Date: Thu, 8 Oct 2015 15:39:35 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Leonardo Fogel <leonardofogel@yahoo.com.br> Cc: freebsd-drivers@freebsd.org Subject: Re: Memory barrier Message-ID: <20151008223935.GH67524@funkthat.com> In-Reply-To: <1441547636.78646.YahooMailBasic@web120801.mail.ne1.yahoo.com> References: <2149458.2Hg3JbBXY3@ralph.baldwin.cx> <1441547636.78646.YahooMailBasic@web120801.mail.ne1.yahoo.com>
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Leonardo Fogel wrote this message on Sun, Sep 06, 2015 at 06:53 -0700:
> Please, what are the correct barriers for the following cases?
>
> Case 1:
> bus_write_1(region_0, ...);
> /* barrier here */
> DELAY(some_time);
>
> Case 2:
> bus_write_1(region_0, ...);
> /* barrier here */
> bus_write_1(region_2, ...);
>
> In the first one, I want the write to reach the device before the thread busy-waits. As I understand it, bus_space_barrier(9) is not adequate, because it does not prevent the processor from executing instructions (or load/store to RAM) before the write completes.
>
> In the second one, I want the write to a device (e.g. power management) to complete before the write to another starts/completes. Again, bus_space_barrier() seems not to be adequate because it can not cover two regions.
>
> Thank you for your time.
If this is a PCI device, you need to do a read from the device before
it is guaranteed that all the bridges have flushed the writes to the
device... A barrier only guarantees from the processor's perspective
that the write "has completed", but not that it will reach the device..
If this is a different bus, then the rules are probably different...
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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