linux-kernel - Re: [PATCH v2 1/1] i3c: Add fallback method for GETMXDS CCC

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Date:   Tue, 14 Nov 2023 09:19:56 +0100
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Joshua Yeong <joshua.yeong@...rfivetech.com>
Cc:     <alexandre.belloni@...tlin.com>, <linux-i3c@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] i3c: Add fallback method for GETMXDS CCC

Hi Joshua,

joshua.yeong@...rfivetech.com wrote on Tue, 14 Nov 2023 11:34:30 +0800:

> Some I3C hardware will report error when incorrect length is received from

					an

> device. GETMXDS CCC are availble in 2 formats; without turnaround time (format

			  available		:

> 1) and with turnaround time (format 2). There is no mechanics to determine which
> format is supported by device. In case sending GETMXDS CCC format 2 resulted

				So in case?

> failure, try sending GETMXDS CCC format 1 instead.

in a

> 
> Signed-off-by: Joshua Yeong <joshua.yeong@...rfivetech.com>
> ---
>  drivers/i3c/master.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
> index 0cdc94e4cb77..2fc04d97f07e 100644
> --- a/drivers/i3c/master.c
> +++ b/drivers/i3c/master.c
> @@ -1099,8 +1099,16 @@ static int i3c_master_getmxds_locked(struct i3c_master_controller *master,
>  
>  	i3c_ccc_cmd_init(&cmd, true, I3C_CCC_GETMXDS, &dest, 1);
>  	ret = i3c_master_send_ccc_cmd_locked(master, &cmd);
> -	if (ret)
> -		goto out;
> +	if (ret) {
> +		/*
		 ^
Wrong alignment of the starts below
		v
> +		* Retry when the device does not support max read turnaround
> +		* while expecting shorter length from this CCC command

period?

> +		*/
> +		dest->payload.len -= 3;
> +		ret = i3c_master_send_ccc_cmd_locked(master, &cmd);
> +		if (ret)
> +			goto out;
> +	}

I believe this is a per-device configuration and we should save this in
order to only fail once, no?

>  
>  	if (dest.payload.len != 2 && dest.payload.len != 5) {
>  		ret = -EIO;


Thanks,
Miquèl