x86, hpet: Restrict read back to affected ATI chipsets

x86, hpet: Restrict read back to affected ATI chipsets

After programming the HPET, we do a readback as a workaround for
ATI/SBx00 chipsets as a synchronization.  Unfortunately this triggers
an erratum in newer ICH chipsets (ICH9+) where reading the comparator
immediately after the write returns the old value.  Furthermore, as
always, I/O reads are bad for performance.

Therefore, restrict the readback to the chipsets that need it, or, for
debugging purposes, when we are running with hpet=verbose.

Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Venkatesh Pallipadi <[email protected]>
LKML-Reference: <20100225185348[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>