy-screen-hw-state" shall be read-only, and can be modified by: - Hardware (e.g. HW kill switch). - Firmware. - (Potentially) needs a notification/irq to the kernel when this
changes (or may be kernel can read it only when userspace queries for it).
- The "privacy-screen-sw-state" shall be read-write, and can only be
modified by user space. - If user space toggles it, the kernel will attempt to "request" the change to hardware. - Whether the request to hardware was successful or not, the "privacy-screen-sw-state" will always reflect the latest value userspace wrote. - If the request to hardware was successful, the "privacy-screen-hw-state" will also change (probably via a separate notification/irq from HW). - We expect the user space to write to "privacy-screen-sw-state" only if it really wants to toggle the value.
Hi,
yes, to my understanding, that seems to be the correct idea from this thread. The hw-state property must reflect the actual hardware state at all times.
Agree on the hw-state prop reflecting the actual hardware state at all times, that one is easy.
However, when userspace sets "privacy-screen-sw-state", the driver should attempt to change hardware state regardless of whether the "privacy-screen-sw-state" value changes compared to its old value or not. Otherwise userspace cannot intentionally override a hardware hotkey setting if possible (or would need two atomic commits to do it).
Ack / agreed.
Mind, the above paragraph is only what I interpreted from this email thread here. Previously I did not think that with atomic KMS, setting a property to a value it already has could trigger anything. But I guess it can?
In a way. My idea for the "privacy-screen-sw-state" is for it to reflect the last requested value, where the request could come from either a firmware controlled hotkey; or from userspace (this seems to be where our ideas of how to handle this diverts).
So what can happen is (with both props being always in sync) -userspace reads privacy screen being off -user toggles privacy screen on through firmware controlled hotkey -kernel gets notified about state toggle, updates both property states to on -userspace commits its old knowledge of the property (off), thereby triggering the kernel to turn the privacy screen back off
So in this case from the kernel pov the property is actually set to a new value, not to "a value it already has".
Note there can be races here of course, but lets ignore those (for now). Both the hotkey event as well as userspace changing the setting will be end-user triggered events and will not happen at high frequency. Also I see no way to completely eliminate racing here. Luckily the side effects of the race or pretty harmless (annoying maybe, but not causing crashes, etc).
This design is based on that it can.
What is not clear to me is if any change to"privacy-screen-hw-state" shall be propagated to "privacy-screen-sw-state"?
- If yes, then I think we are not solving any problems of single property.
- If no, then why do we require userspace to write to sw state only
if something has changed?
No. As already written, the kernel must not change the value of "privacy-screen-sw-state", only userspace can.
So this is where out view of how to handle this differs, I do not see the hotkey changing the state as different from userspace changing it. The reason for me to have both a sw- and a hw-state is in case there is a physical switch (typically a slider style switch) which forces the state to on / off. In this case userspace could still set the "privacy-screen-sw-state" prop and then the 2 could differ.
Lets add one more complication to this, which I think helps. Currently the thinkpad_acpi driver exports the privacy screen as:
/proc/acpi/ibm/lcdshadow
Userspace can write this and then change the privacy-screen setting, this is in shipped kernels and cannot be dropped because it is part if the kernel's uABI now. This means that another userspace process can change the property underneath a kms client. I do not see how this is different from the firmware changing the setting based on a hotkey press. Yet if we stick with your "only userspace can" change the sw-state setting, then does this count as userspace, or do you mean only a kms client can ? And then how is another kms-client changing the setting different ?
So to me to avoid confusion the only valid case where the hw- and sw-state can differ is if userspace requests say "off" as state while the privacy screen is forced on by say a physical switch (or e.g. a BIOS option to lock it?).
Then we would remember the off in sw-state but hw-state would still be on.
I guess that maybe for the enum of the hw-state we need 4 values instead of 2:
Enabled Disabled Enabled, locked Disabled, locked
To indicate to userspace that atm the state cannot be changed.
If userspace then still changes sw-state we cache it and apply this if the privacy screen control gets unlocked.
On hardware where there is no "lock" the 2 properties will simply always be the same.
Let's assume that you have a firmware-implemented hardware hotkey for toggling the shield. The driver also successfully implements "privacy-screen-sw-state" meaning that writing to it will set the hardware shield state. If userspace was writing "privacy-screen-sw-state" even when it does not intend to change hardware state, it would almost immediately override any state set by the hardware hotkey, making the hardware hotkey (randomly) not work.
Right, this is why userspace should not set the property unless it really means to change it, even then things could still race, but as explained above that should normally never happen and luckily the side-effects of hitting the race somehow are not that bad.
This assumes that the hardware hotkey is a momentary switch that does not stop software from controlling the shield too.
This is correct for the Lenovo / thinkpad_acpi case.
If the hardware hotkey can stop software from changing the shield state, then it might not be necessary for userspace to avoid unneeded setting of the property. But that depends on which way the hotkey works and which way users want to use it, so it's still best for userspace to not set the property unless it really intends to apply a change.
Ack.
If possible, it would be good to make this case the prime example of how to correctly implement KMS properties for a hardware feature that can be controlled (and fought over) by both userspace and hardware/firmware. It seems like the same design can also work with hardware switches that force the hardware state to be one or the other, stopping userspace from changing it. Therefore I'd avoid incorporating any specific shield use cases in the design, e.g. "if hw switch is set to shield-on, userspace cannot turn shield off".
I agree that it would be good to make this the prime example of how to deal with similar cases.
Also, it seems to me that in my current patchset, the property I have already behaves like "privacy-screen-sw-state". Do I just need to rename it?
Maybe, it looks like we first need to figure out the exact semantics of all this.
Regards,
Hans