[U-Boot] [PATCH v2] timer: add High Precision Event Timers (HPET) support

[Andy Shevchenko]

On Wed, Apr 4, 2018 at 2:26 AM, Ivan Gorinov <ivan.gorinov at intel.com> wrote:
> On Tue, Apr 03, 2018 at 06:17:42AM -0600, Andy Shevchenko wrote:
>> >> > If readq() is defined as two read operations in 32-bit code, main counter
>> >> > rollover (low part overflow, high part increment) can happen between them.
>> >> And how this contradicts ther current code?
>> > It just does not make the code simpler,
>> ...b/c you misread what I suggested.
>> > rollover check is
>> > still required if U-Boot is compiled as 32-bit code.
>> > Can we do something like the following?
>> Yes, but... why?
>> What's wrong with replacing two sequential 32-bit reads with one 64-bit?
>
> Doesn't readX/writeX imply a single I/O operation?

Not always.
It depends on atomicity you would like to get.

On 32-bit platform it's impossible to do readq() at once.

> It may be misleading to define it as two.
>
> Assuming MMX or SSE2 to be supported by all x86 processors, 64-bit I/O
> registers can be accessed as a single operation even in 32-bit code:
>
> static inline u64 readq(void *addr)
> {
>         u64 value;
>
>         asm volatile ("movq (%0), %%xmm0" : : "r" (addr));
>         asm volatile ("movq %%xmm0, %0" : "=m" (value));
>
>         return value;
> }
>
> I can add these definitions to "asm/io.h".

Please, no. This is wrong approach.

-- 
With Best Regards,
Andy Shevchenko