linux-kernel - Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

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Message-ID: <10624c71-d134-441f-a7e6-d757b60f54f8@stanley.mountain>
Date: Mon, 12 Aug 2024 09:54:41 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Dave Airlie <airlied@...hat.com>, Jocelyn Falempe <jfalempe@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH v2] drm/ast: astdp: fix loop timeout check

On Mon, Aug 12, 2024 at 08:48:16AM +0200, Thomas Zimmermann wrote:
> Hi
> 
> Am 12.08.24 um 08:42 schrieb Dan Carpenter:
> > This code has an issue because it loops until "i" is set to UINT_MAX but
> > the test for failure assumes that "i" is set to zero.  The result is that
> > it will only print an error message if we succeed on the very last try.
> > Reformat the loop to count forwards instead of backwards.
> > 
> > Fixes: 2281475168d2 ("drm/ast: astdp: Perform link training during atomic_enable")
> > Signed-off-by: Dan Carpenter <dan.carpenter@...aro.org>
> > ---
> > v2: In version one, I introduced a bug where it would msleep(100) after failure
> >      and that is a pointless thing to do.  Also change the loop to a for loop.
> > ---
> >   drivers/gpu/drm/ast/ast_dp.c | 12 +++++-------
> >   1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/ast/ast_dp.c b/drivers/gpu/drm/ast/ast_dp.c
> > index 5d07678b502c..9bc21dd6a54d 100644
> > --- a/drivers/gpu/drm/ast/ast_dp.c
> > +++ b/drivers/gpu/drm/ast/ast_dp.c
> > @@ -146,18 +146,16 @@ void ast_dp_power_on_off(struct drm_device *dev, bool on)
> >   void ast_dp_link_training(struct ast_device *ast)
> >   {
> >   	struct drm_device *dev = &ast->base;
> > -	unsigned int i = 10;
> > +	int i;
> > -	while (i--) {
> > +	for (i = 0; i < 10; i++) {
> >   		u8 vgacrdc = ast_get_index_reg(ast, AST_IO_VGACRI, 0xdc);
> >   		if (vgacrdc & AST_IO_VGACRDC_LINK_SUCCESS)
> > -			break;
> > -		if (i)
> > -			msleep(100);
> > +			return;
> > +		msleep(100);
> 
> But we don't want to wait during the final iteration of this loop. If you
> want to use the for loop, it should be something like
> 
> for (i= 0; i < 10; ++i) {
> 
>     if (i)
>       msleep(100)
> 
>     // now test vgacrdc
> }
> 
> Best regards
> Thomas

I feel like if we really hit this failure path then we won't care about the
tenth msleep().  I can resend if you want, but I'd prefer to just leave it.

regards,
dan carpenter