[PATCH 2/3] [v4] ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

[Dave Martin]

On Mon, Aug 24, 2015 at 04:51:44PM -0700, sboyd at codeaurora.org wrote:
> On 08/19, Dave Martin wrote:
> > On Wed, Aug 19, 2015 at 11:16:52AM -0500, Timur Tabi wrote:
> > > On 08/19/2015 05:14 AM, Dave Martin wrote:
> > > >>>So does that mean that ldrb will zero-extend the byte to all 64 bits of x3?
> > > >Yes.  No extra operation is required.
> > > 
> > > So my patch is actually correct as-is?
> > 
> > +static inline void __dcc_putchar(char c)
> > +{
> > +	asm volatile("msr dbgdtrtx_el0, %0"
> > +			: /* No output register */
> > +			: "r" (c));
> > 
> > For safety, you still need to make sure that c is appropriately masked
> > before passing it to the asm.  Something like this should definitely be
> > safe:
> > 
> > 			: "r" ((unsigned long)(unsigned char)c)
> > 
> > GCC can then emit uxtb or not, depending on whether it's needed in each
> > context where the asm is inlined.
> 
> Does this mean we ought to do the same thing in the arm header
> file too?

Probably.

Even though current CPUs don't place special meaning on the upper bits,
it's possible that future CPUs will.

Cheers
---Dave