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

Pierre-Louis Bossart
2022-12-01T20:38:30Z

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@opensource.cirrus.com>
> ---
> 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];

> + }
>
> 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;
>