linux-kernel - Re: [PATCH] i2c: aspeed: Acknowledge Tx ack late when in SLAVE_READ

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Message-ID: <CAOeEDyumVdi-3O3apMUFJ695V3YcZqZQ7wvzYL2YfU88XJ3Dxw@mail.gmail.com>
Date:   Mon, 27 Nov 2023 15:04:51 +0800
From:   Cosmo Chou <chou.cosmo@...il.com>
To:     Andrew Jeffery <andrew@...econstruct.com.au>
Cc:     brendan.higgins@...ux.dev, benh@...nel.crashing.org,
        joel@....id.au, andi.shyti@...nel.org, linux@...ck-us.net,
        wsa@...nel.org, jae.hyun.yoo@...ux.intel.com,
        linux-i2c@...r.kernel.org, openbmc@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        cosmo.chou@...ntatw.com
Subject: Re: [PATCH] i2c: aspeed: Acknowledge Tx ack late when in SLAVE_READ_PROCESSED

Andrew Jeffery <andrew@...econstruct.com.au> wrote on Mon, 2023-11-27
at 11:23 AM:
>
> On Mon, 2023-11-20 at 17:17 +0800, Cosmo Chou wrote:
> > commit 2be6b47211e1 ("i2c: aspeed: Acknowledge most interrupts early
> > in interrupt handler") moved most interrupt acknowledgments to the
> > start of the interrupt handler to avoid race conditions. However,
> > slave Tx ack status shouldn't be cleared before SLAVE_READ_PROCESSED
> > is handled.
> >
> > Acknowledge Tx ack status after handling SLAVE_READ_PROCESSED to fix
> > the problem that the next byte is not sent correctly.
>
> What does this mean in practice? Can you provide more details? It
> sounds like you've seen concrete problems and it would be nice to
> capture what it was that occurred.
>
> Andrew

For a normal slave transaction, a master attempts to read out N bytes
from BMC: (BMC addr: 0x20)
[S] [21] [A] [1st_B] [1_ack] [2nd_B] [2_ack] ... [Nth_B] [N] [P]

T1: when [21] [A]: Both INTR_SLAVE_MATCH and INTR_RX_DONE rise,
INTR_RX_DONE is not cleared until BMC is ready to send the 1st_B:
https://github.com/torvalds/linux/blob/master/drivers/i2c/busses/i2c-aspeed.c#L294
That is, BMC stretches the SCL until ready to send the 1st_B.

T2: when [1_ack]: INTR_TX_ACK rises, but it's cleared at the start of
the ISR, so that BMC does not stretch the SCL, the master continues
to read 2nd_B before BMC is ready to send the 2nd_B.

To fix this, do not clear INTR_TX_ACK until BMC is ready to send data:
https://github.com/torvalds/linux/blob/master/drivers/i2c/busses/i2c-aspeed.c#L302

Cosmo