Re: [PATCH] drm/dp: Use large transactions for I2C over AUX

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Jan 23, 2015 at 06:40:38PM +0000, Simon Farnsworth wrote:
> DisplayPort to DVI-D Dual Link adapters designed by Bizlink have bugs in
> their I2C over AUX implementation. They work fine with Windows, but fail
> with Linux.
> 
> It turns out that they cannot keep an I2C transaction open unless the
> previous read was 16 bytes; shorter reads can only be followed by a zero
> byte transfer ending the I2C transaction.
> 
> Copy Windows's behaviour, and read 16 bytes at a time. Analysis of the
> failure state was provided by Datapath Ltd.
> 
> Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
> ---
> Thierry,
> 
> You put in the comment about "decreased performance", back in December 2013;
> would you mind testing that this still works with the devices you tested?
> 
> Unfortunately, Bizlink are the only game in town for DP->DVI-DL adapters -
> and their firmware is prone to giving up on I2C if we look at it
> wrongly. Even Apple's device is Bizlink designed.
> 
>  drivers/gpu/drm/drm_dp_helper.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 79968e3..b4a9d4a 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -507,16 +507,13 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
>  		err = drm_dp_i2c_do_msg(aux, &msg);
>  		if (err < 0)
>  			break;
> -		/*
> -		 * Many hardware implementations support FIFOs larger than a
> -		 * single byte, but it has been empirically determined that
> -		 * transferring data in larger chunks can actually lead to
> -		 * decreased performance. Therefore each message is simply
> -		 * transferred byte-by-byte.
> +		/* Bizlink designed DP->DVI-D Dual Link adapters require the
> +                 * I2C over AUX packets to be as large as possible. If not,
> +                 * the I2C transactions never succeed.
>  		 */
> -		for (j = 0; j < msgs[i].len; j++) {
> +		for (j = 0; j < msgs[i].len; j+=16) {
>  			msg.buffer = msgs[i].buf + j;
> -			msg.size = 1;
> +			msg.size = min(16, msgs[i].len - 16);

I don't think it's quite this simple. The sink is allowed to ACK
partial data for multi-byte messages. The code doesn't handle that.

>  
>  			err = drm_dp_i2c_do_msg(aux, &msg);
>  			if (err < 0)
> -- 
> 2.1.0
> 
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-- 
Ville Syrjälä
Intel OTC
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