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

[Dave Martin]

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.

Cheers
---Dave