linux-kernel - Re: [PATCH] drm/bridge: nxp-ptn3460: fix i2c_master

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Date:   Mon, 4 Dec 2023 19:59:29 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Neil Armstrong <neil.armstrong@...aro.org>
Cc:     Sean Paul <seanpaul@...omium.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Inki Dae <inki.dae@...sung.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: nxp-ptn3460: fix i2c_master_send() error
 checking

On Mon, Dec 04, 2023 at 02:53:05PM +0100, Neil Armstrong wrote:
> On 04/12/2023 13:29, Dan Carpenter wrote:
> > The i2c_master_send/recv() functions return negative error codes or the
> > number of bytes that were able to be sent/received.  This code has
> > two problems.  1)  Instead of checking if all the bytes were sent or
> > received, it checks that at least one byte was sent or received.
> > 2) If there was a partial send/receive then we should return a negative
> > error code but this code returns success.
> > 
> > Fixes: a9fe713d7d45 ("drm/bridge: Add PTN3460 bridge driver")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
> > ---
> > This is from static analysis and code review.  It's always a concern
> > when you add stricter error handling that something will break.
> > 
> >   drivers/gpu/drm/bridge/nxp-ptn3460.c | 10 +++++-----
> >   1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/bridge/nxp-ptn3460.c b/drivers/gpu/drm/bridge/nxp-ptn3460.c
> > index d81920227a8a..9b7eb8c669c1 100644
> > --- a/drivers/gpu/drm/bridge/nxp-ptn3460.c
> > +++ b/drivers/gpu/drm/bridge/nxp-ptn3460.c
> > @@ -56,13 +56,13 @@ static int ptn3460_read_bytes(struct ptn3460_bridge *ptn_bridge, char addr,
> >   	ret = i2c_master_send(ptn_bridge->client, &addr, 1);
> >   	if (ret <= 0) {
> >   		DRM_ERROR("Failed to send i2c command, ret=%d\n", ret);
> > -		return ret;
> > +		return ret ?: -EIO;
> >   	}
> >   	ret = i2c_master_recv(ptn_bridge->client, buf, len);
> > -	if (ret <= 0) {
> > +	if (ret != len) {
> 
> This is impossible, i2c_transfer_buffer_flags() returns len as-is if no error, so
> ret can only be negative or equal to len. The original code is right.

It works, but it's not "right".  The <= 0 could be changed to < 0.  The
"len" variable is EDID_LENGTH (128).

regards,
dan carpenter