Originally Posted by Chainfire
The stop-gap solution is to disable this caching completely, which is what the 000000deepsleep script does, which you can find mentioned or quoted in many posts around the forum. From SuperSU 2.66 onwards, that script is automatically installed on Samsung devices when systemless root is used.
Please forgive me for posting (in a cf-auto-root thread) and digging afterwards. I had thought I'd just dump the info and forget about it, but I couldn't stop digging...
...which led me to the quoted material.
Digging in the supersu 2.66 update-
binary, I see that you're detecting "samsung" in the build fingerprint, and if true, doing a systemless install AND applying a deepsleep fix. This works for Galaxy S6 devices, but not for some other similar platform devices. In particular, the Note5 has THREE devices that need caching disabled in order for deep sleep to function. (0:0:0:3 as well as :2 and :1.)
My first question is: does the SGS6 even have a file named "/sys/class/scsi_disk/0:0:0:3/cache_type"? If not, just write to all three files and don't worry about it. The third write would fail on the SGS6 and all would be good. It'd be no worse of a work-around than already exists (and I think it's a bad work-around.)
If that file DOES exist in the SGS6, then something would have caching turned off that really shouldn't. Of course, existing or not, automatically tossing in this deepsleep patch for every single device that has "samsung" in the build fingerprint would seem likely break proper caching in some yet unknown samsung device. Perhaps the SGS7 will change things up so that :1 should be left cache flushable, and :2 would be the only one that should block cache flushing.
As well, it's also possible that Samsung will pull in the kernel fix to resolve this issue before they release Android M. (Okay, perhaps it'd be more likely for Samsung to open source touchwiz... but we can always have fantasies.)
My problem with the work-around is expressed above: it can break something in the future (and cause a support headache when some ONE exynos7420 device is fixed, but the rest aren't.) As well, it sets precedent of having platform specific hacks in the generic update.zip (but only allowing for a single platform and not in an easily expandable way.)
Obviously, it would be a maintenance nightmare to have different "00000deepsleep" files for every different device model. (if 'zerolte.*', SGS6. If 'nobellte.*', Note5, etc.)...
In keeping with what I tell other people, I feel I now have an obligation to suggest A Better Way. (a person shouldn't complain about something unless they can make a reasonable suggestion on how it'd be better.)
So, here's my slightly convoluted (but expandable) suggestion:
You currently use /data/.supersu to read certain variables that modify the supersu.zip installer script. Perhaps those "platform specific lines" could be added to that file, and the installer script would put them in place. So, I could do the following in a recovery root shell before installing supersu.zip:
echo PLATFORMSTARTUP='echo "temporary none" > /sys/class/scsi_disk/0:0:0:1/cache_type' >> /data/.supersu
(I'd have included both (or all three) needed lines for samsung deep sleep, but I forget how to include CR in a shell cmdline.. )
Then, the supersu installer script would just read PLATFORMSTARTUP and write it's contents to /su/su.d/00000platformstartup (and set perms.)
Given this type of thing, the existing 000000deepsleep hack would be removed. Then, individual devs could easily create simplistic "pre-installers" for supersu for specific platforms that need changes. Those "pre" installers would just write the needed PLATFORMSTARTUP lines to /data/.supersu...
... and then supersu.zip no longer needs platform specific hacks.
Some random XDA developer could then generate a simple "SGS6-supersu.zip" would only contain an edify script to mount /data and add/edit the .supersu file's PLATFORMSTARTUP variable to contain the two lines needed for deep sleep (and another dev could write a Note5 for the 3 lines needed on that platform... and so on..)