Re: [PATCH] drm/bridge: analogix_dp: Don't return -EBUSY when msg->size is 0 in aux transaction

From: Tomasz Figa <tfiga@chromium.org>
To: zain wang <wzz@rock-chips.com>
Cc: "Sean Paul" <seanpaul@chromium.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Inki Dae" <inki.dae@samsung.com>,
	"David Airlie" <airlied@linux.ie>,
	"Tomeu Vizoso" <tomeu.vizoso@collabora.com>,
	"Mika Kahola" <mika.kahola@intel.com>,
	"Stéphane Marchesin" <marcheu@chromium.org>,
	"Douglas Anderson" <dianders@chromium.org>,
	"Thierry Reding" <treding@nvidia.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Javier Martinez Canillas" <javier@osg.samsung.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
	"open list:ARM/Rockchip SoC..."
	<linux-rockchip@lists.infradead.org>
Subject: Re: [PATCH] drm/bridge: analogix_dp: Don't return -EBUSY when msg->size is 0 in aux transaction
Date: Mon, 20 Feb 2017 11:40:35 +0900	[thread overview]
Message-ID: <CAAFQd5AaCQNP7M4a58Y03y2uY5vQeRfhm5jkyUgJcyQp9SYB4A@mail.gmail.com> (raw)
In-Reply-To: <1486978066-5351-1-git-send-email-wzz@rock-chips.com>

Hi Zain,

On Mon, Feb 13, 2017 at 6:27 PM, zain wang <wzz@rock-chips.com> wrote:
>
> The analogix_dp_transfer() will return -EBUSY if num_transferred is zero.
> But sometimes we will send a bare address packet to start the transaction,
> like drm_dp_i2c_xfer() show:
>         ......
>         /* Send a bare address packet to start the transaction.
>          * Zero sized messages specify an address only (bare
>          * address) transaction.
>          */
>         msg.buffer = NULL;
>         msg.size = 0;
>         err = drm_dp_i2c_do_msg(aux, &msg);
>         ......
>
> In this case, the msg->size is zero, so the num_transferred will be zero too.
> We can't return -EBUSY here, let's we return num_transferred if num_transferred
> equals msg->size.
>

Please see my question inline.

> Signed-off-by: zain wang <wzz@rock-chips.com>
> ---
>  drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c b/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> index 303083a..5384aca 100644
> --- a/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> +++ b/drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c
> @@ -1162,5 +1162,5 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
>                  (msg->request & ~DP_AUX_I2C_MOT) == DP_AUX_NATIVE_READ)
>                 msg->reply = DP_AUX_NATIVE_REPLY_ACK;
>
> -       return num_transferred > 0 ? num_transferred : -EBUSY;
> +       return (num_transferred == msg->size) ? num_transferred : -EBUSY;

I might be missing something but, looking at the code, I don't see any
possibility of num_transferred ever being different than msg->size. To
be honest, it doesn't seem to even make any sense keeping the local
variable there, because msg->size can be simply always returned, as
errors are handled by jumping to aux_error label.

Best regards,
Tomasz