Re: [PATCH] i2c: aspeed: fix master pending state handling

From: Tao Ren <taoren@fb•com>
To: Jae Hyun Yoo <jae.hyun.yoo@linux•intel.com>,
	Brendan Higgins <brendanhiggins@google•com>,
	Wolfram Sang <wsa@the-dreams•de>,
	"Benjamin Herrenschmidt" <benh@kernel•crashing.org>,
	Joel Stanley <joel@jms•id.au>, "Rob Herring" <robh+dt@kernel•org>,
	Mark Rutland <mark.rutland@arm•com>,
	"Andrew Jeffery" <andrew@aj•id.au>,
	Cedric Le Goater <clg@kaod•org>
Cc: "devicetree@vger•kernel.org" <devicetree@vger•kernel.org>,
	"linux-aspeed@lists•ozlabs.org" <linux-aspeed@lists•ozlabs.org>,
	"linux-i2c@vger•kernel.org" <linux-i2c@vger•kernel.org>,
	"linux-arm-kernel@lists•infradead.org"
	<linux-arm-kernel@lists•infradead.org>,
	"openbmc@lists•ozlabs.org" <openbmc@lists•ozlabs.org>
Subject: Re: [PATCH] i2c: aspeed: fix master pending state handling
Date: Thu, 10 Oct 2019 23:11:36 +0000	[thread overview]
Message-ID: <5ce20a28-21ed-ff0c-9e31-bcac3cd7a5fb@fb.com> (raw)
In-Reply-To: <637d4b8b-5113-b07b-f99f-ce66dbe6948c@linux.intel.com>

On 10/10/19 3:04 PM, Jae Hyun Yoo wrote:
> On 10/10/2019 2:20 PM, Tao Ren wrote:
>> On 10/9/19 2:20 PM, Jae Hyun Yoo wrote:
> [...]
>>>           /*
>>>            * If a peer master starts a xfer immediately after it queues a
>>> -         * master command, change its state to 'pending' then H/W will
>>> -         * continue the queued master xfer just after completing the
>>> -         * slave mode session.
>>> +         * master command, clear the queued master command and change
>>> +         * its state to 'pending'. To simplify handling of pending
>>> +         * cases, it uses S/W solution instead of H/W command queue
>>> +         * handling.
>>>            */
>>>           if (unlikely(irq_status & ASPEED_I2CD_INTR_SLAVE_MATCH)) {
>>> +            writel(readl(bus->base + ASPEED_I2C_CMD_REG) &
>>> +                ~ASPEED_I2CD_MASTER_CMDS_MASK,
>>> +                   bus->base + ASPEED_I2C_CMD_REG);
>>
>> Sorry for the late comments (just noticed this line while testing the patch):
>>
>> I assume this line is aimed at stopping the running master commands, but as per
>> AST2500 datasheet, it's NOP to write 0 to MASTER_STOP/MASTER_RX/MASTER_TX bits.
>> Maybe all we need is writing 1 to MASTER_STOP field?
> 
> There could be two pending cases:
> 1. Master goes to pending before it triggers a command if a slave
>    operation is already initiated.
> 2. Master goes to pending after it triggered a command if a peer
>    master immediately sends something just after the master command
>    triggering.
> 
> Above code is for the latter case. H/W handles the case priority based
> so the slave event will be handled first, and then the master command
> will be handled when the slave operation is completed. Problem is,
> this H/W shares the same buffer for master and slave operations so
> it's unreliable. Above code just removes the master command from the
> command register to prevent this H/W command handling of pending events.
> Instead, it restarts the master command using a call of aspeed_i2c_do_start when the slave operation is completed.

Thanks for the clarify, Jae. I mean clearing these bits has no effect to
hardware according to aspeed datasheet; in other word, master command cannot
be removed from command register by this statement.

For example, below is the description for MASTER_STOP_CMD(I2CD14, bit 5):

  0: NOP
  1: Issue Master Stop Command
  This register will be automatically cleared by H/W when Stop Command has
  been issues.

Cheers,

Tao
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists•infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel