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From: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Richard Henderson <rth@twiddle.net>,
	Matt Turner <mattst88@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	alpha <linux-alpha@vger.kernel.org>,
	linux-serial@vger.kernel.org, linux-rtc@vger.kernel.org
Subject: Re: [PATCH 1/2 v3] alpha: add a delay to inb_p, inb_w and inb_l
Date: Wed, 13 May 2020 15:41:28 +0100
Message-ID: <20200513144128.GA16995@mail.rc.ru> (raw)
In-Reply-To: <alpine.LFD.2.21.2005111320220.677301@eddie.linux-mips.org>

On Mon, May 11, 2020 at 03:58:24PM +0100, Maciej W. Rozycki wrote:
>  Individual PCI port locations correspond to different MMIO locations, so 
> yes, accesses to these can be reordered (merging won't happen due to the 
> use of the sparse address space).

Correct, it's how Alpha write buffers work. According to 21064 hardware
reference manual, these buffers are flushed when one of the following
conditions is met:

1) The write buffer contains at least two valid entries.
2) The write buffer contains one valid entry and at least 256 CPU cycles
   have elapsed since the execution of the last write buffer-directed
   instruction.
3) The write buffer contains an MB, STQ_C or STL_C instruction.
4) A load miss is pending to an address currently valid in the write
   buffer that requires the write buffer to be flushed.

I'm certain that in these rtc/serial cases we've got readX arriving
to device *before* preceeding writeX because of 2). That's why small
delay (300-1400 ns, apparently depends on CPU frequency) seemingly
"fixes" the problem. The 4) is not met because loads and stores are
to different ports, and 3) has been broken by commit 92d7223a74.

So I believe that correct fix would be to revert 92d7223a74 and
add wmb() before [io]writeX macros to meet memory-barriers.txt
requirement. The "wmb" instruction is cheap enough and won't hurt
IO performance too much.

Ivan.

  parent reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-06 11:21 [PATCH 1/2] alpha: add a delay between RTC port write and read Mikulas Patocka
2020-05-06 14:20 ` Arnd Bergmann
2020-05-06 17:12   ` [PATCH 1/2 v2] alpha: add a delay to inb_p, inb_w and inb_l Mikulas Patocka
2020-05-07  8:06     ` [PATCH 1/2 v3] " Mikulas Patocka
2020-05-07  8:20       ` Greg Kroah-Hartman
2020-05-07 10:53         ` Mikulas Patocka
2020-05-07 13:30       ` Arnd Bergmann
2020-05-07 14:09         ` Mikulas Patocka
2020-05-07 15:08           ` Arnd Bergmann
2020-05-07 15:45             ` Mikulas Patocka
2020-05-07 15:46             ` [PATCH v4] alpha: add a barrier after outb, outw and outl Mikulas Patocka
2020-05-07 19:12               ` Arnd Bergmann
2020-05-10  1:27                 ` Maciej W. Rozycki
2020-05-10  1:25             ` [PATCH 1/2 v3] alpha: add a delay to inb_p, inb_w and inb_l Maciej W. Rozycki
2020-05-10 18:50               ` Mikulas Patocka
2020-05-11 14:58                 ` Maciej W. Rozycki
2020-05-12 19:35                   ` Mikulas Patocka
2020-05-13 14:41                   ` Ivan Kokshaysky [this message]
2020-05-13 16:13                     ` Greg Kroah-Hartman
2020-05-13 17:17                     ` Maciej W. Rozycki
2020-05-22 13:03                       ` Mikulas Patocka
2020-05-22 13:37                         ` Maciej W. Rozycki
2020-05-22 13:26                     ` Mikulas Patocka
2020-05-22 20:00                       ` Mikulas Patocka
2020-05-23 10:26                         ` [PATCH v4] alpha: fix memory barriers so that they conform to the specification Mikulas Patocka
2020-05-23 15:10                           ` Ivan Kokshaysky
2020-05-23 15:34                             ` Mikulas Patocka
2020-05-23 15:37                               ` [PATCH v5] " Mikulas Patocka
2020-05-24 14:54                                 ` Maciej W. Rozycki
2020-05-25 13:56                                   ` Mikulas Patocka
2020-05-25 14:07                                     ` Arnd Bergmann
2020-05-25 14:45                                     ` Maciej W. Rozycki
2020-05-25 15:53                                       ` [PATCH v6] " Mikulas Patocka
2020-05-26 14:47                                         ` [PATCH v7] " Mikulas Patocka
2020-05-27  0:18                                           ` Maciej W. Rozycki
2020-06-08  6:58                                             ` Mikulas Patocka
2020-06-08 23:49                                               ` Matt Turner
2020-05-25 15:54                                       ` [PATCH v5] " Mikulas Patocka
2020-05-25 16:39                                         ` Maciej W. Rozycki
2020-05-26 14:48                                           ` Mikulas Patocka
2020-05-27  0:23                                             ` Maciej W. Rozycki
2020-05-23 16:44                               ` [PATCH v4] " Maciej W. Rozycki
2020-05-23 17:09                                 ` Mikulas Patocka
2020-05-23 19:27                                   ` Maciej W. Rozycki
2020-05-23 20:11                                     ` Mikulas Patocka

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