Messages in this thread | | | Date | Mon, 6 Aug 2018 04:02:03 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: framebuffer corruption due to overlapping stp instructions on arm64 |
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On Sun, 5 Aug 2018, Florian Weimer wrote:
> On 08/04/2018 01:04 PM, Mikulas Patocka wrote: > > There's plenty of memcpy's in the graphics stack. No one will be rewriting > > all the graphics drivers because of tiny market share that ARM has in > > desktop computers. So if you refuse to fix things and blame everyone else, > > you can as well announce that you don't want to have PCIe graphics on ARM > > at all. > > The POWER toolchain maintainers said pretty much the same thing not too > long ago. I wonder how many architectures need to fail until the > graphics stack is finally fixed. > > Thanks, > Florian
If you say that your architecture doesn't support unaligned accesses at all, there's no problem - the compiler won't generate them and the libc won't contain them.
But if you say that your architecture supports unaligned accesses except for the framebuffer, then you have a problem - the compiler can't know which pointers point to the framebuffer and libc can't know either - you caused this problem by your architectural decision.
You can use 'volatile' to suppress memory optimizations, but it's impossible to go through the whole Linux graphics stack and add volatile to every pointer that may point to videoram. Even if you succeesed, new videoram accesses without volatile will appear after a year of development.
See for example the macros READ_ONCE and WRITE_ONCE in Linux kernel - they should be used when there's concurrent access to the particular variable, but mainstream architectures don't require them, so many kernel developers are omitting them in their code.
If you are building a supercomputer with a particular GPU, you can force the GPU vendor to provide POWER-compliant drivers. If you are building a workstation where the user can plug any GPU, forcing developers will go nowhere. You have to emulate the unaligned accesses and make sure that the next versions of your architecture support them in hardware.
Mikulas
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