Re: [PATCH 2/3] soundwire: cadence: Remove wasted space in response_buf

[Richard Fitzgerald]

On 01/12/2022 18:12, Pierre-Louis Bossart wrote:
> On 12/1/22 07:48, Richard Fitzgerald wrote:
> > The response_buf was declared much larger (128 entries) than the number
> > of responses that could ever be written into it (maximum 8).
>
> Indeed I don't know why we used 128 entries. This is a magic value that
> doesn't appear in any specs I've looked at.
>
> Note that there's 'sniffer' mode when each response takes two
> consecutive 32-words in the FIFO. we've never used this mode though so
> it's not really an issue.
>
> It's also possible that this is related to the automatic command retry,
> where a failed command can be re-issued 15 times. However in that case
> the worst case would be 32 commands * 15 = 480. The value of 128 makes
> no sense at all, unless it was an upper bound for 8 * 15. We don't use
> this hardware retry btw.
>
> See more below...
>
> > Reduce response_buf to 8 entries and add checking in cdns_read_response()
> > to prevent overflowing reponse_buf if CDNS_MCP_RX_FIFO_AVAIL contains
> > an unexpectedly large number.
> >
> > Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/soundwire/cadence_master.c | 6 ++++++
> >   drivers/soundwire/cadence_master.h | 2 +-
> >   2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/soundwire/cadence_master.c b/drivers/soundwire/cadence_master.c
> > index 27699f341f2c..95c84d9f0775 100644
> > --- a/drivers/soundwire/cadence_master.c
> > +++ b/drivers/soundwire/cadence_master.c
> > @@ -774,8 +774,14 @@ static void cdns_read_response(struct sdw_cdns *cdns)
> >   	u32 num_resp, cmd_base;
> >   	int i;
> >   
> > +	BUILD_BUG_ON(ARRAY_SIZE(cdns->response_buf) < CDNS_MCP_CMD_LEN);
> > +
> >   	num_resp = cdns_readl(cdns, CDNS_MCP_FIFOSTAT);
> >   	num_resp &= CDNS_MCP_RX_FIFO_AVAIL;
> > +	if (num_resp > ARRAY_SIZE(cdns->response_buf)) {
> > +		dev_warn(cdns->dev, "RX AVAIL %d too long\n", num_resp);
> > +		num_resp = CDNS_MCP_CMD_LEN;
>
> .... this is different from what the hardware documentation tells me.
> The range of values to RX_FIFO_AVAIL is 0..RX_FIFO_DEPTH + 2.
>
> I don't understand the +2, but we should maybe be more cautious and use
> u32 response_buf[CDNS_MCP_CMD_LEN + 2];

Probably we wouldn't know what to do with those extra 2 responses.
_cdns_xfer_msg() tells cdns_fill_msg_resp() to extract the same number
of responses as the original message length. So it would only extract 8
maximum. But anyway yes I can add the mysterious +2 into this.
Thinking about it, that's probably a good idea anyay for the next patch
that drains the RX FIFO after an error.

> > +	}
> >   
> >   	cmd_base = CDNS_MCP_CMD_BASE;
> >   
> > diff --git a/drivers/soundwire/cadence_master.h b/drivers/soundwire/cadence_master.h
> > index 0434d70d4b1f..c2d817e8e22a 100644
> > --- a/drivers/soundwire/cadence_master.h
> > +++ b/drivers/soundwire/cadence_master.h
> > @@ -117,7 +117,7 @@ struct sdw_cdns {
> >   	struct sdw_bus bus;
> >   	unsigned int instance;
> >   
> > -	u32 response_buf[0x80];
> > +	u32 response_buf[8];
> >   	struct completion tx_complete;
> >   	struct sdw_defer *defer;
> >