Re: [RFC 0/5] drivers: Add boot constraints core

[Stephen Boyd]

On 06/29, Russell King - ARM Linux wrote:
> On Thu, Jun 29, 2017 at 08:28:08PM +0530, Viresh Kumar wrote:
> > On 29-06-17, 13:49, Russell King - ARM Linux wrote:
> > > The thing that concerns me most about this is that typically the LCD
> > > controller will be performing DMA to system RAM.
> > > 
> > > The location of the frame buffer is unknown to the decompressor - and
> > > as the decompressor self-relocates itself (using purely assembly code),
> > > it could relocate itself on top of the frame buffer, causing the "nice"
> > > image to become very colourful.
> > > 
> > > The decompressor doesn't have the information from DT at that point to
> > > know what are safe locations, so it's up to the boot loader to place
> > > the frame buffer somewhere out of the way.  (If people want to write
> > > a DT parser in position independent ARM assembly code that may change.)
> > > 
> > > As long as people realise this, then it's not a problem, but given the
> > > number of problems that we've already encountered with boot loaders and
> > > memory space layout, I don't trust them to get this right.
> > > 
> > > Right now, the ARM kernel booting document requires:
> > > 
> > > - Quiesce all DMA capable devices so that memory does not get
> > >   corrupted by bogus network packets or disk data. This will save
> > >   you many hours of debug.
> > > 
> > > so we would need to modify that to make an exception for LCD controllers.
> > > However, we definitely can't have devices left enabled which are capable
> > > of writing to system memory, or which changing system memory is likely
> > > to cause bad effects (eg, packet ring buffers, USB buffers etc, which is
> > > really what the above requirement is about.)
> > 
> > Well, LCD was just an example here. But yeah, it is one of the most
> > probable case we have.
> > 
> > So, this thing is already working for sure, of course with some out of
> > tree hacks. Every smart phone shows their company's logo (some kind of
> > flash) while the phone boots. How do they get around such issues?
> 
> As far as the memory being used goes, they probably locate the frame
> buffer well away from the kernel or any area that the kernel is likely
> to use during decompression.
> 
> It's probably also marked as a reserved memory region in DT to avoid
> the kernel touching it during boot, or _maybe_ they just locate it
> somewhere in memory that they've tested that the kernel doesn't touch
> until after their kernel has initialised the LCD controller (thereby
> avoiding the memory being permanently consumed.)
> 

Yes. The display controller is typically pointed to a memory
carveout that we treat as reserved in the kernel. I'm fairly
certain that we avoid the "permanently consumed" problem by
making it a carveout for the display controller, so that when the
display controller probes it can take ownership of the memory
from the bootloader.

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