Re: [PATCH] i2c: aspeed: Acknowledge Tx ack late when in SLAVE_READ_PROCESSED

[Quan Nguyen]

On 27/11/2023 14:04, Cosmo Chou wrote:
> Andrew Jeffery <andrew@xxxxxxxxxxxxxxxxxxxx> 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

This looks like the same issue, but we chose to ack them late. Same with 
INTR_RX_DONE.

https://lore.kernel.org/all/20210616031046.2317-3-quan@xxxxxxxxxxxxxxxxxxxxxx/


- Quan