Re: wbflush() abuse for TOSHIBA_RBTX4927

[Atsushi Nemoto <anemo@mba.ocn.ne.jp>]

>>>>> On Wed, 16 Apr 2003 15:46:54 +0200 (MET DST), "Maciej W. Rozycki" 
>>>>> <macro@ds2.pg.gda.pl> said:
>> AFAIK TX49's SYNC instruction correctly flushes the write buffer.

macro>  That would be an overkill; we only need to enforce strong
macro> ordering here.

Hmm... But SYNC is a only TX49 instruction that enforce completions of
preceding read operations.  (TX49 have "non-blocking load" feature
which allows non-cached read/write to overtake preceding cached read)

I can imagine three configurations:

1. Enable CONFIG_CPU_HAS_SYNC and disable CONFIG_CPU_HAS_WB.  In this
   case, wmb/rmb/mb/iob/wbflush macro all issues a SYNC instruction.

2. Disable CONFIG_CPU_HAS_SYNC and enable CONFIG_CPU_HAS_WB, In this
   case, wmb/rmb macro does nothing and mb/iob/wbflush macro calls
   __wbflush().

3. Enable both CONFIG_CPU_HAS_SYNC and CONFIG_CPU_HAS_WB, In this
   case, wmb/rmb macro issues a SYNC instruction, mb/iob macro calls
   __wbflush() and wbflush macro do both.

In the case 2 and 3, __wbflush() must be implemented with SYNC instruction
and/or bc0f loop.

I think the case 1 is good and enough for TX49.

>> No bc0f loop is required.

macro>  But an external buffer may be attached to a TX49 core, IIRC,
macro> so if it is the case for TOSHIBA_RBTX4927, it's obviously
macro> needed.

Although I'm not sure whether TX49 core can be integrated with such an
external write buffer, all TX49XX (not TX39) I have ever seen does not
have it.

---
Atsushi Nemoto